The Double Helix and Biological Functions

Transcript

0:00Introduction

Hello everyone! Let us start with a puzzle. The DNA in a cell is written with just four letters, its four bases. Yet it is the store of the cell's genetic information, including the message for making its proteins. And when the cell divides, each daughter cell gets an identical copy. So how can four letters carry so much, and how does DNA copy itself so exactly? By the end, you will answer both. This topic comes up often in JEE chemistry, so let's begin.

0:32Watson and Crick: the double helix

So, look at the figure. The dark blue lines are the two strands, and the boxes are the bases, with their short names A, G, C and T, okay? In DNA, two polynucleotide strands are coiled about each other into a double helix. This is the Watson and Crick structure; the figure draws the strands straight, like a ladder, to keep it simple. Right? The two strands are held together by hydrogen bonds between pairs of bases. Those are the dashed lines between the boxes. Clear? Now look at the ends. The small numbers with a dash, read five prime and three prime, name the ends of a strand. The left strand has five prime at the top, but the right strand has three prime at the top. So the strands run in opposite directions, just as the first note in the figure says about the sugar phosphate backbones. Got it?

1:24Base pairing: A with T, G with C

Now, the pairing is very specific. Adenine, written A, always pairs with thymine, written T, and the figure shows two dashed lines between them, so two hydrogen bonds. Okay? Now, guanine pairs with cytosine, the letters G and C on screen. Count the dashed lines between them. How many hydrogen bonds is that? That is right, there are three dashed lines, and the third note in the figure agrees: guanine and cytosine are held by three hydrogen bonds. Two for adenine and thymine, three for guanine and cytosine. Students often swap these. Clear? Now read the strands going down. The left one reads A, G, T, C, and the right one reads T, C, A, G, going down. So the strands are not identical; they are complementary. Opposite adenine is always thymine, and opposite guanine, cytosine. Know one strand, and you know the other. With me?

2:23Chargaff's rule

Now, the last note in the figure is about Chargaff. Every adenine faces a thymine, so in DNA, the amount of adenine equals the amount of thymine. This is Chargaff's rule. Okay? In the same way, the amount of guanine equals the amount of cytosine. The figure also says a purine always faces a pyrimidine. So compare all the purines with all the pyrimidines. What do you get? Yes, exactly, they are equal. Adenine plus guanine, the purines, equals cytosine plus thymine, the pyrimidines, since every pair has one of each. But careful, Chargaff does not say adenine equals guanine. Right?

3:05RNA: one strand, three types

Now, RNA is different. It is single-stranded, though it may fold back on itself. So the double helix and Chargaff's rule belong to DNA, okay? And RNA comes in three types, messenger, ribosomal and transfer, as on screen. In the brackets are their short names: the small letter m, r or t, a dash, and then RNA, written together. Keep messenger RNA in mind; we need it soon. Clear?

3:34Heredity: DNA copies itself

Now, the biological functions. First, heredity. DNA is the chemical basis of heredity, and the store of genetic information. Right? Before a cell divides, its DNA is copied, and this is called replication. First, the two strands separate, so each strand is now alone. So how does each lonely strand get exactly the right new partner? Yes, exactly, by the pairing rules we just learnt. Each old strand acts as a template: opposite every adenine goes a thymine, opposite every guanine, a cytosine, and the other way round. So a new complementary strand is built on each. Got it? So identical copies pass to the daughter cells. That is the second half of our puzzle: DNA copies itself so exactly because its strands are complementary. With me?

4:27Protein synthesis: transcription, then translation

The second function is protein synthesis, in two steps. Step one is transcription: the message for a protein is copied from DNA on to messenger RNA, okay? Step two is translation: messenger RNA is read on the ribosomes, and amino acids are joined in order to make the protein. Here is a simple trick. Transcription copies in the same language of bases. Translation changes the language, from bases to amino acids. Clear? So the proteins are made by RNA molecules, but the message comes from DNA, right? Questions sometimes swap these two, so keep them straight.

5:05Codons and DNA fingerprinting

Now, the message itself is written in codons. Three successive bases, called a codon, stand for one amino acid. Each of the three places can hold any one of the four bases. So how many different codons are possible? Each place has four choices, so we get four times four times four. That is four to the power three, the small raised three on screen, which gives sixty four. So four letters make sixty four three-letter words, and a long DNA chain strings many such words together. That is how four letters carry so much. That was the first half of our puzzle, right? One more point: the sequence of bases in DNA is unique to a person, and this is the basis of DNA fingerprinting. Okay?

5:52Example 1: DNA with 20% adenine, find the other bases

Time for example one. Here it is: a sample of DNA contains twenty percent adenine. Find the percentages of the other bases. Pause the video and try it first. Done? Okay, let's check. By Chargaff's rule, thymine equals adenine, so thymine is also twenty percent, where the percent sign means out of a hundred. Right? Now, many students say a hundred minus twenty leaves eighty percent for guanine and cytosine. Is that correct? That is not correct, because it forgets thymine. Adenine and thymine together, in the bracket, take forty percent. So guanine plus cytosine is a hundred minus forty, which is sixty percent. Okay? And guanine equals cytosine, so they share it equally: sixty over two gives thirty percent each. So the answer is thymine twenty percent, guanine thirty, and cytosine thirty.

6:49Example 2: amino acids from m-RNA AUGGCUUAA with stop codon

Clear? Now a harder one. This messenger RNA is translated. Its nine bases run from the five prime end to the three prime end, and UAA is a stop signal. How many amino acids does the peptide formed contain? Pause the video and try it first. Done? Okay, let's check. Split the nine bases into codons, three at a time, in order. The first codon is AUG, the second is GCU, and the third is UAA, okay? There are three codons, and each codon stands for one amino acid. So the peptide has three amino acids, right? Be careful here, because that is the trap, so read the question again. UAA is a stop signal, and it ends the chain, so it adds no amino acid. Clear? Only the codons AUG and GCU are read, so the peptide contains just two amino acids. Nine bases, but only two amino acids, so always read the whole question, okay?

7:59Quick revision: base pairing, Chargaff, replication, codons

So, quick revision. DNA has two complementary strands, running in opposite directions. Adenine pairs with thymine by two hydrogen bonds, guanine with cytosine by three, and Chargaff's rule gives equal amounts within each pair. Replication copies DNA, transcription and translation make proteins, and there are sixty four codons. Now go and try the practice problems on this page. All the best!