AIEEE and Earlier JEE Main Questions

Transcript

0:00Introduction

Hello everyone! Let us start with an old exam question. DNA and RNA are both long chains of nucleotides, and their sugars differ by just one small group, a single hydroxyl group, O H, on one carbon atom. So which carbon is it? Many students know both sugars, and still get stuck here. Today we go through the old A I triple E and JEE Main questions of this chapter, on vitamins, nucleic acids and hormones. Most of them test one clean fact, so these are marks you can be sure of. Let's begin.

0:38Vitamins: fat-soluble or water-soluble?

So, first, vitamins. They come in two classes. The fat-soluble vitamins are A, D, E and K, and the body stores them in the liver and in adipose tissue, which, as the brackets say, is fat-storing tissue. Okay? The water-soluble ones are vitamin C and the B group. They are readily excreted in urine and not stored, so the diet must supply them regularly. Right? Now, a JEE Main question from two thousand and fifteen. Which of these vitamins is water-soluble: vitamin E, vitamin K, vitamin C, or vitamin D? Yes, exactly, it is vitamin C, the only one here from the water-soluble class. Vitamins E, K and D all belong to the fat-soluble four. Clear? And remember one exception. Vitamin B twelve, written as B with a small twelve below it, is water-soluble, and yet it is stored in the liver. Got it?

1:37Nucleic acid bases: purines and pyrimidines

Now, nucleic acids. Their bases contain nitrogen, and they come in two families. The purines have two fused rings, two rings sharing one side: adenine and guanine. The capital letter in brackets is the short name of each base. With me? The pyrimidines have just one ring: cytosine, thymine and uracil. So, two rings for a purine, one ring for a pyrimidine. Okay? Now, DNA and RNA share three of these bases, and each has one more of its own. Which base is found only in DNA, and which only in RNA? Very good, only DNA has thymine, and only RNA has uracil. So DNA has adenine, guanine, cytosine and thymine, while RNA has adenine, guanine, cytosine and uracil. Right? This one table answers old questions straight away. From A I triple E two thousand and six, the pyrimidine bases present in DNA are cytosine and thymine. Uracil is a pyrimidine too, but it is not in DNA, so do not add it. Clear? And from JEE Main two thousand and fourteen: among quinoline, adenine, cytosine and thymine, the one not present in DNA is quinoline. Quinoline is not one of our five bases at all. Got it?

3:00What holds the double helix together?

Next, the figure. DNA has two polynucleotide strands coiled about each other into a double helix, and the thick vertical lines are their sugar phosphate backbones. The end labels, five prime and three prime, with the small dash read as prime, show that the strands run in opposite directions. Okay? Now, an A I triple E question from two thousand and three. The double helical structure of DNA is due to which interaction? Look at the dashed lines between the bases. Yes, exactly, it is hydrogen bonding. Each dashed line in the figure is one hydrogen bond, between a pair of bases, one base from each strand. Right? And the pairing is specific. Adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three. Count the dashed lines in the figure, two and then three. The last note in the figure adds that a purine always faces a pyrimidine. And since every adenine is paired with a thymine, and every guanine with a cytosine, in DNA the amount of adenine equals that of thymine, and guanine equals cytosine, which is Chargaff's rule. With me?

4:11Hormones: chemical messengers

Last idea before our examples: hormones. They are molecules that act as chemical messengers between cells. They are made in the endocrine glands, ductless glands as the brackets say, and poured into the blood, which carries them to where they act. Okay? Now, from A I triple E two thousand and four. Insulin production and its action in the body are responsible for the level of diabetes. So, is insulin an enzyme, a hormone, a co-enzyme or an antibiotic? It is a hormone, that is correct. Chemically, insulin is a polypeptide, and it is made in the pancreas. Clear? Its job is to lower the blood glucose level, while glucagon, also from the pancreas, raises it. Right?

4:59Example 1: which sugar carbon OH separates DNA from RNA

Time for Example one, from A I triple E two thousand and eleven, and it is our puzzle from the start. The presence or absence of a hydroxyl group, on which carbon atom of the sugar, differentiates RNA from DNA? Pause the video and try it first. Done? Okay, let's check. In the figure, the pentagon is the sugar, and five prime and one prime beside it number its carbons, while the circle marked P is the phosphate and the box is the base. In RNA the sugar is ribose, C five H ten O five, where the small numbers written low are subscripts. Okay? In DNA, the sugar has a longer name, two deoxyribose. So what does the word deoxy tell you? Yes, exactly, deoxy means without an oxygen. Two deoxyribose is C five H ten O four, with one oxygen fewer than ribose. Right? And the two in its name tells you where. Ribose has an O H group on carbon two prime, and C dash two prime on screen simply means carbon two prime. Deoxyribose has only a hydrogen there. Now careful, some students flip this and give the O H to DNA, but deoxy tells you the oxygen is missing in DNA, okay? So the answer is the second carbon of the sugar, carbon two prime. And that solves our puzzle, clear?

6:25Example 2: sugar carbons holding the base and the phosphate

Now Example two, a little harder, from A I triple E two thousand and five. In both DNA and RNA, at which carbons of the sugar molecule are the heterocyclic base and the phosphate ester linkage, respectively? Heterocyclic simply means the ring of the base holds nitrogen as well as carbon. Pause the video and try it first. Done? Okay, let's check, with the left part of the figure. The base, the box, is joined to the sugar at the carbon marked one prime. Base plus sugar is called a nucleoside, the shorter brace in the figure. With me? Now add the phosphate, the circle marked P, on carbon five prime. That makes a nucleotide, the longer brace. Okay? On the right, nucleotides join into a chain, from the five prime end at the top to the three prime end at the bottom, and as the note says, phosphodiester links join carbon three prime of one sugar to carbon five prime of the next. But each nucleotide carries its own phosphate on carbon five prime, so five prime is the carbon we want. Clear? Now the last trap is one word: respectively. Some students see the phosphate first in the figure, and write five prime first. So which carbon should come first in the answer? The carbon of the base, one prime, because the question names the base first, and respectively means in the same order. So the answer is carbon one prime and carbon five prime, the same for DNA and RNA, got it?

8:00Quick revision: vitamins, DNA vs RNA, base pairing

So, quick revision. First, the fat-soluble vitamins A, D, E and K are stored, while the water-soluble B group and vitamin C are not, except vitamin B twelve. Second, DNA has the sugar two deoxyribose, with no O H on carbon two prime, and the base thymine, while RNA has ribose and uracil. And third, the base sits on carbon one prime and the phosphate on carbon five prime, and hydrogen bonds hold the double helix. Now go and try the practice problems on this page. All the best!