Question

Answer the following questions:

1. Which bases are purines? Pyrimidines?

2. Why does DNA need to replicate?

3. When in the cell cycle does DNA replication occur?

4. Does DNA replication occur in mitosis and meiosis? How does it differ in each?

5. Meselson and Stahl discovered that DNA replication is semi-conservative. Explain what that means.

6. DNA strands run in the opposite directions. What is this called and why is this an issue in DNA replication?

7. Why are the two strands of DNA said to be complementary?

8. Explain the difference between leading and lagging strand replication.

9. What are Okazaki fragments and why are they needed?

10. Why does the lagging strand require a primer?

11. DNA synthesis always occurs in the __________ direction, so one new strand is synthesized continuously towards the replication fork, producing the __________ strand. The other strand, known as the __________ strand, forms away from the replication fork in small fragments.

12. Explain the process of DNA replication, include all parts of initiation, elongation, termination, and the enzymes involved in each.

Answer 1

1. Adenine (A) and Guanine (G) are purines.

Thymine (T), Cytosine (C) and Uracil (U) are pyrimidines.

2. DNA needs to replication because existing cells divide to produce new cells. So, DNA is copied during S-phase of interphase cell cycle so that all the instructions in the parent cell are passed on to the daughter cells.

3. DNA replication occurs in the S-phase of Interphase.

4. Cell cycle is the ordered sequence of events. Cell cycle has two main phases namely interphase and M-phase.

Interphase is further divided into G1, S and G2. DNA synthesis takes place in S-phase.

Hence, DNA replication occurs during cell cycle but not in M-phase. So, there is no difference in DNA replication of mitosis and meiosis.

5. Meselson and Stahl discovered that DNA replication is semi-conservative. This means that two DNA stand unwind from each other and each acts as a template for synthesis of a new complementary strand. This gives two DNA molecules having one original strand and one new strand.

6. DNA strands run in the opposite directions. This is called anti-parallel nature of DNA or anti-parallel polarity, that is, one chain has polarity 5'-3' and the other has 3'-5'.

The anti-parallel structure of DNA for DNA replication because it replicates the leading strand one way and the lagging strand the other way.

7. The two DNA chains are held together by complementary base pairing, where A pairs T and G pairs with C. A and T are bond to each other by two hydrogen bonds and C and G are bound to each other by three hydrogen bonds. This is referred to as complementary base pairing.

8. Leading strand is replicated continuously which requires primer for synthesis while lagging strand is replicated discontinuously which requires a new primer to start each okazaki fragement.

Lagging strand requires DNA ligase to join the short strands of DNA synthesized while leading strand does not require DNA ligase as there is a continuous synthesis of DNA.

9. Okazaki fragments are short newly synthesized DNA sequences. The Okazaki fragments are important for DNA synthesis because no DNA polymerase can use 3' to 5' strand of DNA to use as a continuous template. Hence, okazaki fragments allow discontinuous DNA synthesis allowing DNA polymerase to work in backward direction moving away from replication fork. It then jumps back to the fork for next RNA primer to begin a new okazaki fragment.

10. Lagging strand is a discontinuous strand. As the lagging strand is oriented the opposite direction, the polymerase enzyme cannot accommodate it in the space due to the shape and location of the active site. It needs a primer to start each fragment. These oakazaki fragments are joined by ligase enzyme.

11. DNA synthesis always occurs in the 5’ TO 3’ direction, so one new strand is synthesized continuously towards the replication fork, producing the LEADING strand. The other strand, known as the LAGGING strand, forms away from the replication fork in small fragments.

12. DNA replication is the basis of biological inheritance. DNA is a self-replicating material present in almost living organisms on the chromosomes. It is present in a highly coiled structure within the nucleus. It consists of four bases (A, T, C and G), long backbone of sugar and phosphate.

The three main steps of DNA replication are: Initiation, Elongation and Termination.

Initiation

DNaA protein binds to the OriC which begins at the site of origin of replication. Helicases unwind the double helix by breaking the hydrogen bonds between complementary base pairs. Topoisomerase proteins surround the unzipping strands and relax the twisting to prevent the DNA from damage. Cell creates a short sequence of RNA known as RNA primers that are needed for elongation of the DNA.

Elongation

DNA polymerase catalyse the step by step addition of deoxyrionucleotide units to a DNA chain. At each growing fork one strand is called leading strand which is synthesized continuously from a single primer on the leading strand template and grows in 5’to 3’ direction. The other strand is lagging strand. Existing strand is called as template strand. Here, A pairs with T, C pairs with G.

DNA polymerase can add nucleotides only to 3’end of the growing strand. After 1000-2000 nucleotides of the leadings strand have been replicated, the first round of discontinuous strand synthesis on the lagging strand begins. Short pieces of DNA are repeatedly synthesized on the lagging strand. These short pieces of DNA are called Okazaki fragments. These fragments are joined by DNA ligase.

Termination

After elongation is complete, two new double helices have replaced the original helix. Replication terminates at the terminus region containing mutilple copies of about 23 bp sequences called Ter sequences. The lagging strand has a telomere section which contains a repeating non-coding sequence of bases. Telomeres are the regions of repetitive nucleotide sequences at the end of a chromatid which protect the end of the chromosomes from fusion with neighbouring chromosomes. Enzyme removes telomere at the end of the replication. Finally, nucleases proofread the new double helix structures. DNA polymerase fills the gaps created by the excised bases.

Each of these two daughter helices is a nearly same as that of the parental helix.