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Lecture Comments (2)

0 answers

Post by Kunj Kashyap on May 12, 2017

where can I find the questions?????????

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Post by Connor McRobert on May 9, 2015

Could you please be better prepared in each lecture for the future?

AP Biology Practice Exam: Section I, Part A, Multiple Choice Questions 1-31

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • AP Biology Practice Exam 0:14
  • Multiple Choice 1 0:40
  • Multiple Choice 2 2:27
  • Multiple Choice 3 4:30
  • Multiple Choice 4 6:43
  • Multiple Choice 5 9:27
  • Multiple Choice 6 11:32
  • Multiple Choice 7 12:54
  • Multiple Choice 8 14:42
  • Multiple Choice 9 17:06
  • Multiple Choice 10 18:42
  • Multiple Choice 11 20:49
  • Multiple Choice 12 23:23
  • Multiple Choice 13 26:20
  • Multiple Choice 14 27:52
  • Multiple Choice 15 28:44
  • Multiple Choice 16 33:07
  • Multiple Choice 17 35:31
  • Multiple Choice 18 39:43
  • Multiple Choice 19 40:37
  • Multiple Choice 20 42:47
  • Multiple Choice 21 45:58
  • Multiple Choice 22 49:49
  • Multiple Choice 23 53:44
  • Multiple Choice 24 55:12
  • Multiple Choice 25 55:59
  • Multiple Choice 26 56:50
  • Multiple Choice 27 58:08
  • Multiple Choice 28 59:54
  • Multiple Choice 29 1:01:36
  • Multiple Choice 30 1:02:31
  • Multiple Choice 31 1:03:50

Transcription: AP Biology Practice Exam: Section I, Part A, Multiple Choice Questions 1-31

Welcome to I am Dr. Carleen Eaton.0000

In today's lesson, I am going to be reviewing a practice AP Biology Exam.0005

And I will be starting with section 1, part A, multiple choice questions.0010

You can find the questions written out in Barron's AP Biology, 4th Edition by Deborah Goldberg, and I will be reviewing model test 1.0016

So, I am not going to read out each question. You can follow along in the book.0030

But I am going to review the information and explain it and how to get to the correct answer.0033

Question 1 discusses chemical messengers, hormones, specifically, testosterone which is a steroid hormone.0045

And you are given a sketch of the structure of this hormone, and because it is a steroid hormone, it is a derivative of cholesterol.0053

And cholesterol has a characteristic structure of four linked hydrocarbon rings.0061

And then, in this question, you are supposed to pick the statement that explains an action from testosterone.0067

So, looking at the first answer A, it discusses binding to a cell surface receptor.0077

However, because testosterone is a steroid hormone, it is actually able to enter the cell.0084

It is soluble to the cell membrane, and because it can enter the cell, the receptor for testosterone is in the cytoplasm.0092

It is a cytoplasmic receptor.0101

So, what happens is testosterone enters the target cell, binds to its receptor.0103

And the receptor hormone complex travels to the nucleus where that complex can turn genes on or off.0108

So, it can act directly on the DNA because it is able to enter the cell.0115

Therefore, A and B are both incorrect because they discuss binding to a cell surface receptor.0121

C is correct. It describes the action of testosterone.0128

Again, D is incorrect. It is talking about the binding cyclic-AMP.0133

Cyclic-AMP often acts as the second messenger within the cell, so the correct answer for 1 is C.0137

This second question talks about the difference between ectotherms and endotherms regarding the control of body temperature.0147

And recall that like a mouse or a human being or a horse, endotherms maintain a constant internal temperature despite changes in the environmental temperature,0156

whereas ectotherms such as lizards have body temperature that changes based on the surrounding temperature.0168

So, looking at these different graphs, in the first graph, the mouse is shown to have a constant0175

body temperature, stable body temperature despite changes in the environmental temperature.0185

What is happening with the lizard though is that as the temperature is increasing, in answer A, the lizard's0191

body temperature is decreasing which does not make sense because I would expect the lizard's temperature0199

to increase as environmental temperature increased, and that is actually what is happening in answer B.0204

In answer B, as the surrounding temperature, the ambient temperature is increasing, the lizard's temperature is increasing as well.0211

The mouse has a stable body temperature across a wide environmental temperature range which is expected, so that is the correct answer.0222

In C, again, the mouse has a stable body temperature.0230

However, it is showing that the lizard is able to maintain its body temperature across a certain temperature range.0234

And then, it goes up with the surrounding temperature, but that is not accurate depiction of the situation.0239

And then, in the final answer, the mouse has some control of its body temperature.0245

But at higher temperatures, the mouse's body temperature starts increasing, and then, the lizard's temperature is changing in sort of a stepwise fashion.0250

So, it is showing periods of stability which would be inaccurate.0258

I would not expect the lizard to be able to maintain its body temperature than have a jump-up, maintain and then a jump-up, so again, the answer is 2.0261

