Short Answer Type(a) What do 'Y' and 'B' stand for in 'YAC' and 'BAC' used in Human Genome Project (HGP). Mention their role in the project.
(b) Write the percentage of the total human genome that codes for proteins and the percentage of discovered genes whose functions are known as observed during HGP.
(c) Expand 'SNPs' identified by scientists in HGP.
Long Answer Type(a) Name the stage in the cell cycle where DNA replication occurs.
(b) Explain the mechanism of DNA replication. Highlight the role of enzymes in the process.
(c) Why is DNA replication said to be semiconservative ?
(a) DNA replication takes place in the S phase or Synthetic phase of the Cell cycle.
(b) The process of Replication;
1. The double-helix structure of the DNA unzips. This is carried out by an enzyme called helicase which breaks the hydrogen bonds holding the complementary basesof DNA together
2. The DNA do not separate completely but at some point. The separation of the two single strands of DNA creates a ‘Y’ shape called a replication ‘fork’. The two separated strands act as templates for making the new strands of DNA.
3. One of the strands is oriented in the 3’ to 5’ direction and is called the leading strand. The other strand is oriented in the 5’ to 3’ direction and is the lagging strand. As a result of their different orientations, the two strands are replicated differently:
4. Leading Strand:
A short piece of RNA called a primer (produced by an enzyme called primase) comes along and binds to the end of the leading strand. The primer acts as the starting point for DNA synthesis.
DNA polymerase binds to the leading strand and then ‘walks’ along it, adding new complementary nucleotide bases (A, C, G and T) to the strand of DNA in the 5’ to 3’ direction. This sort of replication is called continuous.
Lagging strand:
Numerous RNA primers are made by the primase enzyme and bind at various points along the lagging strand. Chunks of DNA, called Okazaki fragments, are then added to the lagging strand also in the 5’ to 3’ direction. This type of replication is called discontinuous as the Okazaki fragments will need to be joined up later.
5. Once all of the bases are matched up (A with T, C with G), an enzyme called exonuclease strips away the primer(s).
6. Finally, an enzyme called DNA ligase seals up the sequence of DNA into two continuous double strands. The result of DNA replication is two DNA molecules.
(c) The newly formed two DNA strands consist of one new and one old chain of nucleotides. This is why DNA replication is described as semi-conservative, half of the chain is part of the original DNA molecule, half is new
Short Answer TypeDescribe the experiment that helped demonstrate the semi-conservative mode of DNA replication
Long Answer Type(a) How are the following formed and involved in DNA packaging in a nucleus of a cell?
(b) Differentiate between Euchromatin and Heterochromatin
Short Answer TypeA. Name the enzyme that catalyses the transcription of hnRNA.
B. Why does the hnRNA need to undergo changes ? List the changes hnRNA undergoes and where in the cell such changes take place.
Unambiguous, universal and degenerate are some of the terms used for the genetic code. Explain the salient features of each one of them.
A number of passengers were severely burnt beyond recognition during a train accident. Name and describe a modern technique that can help hand over the dead to their relatives.
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