The enzymes involved in DNA replication other than DNA polymerase and ligase are listed below with their functions:

Adenine,uracil,guanine,cytosine

A substance which stores biological information,refers it to the next generation,and expresses it in the offspring

1. In prokaryotes, the mRNA does not require any processing to become active.
2. Transcription and translation occur in the same compartment-cytosol, as there is no well-defined nucleus. So coupling of transcription and translation is possible.
3. The mRNA has to be processed before becoming active (splicing is necessary).
4. Since nucleus and cytoplasm are well separated, in eukaryotes the RNA has to be transported to the cytoplasm for translation.
5. So coupling of transcription and translation is not possible.

1. The enzyme DNA polymerase polymerises the nucleotides in the   5' -> 3' direction in both continues and discontinuous synthesis.
2. The enzyme DNA-ligase joins the short stretches of DNA of discontinuous synthesis.

The possible levels of regulation in eukaryotes could be at
(i) Transcriptional level i.e. formation of primary transcript.
(ii) Processing level i.e. regulation of splicing
(iii) During transport of mRNA from the nucleus to the cytoplasm
(iv) translation level

(a) (i) Methionine.
(ii) Phenylalanine.
(iii) It is a termination codon that does not code for any amino acid.
(b) AUG - TAC,UUU - AAA and UAG- ATC.

(i) The split-gene arrangement in eukaryotes represents probably an ancient feature of the genome
(ii) The presence of introns is reminiscent of antiquity.
(iii) The process of splicing represents the dominance of RNA-world.

(ii) DNA replication occurs in S-phase of cell cycle in eukaryotes.If cell division is not followed after DNA replication then the replicated chromosomes (DNA) would not be distributed to daughter nuclei. A repeated replication of DNA without any cell division results in the accumulation of DNA inside the cell.thereby causing cell expansion.

1. It was to show that DNA replication is semi conservative.
2. After completion of replication, each new/daughter molecule of DNA has one parental strand and one newly synthesised strand; this is called semiconservative replication of DNA.
3. Experiment
Mathew Meselson and Franklin Stahl performed the experiment.
4. They grew E.coli. in a medium containing \(^{ 15 }{ { NH }_{ 4 } }Cl,\) until \(^{ 15 }N\) was incorporated in the two strands of newly formed E.coli cells; this 'heavy' DNA can be separated from the normal \(^{ 14 }N-DNA\) by centrifugation in cesium chloride (CsCI) density gradient.
5. Then they transferred the cells into a medium with normal \(^{ 14 }{ { NH }_{ 4 } }Cl\) and took out samples at various time intervals.
6. They extracted the DNA and centrifuged it to measure the densities.
7. The DNA extracted from cells after one generation after transfer from the \(^{ 15 }{ { N } }\) medium to \(^{ 14 }{ { N } }\) medium (i.e. after about 20 minutes), had an intermediate/hybrid density.
8. The DNA extracted after two generations (i.e. after 40 minutes) consisted of equal amounts of 'light' DNA and 'hybrid' DNA.
This proves that after replication, each DNA molecule has one parental strand and one newly synthesised strand, i.e. replication is semiconservative.

Genetic Code
Genetic code refers to the relationship between the sequence of nucleotides on mRNA and the amino acids in the polypeptide.
It was George Gamow, who suggested that the code must be made of three bases, in order to code for the twenty different amino acids, with only four bases; this would generate \(\left( { 4 }^{ 3 } \right) \) or (4 x 4 x 4) = 64 triplet codons.
Har Gobihd Khorana could synthesize RNA molecules with definite combinations of bases (homo polymers and copolymers).
Marshal Nirenberg made a cell-free system for protein synthesis, that helped in deciphering the code.
Ochoa discovered enzyme polynucleotide phosphorylase, that could polymerise RNA with definite sequence in template-independent manner.