This next question is talking about depolarization with a presynaptic membrane.0270

So, what we are really talking about here is action potentials and the various steps in an action potential.0279

So, looking at a presynaptic cell and a postsynaptic cell, depolarization is a very early step in the generation of an action potential.0285

And what happens is when the presynaptic cell is depolarized, calcium channels open up and calcium enters the cell.0310

When calcium enters the cell, that triggers the fusion of vesicles containing neurotransmitters with the cell membrane.0324

And when those vesicles fuse, the neurotransmitters are released into the synaptic cleft.0333

So, here is the neurotransmitter, and the postsynaptic cell will contain receptors. For this neurotransmitter, the receptors will bind.0341

And that binding can stimulate the cell, trigger depolarization and the postsynaptic cell and an action potential.0353

So, I have gone through all these various steps, but what this question is asking about is really the next step after depolarization.0360

And as I mentioned, that next step would be the opening of calcium channels, voltage-gated calcium channels0367

causing in the influx of calcium into the presynaptic cell, and that is what answer C is describing.0374

The other answers, A, B and D are all describing various steps that occur in the action potential0384

and the stimulation of post synaptic cell, but they are the later steps, so the next step, again, is answer C.0393

Going on to question 4. Here is a system involving an enzyme succinate dehydrogenase and an inhibitor, malonate.0405

But we do not know what kind of inhibitor it is, and recall that there are two kinds of inhibitors.0416

In the competitive inhibitor, you have got an enzyme. So, here is the enzyme, and its active site is right here.0426

And the substrate combines the active site.0439

However, there is also an inhibitor and I will call it C for competitive inhibitor that can bind that site as well, that active site.0442

And binding by the competitive inhibitor would block binding of the substrate.0451

Non-competitive inhibitor, recall, works differently.0457

There are multiple sites on this enzyme, so there is the active site. This is where the substrate binds.0462

Then, there is the second site wherein an inhibitor combines, so N for non-competitive, and it can bind in here.0471

Binding of a non-competitive inhibitor, it causes a conformational change in the enzyme that could block binding of the substrate.0480

So, to determine what type of inhibitor malonate is, one thing that you could do would be to add a very high level of substrate.0493

So, since these two, the substrate and the inhibitor, are directly competing for the active site, if you add a lot of substrate,0505

you can look at it as it is diluting out the inhibitor, and the chances are that the substrate is going to bind rather than the competitive inhibitor.0513

So, what I would expect to happen, or the correct statement here is actually A, that if concentration of0524

substrate is increased, the rate of reaction will increase if malonate is a competitive inhibitor.0533

Now, if I were to increase substrate concentration, and it was a non-competitive inhibitor,0542

it would not affect the reaction rate because there is no competition for this site.0547

Adding substrate is not going to affect the binding of a non-competitive inhibitor over here at this distant site.0551

So, the only one that is an accurate statement here is A.0558

Question 5 gives you various graphs showing temperature and precipitation by month.0568

And try to figure out which of these various climates is going to be best for plant growth.0576

And a climate that is warm for more of the year is going to have a longer growing season, so that is one thing I am going to look for.0584

Also, since, we are given that precipitation is a limiting factor, I am going to look for a climate that has a high level of precipitation.0596

Looking at this first climate that is described, it has a cold winter.0606

So, there are some cold months in there, and plant growth is going to be very slow or minimal during those months.0611

In addition, there are some very dry months in there where precipitation drops down to zero.0619

So, A is not an optimal climate out of these. Looking at B, just right away, I am seeing that precipitation is very low throughout the year in this climate.0625

C: looking at the graph, this temperature actually goes all the way down to -10°C. Again, some cold winter months.0640

And precipitation is low during certain months, higher during others.0649

But compare that to D, D is actually the best answer because if you are looking for what is going to give a good growing season, there is a stable0655

relatively warm temperature about 25 or so degree Celsius throughout the year and high levels of precipitation ranging from about a 100 all the way up to 400.0664

So, those are both very good to allow for plant growth, the warm stable temperature throughout the year and high levels of precipitation.0679

So, the answer to 5 is D.0688

Another graph in question 6, and in this one, we are being asked about photosynthesis.0692

And in the graph, what you can see is that as light intensity increases, the rate of photosynthesis increases to a certain point.0700

After a certain point, even if you increase the intensity of the light, the rate of photosynthesis remains stable.0709

So, let's figure out the explanation for this.0717

Really, looking at the graph, the only one that fits is D because the explanation0723

here is fitting an idea of the rate increasing as light increases and then, levelling off.0731

And that levelling off can be explained by the fact that the light harvesting apparatus is maxed out.0738

It has been saturated so that even if you provide greater light intensity, the cell machinery cannot work any faster, so it is D.0743

These other answers are referring to photosynthesis stopping or slowing down.0756

And it is actually not stopping nor is the rate decreasing. The rate is actually just levelling off, so D fits with that scenario.0764

