(i) In eukaryotes, chromatin is formed by a series of repeating units called nucleosomes.
(ii) Korenberg proposed a model for the nucleosome, in which 2 molecules of the four histone proteins H₂A, H₂B, H3 AND H4 are organized to form a unit of eight molecules called histone octamere.
(iii) The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome. A typical nucleosome contains 200 bp of DNA helix.
(iv) The histone octameres are in close contact and DNA is coiled on the outside of nucleosome.
(v) Neighbouring nucleosomes are connected by linker DNA (Hl) that is exposed to enzymes. The DNA makes two complete turns around the histone octameres and the two turns are sealed off by an HI molecule
(vi) Chromatin lacking HI has beads on a string appearance in which DNA enters and leaves the nucleosomes at random places.
(vii) HI of one nucleosome can interact with HI of the neighbouring nucleosomes resulting in the further folding of the fibre.

1. They wanted to know whether the proteins coat or the DNA core of the virus entered , into the infected cells. This would help them to determine whether genes were made up protein or of DNA.
2. They used radioactive isotopes of phosphorous - 32 and sulfur - 35 as markers. So they could trace the proteins and DNA to see what they were doing to the cell.
3. They were perfect because proteins contain almost no phosphorous and DNA contains no sulfur, so the radioactive isotopes would show up in the cells if the P-32 or S-35 appeared. If S-35 shown up in the bacteria. It meant the viruses protein had been injected into the bacteria, if the P-32 was found in the bacteria the DNA had been injected into the bacteria.
4. By that they concluded the genetic material of the bacteriophage was DNA, not protein.
5. If in Hershey and Chase's experiments, the phages were labeled with radioactive carbon, the label would be found in all components of the phages and of the phase ghosts (empty capsids) and possible in all components of the bacteria because carbon is found in all organic material.
6. If Hershey and Chase used radiolabeled nitrogen they would see that the cells would be labelled in the nucleus, in the membrane and possibly throughout the cell. Additionally, numerous components of the phage ghosts and the progeny phage would be labelled.
7. Nitrogen is found in proteins and in DNA so they were not able to get actual result that the genetic material of the phage was DNA, if they use radio isotopes labelled with carbon and nitrogen.

1. The process protein synthesis actually consists of two stages-transcription and translation.
2. In eukaryotic cells, transcription takes place in the nucleus. During transcription, DNA is used as a template to make a molecule of messenger RNA (mRNA). The molecule of mRNA then leaves the nucleus and reaches ribosome in the cytoplasm, where translation occurs.
3. During translation, the genetic code in mRNA is read and used to make a protein.
4. These two processes are summedup by the central dogma of molecular biology

(a) Replicating fork of DNA
(b)

(c) Source of Energy:
NTP's are used in the synthesis of RNA primers and ATP is used as an energy source for some of the enzymes, needed to initiate and sustain DNA synthesis occurs at the replication fork.
Enzymes involved in replication :
(i) DNA Helicase
(ii) DNA polymerase
(iii) Topoisomerase/DNA Gyrase
(iv) DNA ligase
(v) Primase
(d) Both the strands of (parental) DNA act as template for the synthesis of new strands
1. On the templates strand with \(3^{\prime} \rightarrow 5^{\prime}\) polarity the new strand (leading strand) is synthesized as a continuous stretch. It is called continuous synthesis (DNA polymerase carryout polymerisation).
2. On the other strand \(5^{\prime} \rightarrow 3^{\prime}\) polarity DNA synthesized as short stretches; it is called discontinuous synthesis.
3. Later the short stretches of DNA are joined by DNA-ligases into a continuous strand called lagging strand.

Structural difference between DNA and RNA.

Human genome project was considered to be a mega project because
1. It had a specific goal to sequence for everybase pair present in the human genome. (Human Genome has 3.1647 billion nucleotide base pairs).
2. It took around 13 years of completion and got accomplished in year 2006.
3. It was a large-scale project, which aimed at developing new technology and generating new information in the field of genomic studies.
4. As a result of it, several news areas and avenues have opened up in the field of genetics, biotechnology and medical sciences.
5. It provided clues regarding the understanding of human biology.