DNA Packaging and Replication

DNA

Deoxy ribonucleic acid; it is a chain of polynucleotide

Helical structure

Watson and Crick

Nucleotide

1. Pentose sugar (5 carbon)

2. Nitrogenous base

3. Phosphate group

Where was DNA first observed?

Puss cells

Phosphodister bond

Bond between P and OH

Glycosidic bond

Bond between N and Pentose sugar

Nitrogenous bases

• Purine

  • Adenine

  • Guanine

• Pyrunidines

  • Thymine

  • Cytosine

Chargoff rule

1. Amount of A = T and G = C

2. Ratio of A/T = G/C

Nucleosome model of DNA packaging

• Prokaryotes have [ DNA + Protein ] in cytoplasm

• Eukaryotes have [ DNA + Histone proteins ]

Types of histone proteins (histo-proteins)

1. H1

2. H2A

3. H2B

4. H3

5. H4

Octamers

8 proteins are arranged together and around each octamer DNA is attached

Linker DNA

DNA which connect 2 octamers

Why is there strong between DNA and Protein?

DNA → -vely charged

Protein → +vely charged (because they are rich in special enzymes- Arginine and Lysine)

Chromatin

Thread like structure present in nucleus which changes into chromosomes at the time of cell division. (2 types)

Euchromatin

1. Loosely packed

2. It will be lightly stained

3. Transcriptionally active

Heterochromatin

1. Tightly packed

2. It will be darkly stained

3. Transcriptionally inactive

DNA replication

Making copies of DNA which are formed in S-phase (synthesis phase)

DNA polymerase works properly in which direction?

5’ → 3’

Enzymes required for DNA replication

1. Topoisomerase: reduces tension on the strands

2. Helicase: unbinds / opens the DNA strands or helix of DNA

3. DNA polymerase: helps in synthesis of DNA strands

4. RNA primer: small segment of RNA which is required to start formation of DNA

5. SSB Protein: single strand binding protein

6. Ligase: enzyme used for joining the DNA fragments

Why is DNA replication semi-conservative in nature?

Because there are half characters of parents and half characters are new.