The lac operon is a group of genes found in E. coli that are responsible for synthesizing0776

the enzymes that metabolize lactose sugar, that allow E. coli to break down lactose sugar.0783

The lac operon is turned off when a repressor protein is bound to it.0790

However, when lactose is present, the repressor is blocked from binding, so the repressor cannot0797

bind to the lac operon, and because the repressor is not bound, transcription can take place.0803

And this is a very well-studied set of genes, and the correct answer is A.0811

It is studied because it allows us to look at one model of the regulation of transcription.0818

Looking at answer B, that is scaling way up talking about what is going on in a mammal with utilization of lactose, E. coli or bacteria.0829

So, we are just looking at a very basic molecular mechanism when we study this.0840

RNA processing occurs after transcription.0845

The lac operon is studied to really see how transcription itself is regulated not necessarily processing that takes place after the transcript is created.0850

Cell cycle is related to cancer. However, this model, this lac operon is not really giving us insight into the cell cycle.0861

So, remember that this regulation of the cell cycle can cause out of control growth in cells, and that is one cause of cancer.0870

Question 8 provides you with an equation for photosynthesis.0884

And recall that in photosynthesis, carbon dioxide and water are used by plants to generate glucose using light energy.0889

Other product are water and oxygen, so those are other products of photosynthesis.0901

And we are asked to follow an atom and see where this atom is going to end up in the products.0908

The first three answers all are indicating an oxygen atom on water.0917

Now, in the light-dependent reaction...remember that there is light-dependent reactions and light-independent reactions in photosynthesis.0924

In the first set of reactions, the light-dependent reactions, water is split, so that process is called photolysis.0933

Recall the splitting of water using light. In this, water is split into two hydrogen ions plus one half of an O2 molecule.0942

Splitting water provides a source of electrons that are needed for the light-dependent reactions.0961

And what ends up happening when water is spit is that this one half O2 combines with another oxygen molecule to create O2 gas0968

which is released from the plant into the atmosphere, and the only one that is describing that is B.0978

B is indicating that the oxygen on water, on this twelve water molecules is oxygen as part of water will end up being released as oxygen gas.0986

A and C both show the oxygen on water, but they show it ending up...1000

In A, it ends up in the glucose, and in C, it ends up in water again. That is not correct.1007

D shows the oxygen from CO2 as being the source of oxygen released in the plant, which is not correct.1013

No. 9 is talking about transport across cell membranes and various molecules or mechanisms of transport.1030

So, we have to look at which one of these is correct.1041

The statement in A is inaccurate. Carbon is actually a symmetrical molecule that is non-polar, and it is non-polar because the electrons are equally shared.1043

And because it is a small non-polar molecule, it is permeable to the cell membrane. However, answer A is stating that carbon dioxide is polar.1055

B, starch is a large molecule, so it cannot diffuse across the cell membrane. C is the correct answer.1065

While water can diffuse across cell membranes also across many species of organisms, their cells contain aquaporins.1073

Or they are also called water channels to allow much more rapid uptake of water that the cell may need.1084

D is also inaccurate. Cristae are infoldings of the inner mitochondrial membrane.1094

And these infoldings actually increase the surface area of the inner mitochondrial membrane.1100

Protons can go through ATP synthase channels, so protons can pass through the cristae. However, oxygen does not.1107

So, the only accurate answer here is C which discusses the function of aquaporins or water channels.1115

Talking a little bit more about transport across cell membranes, recall that active transport requires the input of energy.1124

It transports molecules against their concentration gradients.1134

So, it requires energy transport protein, and it moves molecules against their concentration gradients.1142

So, a few things to think about as you are looking at these answers.1147

Glucose transport in A, glucose transport occurs by diffusion, and diffusion is a passive transport process. It does not require the input of energy.1154

Facilitated diffusion indicates that there is a carrier protein but facilitated diffusion is still a passive process1164

in which as this answer says, the molecules being moved down or along its concentration gradient.1175

The correct answer B discusses pumps. Protists have vacuoles to remove water, and these pumps do require energy.1181

That is a form of active transport, B.1193

Counter current exchange is actually a passive process. It is the passive diffusion of oxygen.1197

D is describing osmosis.1208

Remember that if you place a cell in a hypotonic solution, so a solution in which the solute concentration is1211

lower than the concentration of solutes inside the cell, water is going to rush in, and the cell will increase in volume.1221

And if that continues happening, the cell will eventually lyse or burst, but that again, this process is passive.1232

So, the only active transport described here is B, so 10, the answer is B.1240

Looking at structure of an animal cell. This is the cross section of a cell, and there are1250

four different structures pointed out and statements made about each of them.1257

Structure A is actually centrioles. That is a cross section of centrioles.1263

Centrioles are composed of microtubules. They are part of the spindle apparatus, and they allow the movement of chromosomes during cell division.1270

