DNA ligase facilitates the joining of okazaki fragmente in lagging DNA strands together by catalysing the formation of phosphodiester bond. It also play a role in repairing single-strand breaks in duplex DNA.

A complete set of gene contained in the haloid does of chromosomes and inherited as a unit from one parent is called geome

DNA expresses its genetic information by transcribing mRNA and synthesising proteins

(i) The deoxyribonucleoside triphosphates are the building blocks for the DNA-strand(polynucleotide chain), as substrate.
(ii) These also serve as energy source in the form of ATP and GTP from there two terminal phosphates.

(i) It is attached at the 3' end of tRNA.
(ii) UUU and UUc.
(iii) Aminoacyl-tRNA synthetase.

1. Central Dogma of molecular biology
2. It is the present day representation of the central dogma of molecular biology.
3. RNA is made from DNA by the process of transcription.
4. With the discovery of retroviruses, there is reverse transcription, i.e. the synthesis of DNA on RNA template, catalysed by the enzyme reverse transcriptase
5. RNA is translated into a polypeptide, making use of amino acids, ribosomes and enzymes.

(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.

1. A single DNA dependent RNA polymerase catalyses the formation of mRNA, tRNA and rRNA in bacteria.
2. The enzyme is capable of catalysing only the elongation step of transcription.
3. It combines transiently to the initiation or sigma factor and binds to the promoter and initiates transcription.
4. It somehow facilitates the opening of the DNA helix and catalyses the polymerisation of ribonucleoside triphosphates in a template-depended fashion, i.e. elongation.
5. When it reaches the terminator sequence, the enzyme associates transiently with the termination or rho(p) factor and terminates transcription, the RNA and the enzyme fall off the template.

Alfred Hershey and Martha Chase proved that DNA is the genetic material.
Experiment of Hershey and Chase
1. They made two different preparations of the phage; in one, the DNA was made radioactive with \(^{ 32 }P\) and in the other, the protein coat was made radioactive with \(^{ 35 }{ { S } }\).
2. These two phage preparations were allowed to infect the bacterial cells separately.
3.Soon after infection, the cultures were gently agitated in a blender to separate the adhering protein coats of the virus from the bacterial cells.
4.  The culture was also centrifuged to separate the viral coat and the bacterial cells.
5.  It was found that when the phage containing radioactive DNA was used to infect the bacteria, its radioactivity was found in the bacterial cells (in the sediment) indicating that the DNA has been injected into the bacterial cell.
6. So, DNA is the genetic material and not proteins.

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.

Sequencing of a Genome
1. The methods involve two major approaches:
(i) One approach called Expressed Sequence Tags (ESTs), focuses on identifying all the genes that are expressed as RNAs.
(ii) Second approach called Sequence Annotation, is to simply sequence the whole set of genome, that includes all the coding and non-coding sequences and then assigning functions to different regions in the sequence.
2. The total DNA from the cell is isolated and converted into random fragments of relatively smaller sizes.
3. These fragments are then cloned in These fragments are then cloned in suitable hosts using specialised vectors; the commonly used hosts are bacteria and yeast and the vectors are bacterial artificial chromosomes (BAC) and yeast artificial chromosomes (YAC).
4. The fragments are then sequenced using automated DNA sequences.
5. The sequences are then arranged on the basis of certain overlapping regions present in them; this requires the generation of overlapping fragments for sequencing.
6. Specialised computer programmes are developed for alignment of the sequences.
7. These sequences are annotated and assigned to the respective chromosomes.