Question

1. Outline the major points in DNA replication. What are some of the biochemical issues (e.g. separating dna strands, discontinuous replication) and how are they resolved?

2. Describe the basic structure of a gene.

Answer 1

The basic mechanism of DNA replication is similar in all organisms. Dna relplication is semiconservative method of replication . Major points in Dna replications are

• Each strand of a double helix acts as a template for the synthesis of a new ,complementary strand .so it is calledsemiconservative ,a daughter DNA has one strand from the parent and other is newly synthesised.
• Enzyme DNA polymerase is involved in this, which requires a template and a primer (starter) and synthesis DNA in 5'-3' direction
• During DNA replication one strand is made from continuous ( leading strand ) and other is lagging strand ( made in small pieces).
• Dna replication requires other enzymes like DNA primase( Synthesis of RNA primer),DNA helicase(Unwind DNA helix), DNA ligase(seals nicks left after DNA primase and fill gaps left by removal of RNA primer),and topoisomerase (relieve supercoil in front of replication fork).

biochemical issues

• Discontinous replication - the synthesis of new strand through aseries of fragments are called discontinous replication , this strand is called lagging strand . These fragments are called Okazaki fragments. synthesis of okazaki fragments from RNA primer is carried out by primase and DNA ligase needed to join these fragments.
• Polymerisation takes place in 5'-3' direction only- it enables the proof reading , the polymerisation carried out by DNA polymerase enzymes. if it happen 3'-5' direction it effects proof reading
• Unwinding of DNA helix correctly done by Helicase enzymes.

basic structure of gene

A gene is a sequence of DNA that codes for protein or catalutic RNA. gene consists of a promoters ( highly specific DNA sequence ,it allows transcription factor to bind and recruit RNA polymerase II to the site.) , Enhancer ( it boost expression of genes in certain cell types,) Transcription initiation site( made from DNA template ,) , Transcription stop site ( the region of gene after the coding sequence has ended), coding sequence(the unique coding sequence contains exons that will translated in to protein )