So, A is not correct. Centrioles do not function in detoxification, so that is not a correct answer.1284

Looking at what structure B is, structure B is a series of flattened sacs known as the golgi apparatus.1293

That is the correct answer. B is the Golgi apparatus, and the Golgi apparatus is the site of modification of protein and lipids.1302

And then, these proteins or lipids can be packaged for storage or for export from the cell, and B is actually describing that export process.1311

Looking at C, what structure is being described is the lysosome. Lysosomes contain hydrolytic enzymes, and the function is to break down old organelles,1324

macromolecule structures in the cell down to their just basic building blocks, component parts that can then be reused by the cell.1337

RNA synthesis does not take place inside lysosomes.1346

And then, finally, answer D is pointing out the endoplasmic reticulum. It is not part of the cytoskeleton.1352

The endoplasmic reticulum is an organelle that is a network of tubules and sacs.1359

Remember that there are two types of endoplasmic reticulum.1364

The rough endoplasmic reticulum has ribosomes attached to it, and it can synthesize proteins.1368

The smooth endoplasmic reticulum synthesizes lipids and steroid hormones, and the smooth endoplasmic reticulum in the liver is the site of detoxification.1376

So, actually A was talking about detoxification that would actually apply to the smooth ER.1387

Therefore, the only structure in the cell that is described correctly, that the function is described correctly is B.1393

Question 12 describes Cushing syndrome.1406

So, before we talk about Cushing syndrome, let's talk about how the pituitary gland normally affects the adrenal gland and the release of cortisol.1411

Let's talk about what is happening in a normal individual.1421

So, in a normal individual, the pituitary gland is going to release adrenocorticotrophic hormone, ACTH.1425

ACTH is then going to stimulate the adrenal glands which are located right by your kidneys to release cortisol.1434

Individuals with Cushing syndrome have a high level of cortisol release, and that causes a variety of symptoms.1448

There is a test used called the dexamethasone suppression test that is being described here.1456

And it allows physicians to figure out if the cause of this over secretion of cortisol is due to a1463

problem with the pituitary over producing ACTH or a problem with the adrenal glands directly.1470

Let's look at what happens in a normal patient here. In the normal patient there is a level of cortisol on the blood, normal level.1477

They do not give what that level is, but it just shows the baseline.1486

When dexamethasone is given in a normal patient, you will see decreased levels of cortisol in the blood.1490

Now, one action of dexamethasone is actually to block ACTH.1498

So, if dexamethasone is given and ACTH is blocked then, the adrenal glands will not be stimulated, and cortisol levels will decrease.1506

That is what you would expect to happen in a normal patient, and that is what does happen.1520

Now, let's look at this patient with Cushing syndrome.1525

They have a very high level of cortisol at baseline, and when dexamethasone is given, that level remains high.1527

And what that shows is that the issue is not at the level of the pituitary. The problem is the adrenal glands.1534

The adrenal glands are not under the normal control of ACTH. So, even when ACTH is blocked, they just keep secreting cortisol.1540

So, the problem is with the adrenal glands, and that aligns with answer C, which discusses the problem1548

in this particular patient, the cause of Cushing syndrome being the adrenal.1563

Even though ACTH is being blocked, adrenals are still producing high levels of cortisol.1567

Question 13 talks about the conversion of malate to oxaloacetate and gives various statements about it, and you are trying to find a true statement.1583

So, an allosteric enzyme changes its shape after it binds to an effector molecule.1598

But from the information given, we cannot determine whether this malate dehydrogenase is an allosteric enzyme or not.1604

Looking at B, that is a correct statement.1615

This is an exergonic reaction. This is a reaction that actually releases energy.1618

C is inaccurate. Remember that oxidation is the loss of electrons, whereas reduction is the gain of electrons.1625

And what you are seeing in this overall reaction is actually an oxidation reaction, the loss of the electrons with the removal of the hydrogen atoms.1642

D is also an inaccurate statement. It is stating that NAD+ is oxidized into NADH but actually NAD+ is reduced to NADH.1651

So, the only true statement here is that what you are seeing is an exergonic reaction.1663

Thinking about DNA in a particular cell type here and looking at which statement is accurate, and this is a fairly straightforward question.1672

The correct answer is A.1686

All cells in a particular individual, skin cells, liver cells, neurons all contain the same DNA.1692

However, different genes are expressed in different cell types, so depending on the function of the cell, particular genes will be expressed.1699

They will be transcribed and proteins will be made, but that same basic genetic material is in every cell, so the correct answer for 14 is A.1709

15, this is a fairly long question if you just break down each part, and it is talking about the topic of epistasis.1725

And what epistasis refers to is one set of genes affecting phenotype that results from another set of genes.1737

You can think of this as one set of genes modifying the effect of another set of genes.1752

And it really is most easily understood through an example, and this is a really great example here, and it is coat color in Labradors.1760

So, there are two sets of genes that you need to look at here, and the first one is for the amount of melanin present.1767

And the gene that controls that has two different alleles:1775

big B which is dominant, and that codes for black fur and then, the recessive allele which codes for brown hair.1780

However, there is a second set of genes. So, this is for melanin deposition.1791

Excuse me, how much melanin is present, the amount of melanin present. That is the set of genes.1800

Now, there is a second set of genes that is epistatic to this first set.1805

So, it actually exerts an effect that will influence the phenotype, and this set of genes influences pigment deposition.1810

If big E is not present, coat color will be yellow, so no big, no dominant allele for E equals yellow.1832

Looking at the various combinations that you are given, if an animal has this combination, big B big B, big E little E...let's look at each set here.1849

Big B big B is going to code for black coat color, and since big E is present, that is expressed.1865

So, I would expect this animal to have black coat color.1876

Now, answer A states that this combination gives brown coat color, so that is incorrect.1881

Looking at the next genotype given, big B big B, so heterozygous for both loci.1889

Because black is dominant to brown, the phenotype that I would expect is black, and since big E is present, that phenotype can be expressed.1901

So, I would expect color to be black. Therefore, B is incorrect.1914

C, here, again, heterozygous for melanin production, and seeing that big B there, I would say OK dominant, coat color is going to be black.1921

However, notice that there is no big E present, and remember, if there is no big E present,1935

it does not matter what is going on over here with the big B little B that coat color is going to be yellow.1943

So, this third combination, the coat color should be yellow.1950

So, the first three are incorrect. They are all giving incorrect phenotypes for the given genotype.1952

The correct answer is D. In D, what you see is big B big B which I would expect to produce a black coat color except there is no big E present.1957

So, again, if you see little E little E, coat color is yellow, and that is what the answer says, and that is the correct statement. 15 is D.1970

16, with 16, the important thing is to keep transcription and translation straight.1983

So, remember that going from DNA to RNA is the process of transcription.1994

When mRNA is used as a template for the production of protein, what you are talking about is translation.2009

And what this question is asking about is protein synthesis using an mRNA molecule, so it is talking about translation.2021

Answer A is talking about the suppression of expression of genes. That is transcription.2033

Again, here, the question is asking about translation, so A is not correct.2039

B, again, talking about transcription factors and things...well, it mentioned...2046

Actually this answer, it is pragmatic on various levels because it is talking about initiation of transcription which is going to produce RNA.2060

And then, it is saying that, that is going to determine the protein.2069

But they are actually talking about if you have a particular mRNA molecule, what is going to happen.2075

And the accurate answer is C because if messenger RNA is more quickly broken down,2082

there will be less opportunity for ribosomes to translate that RNA into protein.2094

D says that there is just one factor here. Ribosomes are the sole factor influencing the production of protein which is not correct.2106

So, the correct answer is C.2114

If you are looking at a particular mRNA molecule, one factor that affects the amount2115

of protein you get out of it is how quickly that messenger RNA is degraded.2120

17 shows a couple different hemoglobin oxygen saturation curves.2136

And looking at the graph and thinking about your background knowledge of this topic, which one of these is true?2146

The first answer compares the affinity of hemoglobin B for oxygen with that of hemoglobin A, and it is saying that it has a greater affinity.2155

Now, let's check that out on the graph.2164

If I look at a particular PO2, I am going to look at 60. At a PO2 of 60, the curve showing A shows that hemoglobin is about 90.2166

The saturation is 90. The oxygen saturation is 90.2179

Now, If I look at B at that same PO2 of 60, so looking at PO2 of 60, hemoglobin A, the O2 sat is about 90.2183

Now, if I look at that same PO2 of 60, hemoglobin B, the O2 saturation is only about 80.2199

What that suggests is that the affinity for hemoglobin B is actually lower than for hemoglobin A.2209

That is the opposite of what answer A is saying, so answer A is incorrect.2218

B cannot be concluded from this either.2224

If an animal has evolved in a very high oxygen level, it does not need to have such a high affinity for oxygen actually.2226

If an organism in a situation where oxygen is scarce, then, it needs to have hemoglobin2236

that really is sufficient at binding and grabbing onto the oxygen that is there.2242

If anything, I would actually think the opposite.2247

D, you cannot conclude that from this graph.2250

The correct answer is C. So, let's think out the rationale for this.2256

Now, looking at pH, the curve for A shows a pH of 7.4, and then, the curve for B is showing a more acidic environment.2262

Hemoglobin B, it is showing a pH of 7.2, so this is a more acidic environment.2279

And I already mentioned that hemoglobin B has a lower affinity for oxygen. This actually makes a lot of sense.2290

So, again C is the correct answer.2301

And the idea is that you need to think about wearing your body that would be in a slightly more acidic environment.2303

In tissues and in cells that are very active, that are metabolically active, like a muscle cell when you are exercising,2311

they are going to need a lot of energy, be very metabolically active, and as a product of that, they are going to produce CO2.2323

When CO2 combines with water, it produces carbonic acid.2335

Therefore, in regions where cells, tissues are using a lot of oxygen, the environment becomes more acidic.2340

So, what the body wants to do is have the hemoglobin carrying the oxygen travel to those2350

areas of the body and release, let go, deliver the hemoglobin to those areas.2358

So, when hemoglobin is in an environment that is acidic, where the oxygen is needed, you are going to want2364

the hemoglobin to have a lower affinity so it releases it, and that is exactly what you see with curve B, so again, answer C is correct.2371

18: triploid means that there are three sets of chromosomes, and it is true that bananas plants are triploid. They are sterile.2385

And the rationale for this is that during meiosis, homologous pairs of chromosomes line up at the metaphase plate.2396

However, there cannot be correct pairing of these chromosomes if there are three sets instead of two. Therefore, answer A is correct.2409

Just pointing out something with answer C, answer C is talking about mitosis. However, gametes are produced through the process of meiosis.2422

Looking at metabolic rate per gram of body weight versus mass and looking at various animals,2438

what statement here among these four answers is correct?2449

Well, A is incorrect because if it was a direct relationship then, what would I expect is that animals with larger2454

mass would actually have a higher metabolic rate per gram of body weight, and I am seeing the opposite here.2462

Large animals like a horse or an elephant actually have lower metabolic rates, so A is not correct.2471

B is again stating that this is a direct relationship which is not true.2479

And then, it goes on to talk about that an animal with a larger body mass would have a lower respiratory rate and a higher BMR.2484

And it is not fully giving an accurate description here.2499

I mean, the main issue is that as an animal gets larger, its metabolic rate actually gets lower.2503

So, while a lower metabolic rate would translate to a lower breathing rate, it would not translate to a higher BMR.2510

C is correct. Instead of a direct relationship, the relationship we see here is that metabolic rate, BMR and body mass are inversely proportional.2520

So, as body size increases, metabolic rate decreases. Very large animal like an elephant has a low BMR.2530

D is inaccurate. Larger animals have lower metabolic rates.2541

They have lower heart rates and lower respiratory rates than very small animals2548

like mice and rabbits that have high BMR, higher respiratory rate and higher heart rate.2552

Another graph and this one dealing with the immune system's response to antigens.2567

And on first exposure to an antigen, the body will produce a response.2576

And to get to the peak of the response as this graph is showing, it takes a couple weeks.2583

However, if the body sees that it is exposed to the antigen a second time, the response is much quicker and much stronger.2589

And thinking of this in real world terms, let’s say someone who has not been vaccinated for chicken pox is exposed to chicken pox.2597

The first time their body sees that virus, they are going to mount.2606

It is going to take some time, couple of weeks, and they are going to mount in a new response, and it is going to kind of lower.2612

If they are later on in life exposed to chicken pox again, they are going to have a very immediate strong response and be able to fight that off.2617

Now, what happens with this second antigen here?2625

Again, analogous to this real world example, if the person who had chicken pox is also then exposed to influenza virus,2630

the fact that they have been exposed to chicken pox is not going to affect their body's response to influenza virus.2641

And that is because the antibody response is part of the specific immune system, so it is specific to the antigen.2647

Memory cells that have been activated by a particular and recruited by a particular antigen will remain in the body.2657

And then, if that antigen comes back again, there is a re-exposure, these memory cells are ready to go but only for that particular antigen.2664

So, immunity or exposure to chicken pox is not going to help you fight off another type of virus. That situation is accurately depicted by B.2674

What B shows is that exposure to antigen A elicits an initial immune response, and then, a second exposure elicits a quicker, larger immune response.2688

The first exposure, the primary exposure to antigen B has a similar curve to the primary exposure to antigen A2702

in terms of the response, that there will be a response, but it is going to be relatively small and take a little longer to occur.2711

These other, like A and C is showing that antibody B would be rapidly produced in large amounts upon initial exposure which is not correct.2723

This is a first exposure to antigen B.2734

And then, for some reason in D, that individual is not producing antibodies to B despite exposure2738

which there is no explanation for that in this case, so the correct answer is B.2747

This question eludes back to epistasis which we discussed in question 15.2760

So, it discusses actually two different things: incomplete dominance and epistasis.2767

So, recall that in question 15, we talked about coat color of Labs, and that is controlled by two sets of genes.2773

And those two sets of genes were big B little B, and they had to do with melanin production and big E little E which had to do with pigment deposition.2784

And the presence of big E is required in order for the big B little B genes to be expressed.2795

So, this second set of genes is exerting an effect on this first set in terms of their expression. That is epistasis.2805

Incomplete dominance is a little bit different. Before we talk about incomplete dominance, let's talk about complete dominance.2814

With complete dominance, a good example is Mendel's pea plants, and one of the traits that he studied is height.2824

And the gene for height has two alleles: big T dominant for tall and little T for short or dwarf plants.2832

Big T, tall, is completely dominant to little T, so if you have a plant that is homozygous dominant, it is going to be tall, the phenotype will be tall.2844

If the genotype is heterozygous, the plant will also be tall.2855

You are not going to end up with a medium plant because big T is completely dominant over the little T, short phenotype.2860

Little T little T, the phenotype will be short. This is complete dominance.2871

Now, let's look at incomplete dominance.2878

The example here is with flower color in snapdragons, and the gene for this has two alleles. We are going to call them big R and little R.2890

Big R codes for red flower color. Little R codes for white flower color, so let's go through and see what happens.2906

If the plant is homozygous dominant, the plant is red. That is what I would expect.2915

If the plant is homozygous recessive, I would expect a white plant, and that is what I see.2922

Something interesting happens with the heterozygote, so big R and little R, these plants are actually pink.2928

Now, if there was complete dominance of big R over little R, big R would completely mask the expression of little R.2935

And I would expect the plant to be red, but the dominance of big R over little R is incomplete so the plant is pink.2942

So, now, after that discussion of epistasis versus incomplete dominance, let's go ahead and look at this question no. 21.2952

And D describes what I just discussed, that with incomplete dominance, you are just looking at two different alleles for a single gene.2961

Whereas with epistasis, there are multiple sets of genes, and one can exert an effect on the other. Therefore, the correct answer is D.2975

22 talks about why there was this surprising finding, that scientists expected to find many more genes2995

in the human genome than they actually discovered when they sequence the genome, so what would explain this?3006

There is so many traits in a human being controlled by relatively few genes. This is more just factual knowledge-based type question.3012

And first of all A is inaccurate. When histones are modified, what we usually see is actually a decrease in the function of proteins, so that is incorrect.3026

Epigenetics is a very hot term right now, a very hot area of research.3036

Epigenetics means over or above genetics, so epigenetics over or above genetics.3042

And what this refers to is a change that can increase or decrease the expression of a gene, but it does not alter the DNA sequence.3054

One example of epigenetics is methylation. So, methylation of DNA can supress the expression of genes.3066

And epigenetics refers to changes that are not in the DNA sequence itself, but they are heritable.3074

And these factors can also be influenced by environment.3083

So, anyways, that is the description of epigenetics, but B is not an accurate statement about epigenetics.3087

Pseudogenes are closely related to genes, but they do not lead to the production of a protein. Introns are not expressed either.3094

That is inaccurate. D is correct.3103

Remember that you have got a DNA sequence, and there will be exons, parts of the DNA that code for gene.3107

And then, there are these introns. You can think of them as just sequences that just interrupt the genes.3118

And so you have these various exons and introns.3127

And transcription occurs leading to the production of pre-mRNA, and pre-mRNA is going to contain the sequence for both the exons and the introns.3132

And remember that some modifications are going to occur in order...processing will occur to convert pre-mRNA to mRNA such as the addition of a poly-A tail.3147

Another type of processing that occurs is splicing, and there are alternative ways that this transcript can be spliced.3162

One way would be to splice it like this so that you will end up with mRNA like this. However, there is another possibility.3171

Instead of splicing it like this, let's look at the same pre-mRNA and splice it a little bit differently so that this exon is removed.3188

As you can see then with the sequence, you can get different combinations through alternative splicing or differential splicing as it sometimes called.3205

Therefore, the correct answer for 22 is D.3216

23: so we are back to talking about photosynthesis and in particular, the light-dependent reactions.3224

Remember that in the light-dependent reactions, photolysis occurs, and that is the splitting of water. That is what is described in C.3231

So, overall in the light-dependent reactions, light energy is used to produce ATP.3243

As a part of this, you need a source of electrons, and water provides that source, so water is split to provide a source of electrons.3250

The other response is here referred to the light-independent processes.3258

For example A is talking about CO2 being used as a source of carbon to produce glucose.3265

Again, that happens in the second part of photosynthesis, the light-independent reactions.3273

However, remember that the light-independent reactions actually require ATP that is produced from the light-dependent reactions.3279

OK, B also describing the production of a product from the light-independent reactions.3288

And then, D, the light-independent reactions take place in the stroma. The light-dependent reactions take place in the grana, OK.3297

So, C is the only one that refers to light-dependent reactions.3308

The acrosome is found at the head of the sperm, and it releases enzymes that allow a sperm to penetrate an ovum so that fertilization can occur.3317

Now, immediately upon fertilization, cortical granules are released from the ovum.3329

And these granules actually help to form a barrier around the egg so that additional sperm cannot fertilized that egg.3336

So, what is being described here with these reactions is actually C, preventing multiple sperm from fertilizing a single egg- C.3345

25 is a very broad question. Actually, the only response here that fits is A.3359

All cells are surrounded by a cell membrane or plasma membrane that separates the cell from its surrounding environment.3371

Not all cells have a nucleus. Remember that prokaryotic cells do not contain membrane-bound organelles.3379

They do not contain a nucleus, so B is not correct.3390

Again, mitochondria, that is a membrane-bound organelle, not every cell has that.3394

And the surface to volume ratio of cells actually varies, so A is the correct response.3398

26 is talking about translation, and you are given a sequence of an mRNA and ask what would happen with a mutation occurred.3410

And you are given this table of the codons and what amino acids or what they code for.3418

Now, this refers to an alteration of bold phased A, but actually what they show here is that the mutation has already taken place.3429

So, it is showing the U which is presumably an A before, and now, post mutation is a U.3437

And if you look at the middle codon UAA, and you look at what it codes for3444

using that chart that you are given, you will see that UAA is a stop codon.3452

What this means is that when the ribosome encounters that sequence UAA, it is going to dissociate.3458

It is going to stop translation, and that polypeptide chain will be finished.3465

So, even though there are additional codons after that, those will not be translated.3470

So, this protein is going to be non-functional. This is a very serious kind of mutation, and the correct answer is C.3476

This next question talks about clear cutting, and looking at a negative consequence,3492

a bad outcome that could occur due to clear cutting in a tropical rain forest area.3501

One very negative consequence that they are discussing here is potentially less precipitation.3512

And that is because trees are an important part of the water cycle.3518

Recall that in the water cycle, water evaporates from bodies of water such as lakes, rivers and oceans, so it evaporates up into the atmosphere.3523

However, water also evaporates from trees, so it is taken up by the trees via the roots and then, evaporates in the process of transpiration.3535

So, that water that has evaporated from bodies of water or transpired back into3546

the atmosphere is actually carried higher up into the atmosphere by air currents.3550

Cooler temperatures higher up in the atmosphere cause that water vapor to form into clouds.3556

Eventually, precipitation occurs, and the water is released from the clouds.3561

It returns back to the ground, into the soil, into the bodies of water, so come full circle.3566

Clear cutting can disrupt the water cycle by eliminating trees and therefore, transpiration. Therefore, the correct answer is B.3574

OK, so, DDT is a pesticide, and we are supposed to figure out why some mosquitoes...3594

the mechanism by which an explanation for them becoming resistant to this pesticide.3606

So, initially, pesticide was used very, very effective, but eventually, populations of mosquitoes arose that did not die when they were exposed to DDT.3609

And the explanation for that is B. This is really just describing the process of natural selection,3619

that in any population whether it is mosquitoes or mice or elephants or humans, in any population there is a variety of traits.3627

So, some mosquitoes were resistant to DDT.3635

They carried a particular gene or set of genes that gave them a phenotype that made them resistant.3640

So, when DDT was sprayed, susceptible mosquitoes were killed by the DDT, and those mosquitoes carrying a trait that made them resistant survived.3647

And so that is the explanation for B.3659

Then, what would happen is the mosquitoes that survived would have offspring.3662

Then, they may pass on the genotype for resistance to DDT to their offspring.3667

And therefore, that trait increases in frequency in the population.3673

And eventually, the surviving mosquitoes, the majority of them would have that trait if they have had this selective pressure put upon the population.3677

So, again, the answer is B.3688

29: so, we are looking at humans and chimpanzees in terms of similarities in genetic sequences.3692

What would explain the fact that there are these many similar sequences between humans and chimpanzees.3707

An explanation would be C. If humans and chimpanzees diverged evolutionary relatively recently,3716

so they have a recent common ancestor, that could explain why they have so many similar sequences in their genomes.3724

Looking at D, if they were branched off 4 billion year ago from a distant common ancestor,3731

I would actually expect them to have relatively few sequences in common on their genome.3740

So, the correct answer for 29 is C.3746

30: You are given a description of a situation involving natural selection.3752

So, there was a drought, and therefore, there was less food, less seeds available to birds.3760

And the seeds that were available were large ones that were maybe harder for the birds with smaller beaks to eat.3768

So, what would happen is that those finches that happen to have larger beaks and could eat these3777

bigger tougher seeds were more likely to survive, and they were more likely to reproduce.3784

And when they reproduced, they passed on the genes for larger beak to their offspring.3792

And eventually, there were higher frequency of these larger beaked birds in the population.3798

And the average size of beaks within that population would increase when you went in and measured it after this drought, so D.3803

Again, we are just talking about natural selection here and how it can favor individual with the particular trait, here, beak size.3813

Those individuals survive. They reproduce, and then, that trait becomes more common in the population.3821

Really, I have also explained 31 at the same time because it is asking about what is behind this change, the mechanism for the change which is B.3828

The pressure exerted on the finch population favored larger beaks. It favored individuals carrying the genes for larger beaks.3839

Those genes were then passed on to their offspring. Birds that did not carry that trait were less likely to survive and reproduce.3848

So, the frequency of genes for a large beak size increased in the population.3856

That concludes this lesson for Thank you for visiting.3864