TOPIC: DNA as a Genetic Material Material,

The identification of DNA as genetic material was done by Frederick Griffith involving the bacterium Streptococcus pneumoniae in 1928.

This bacterium, which causes pneumonia in humans, is normally lethal in mice.

Griffith used two strains that are distinguishable by the appearance of their colonies when grown in laboratory cultures.

o In one strain, a normal virulent type, the cells are enclosed in a polysaccharide capsule, giving colonies a smooth appearance; this strain is labelled S.

o In the other strain, a mutant nonvirulent type that is not lethal, the polysaccharide coat is absent, giving colonies a rough appearance; this strain is called R.

DNA as a genetic material Griffith Experiment

This experiment showed that something passed from dead bacteria into nearby living ones, allowing them to change their cell surface.

However, mice injected with a mixture of heat-killed virulent cells and live nonvirulent cells did die. Live cells could be recovered from the dead mice; these cells gave smooth colonies and were virulent on subsequent injection. Somehow, the cell debris of the boiled S cells had converted the live R cells into live S cells. The process is called transformation.

STEPS AND RESULTS OF GRIFFITH’S EXPERIMENT

In 1944, Avery, MacLeod and McCarty published results of a study that identified the transforming principle from S. pneumoniae.

Chemically separate the components (e.g., protein, nucleic acids) and determine which was capable of transforming live S. pneumoniae cells.

Only the nucleic acid fraction was capable of transforming the bacteria.

Nucleic acid fraction was contaminated with proteins. The researchers treated this fraction with either RNase or

protease and still found transforming activity, but when it was treated with DNase, no transformation occurred, indicating that the transforming principle was DNA.

Avery’s Transformation Experiment

Hershey-Chase experiment

1. More evidence for DNA as the genetic material came in 1953 with Alfred Hershey and Martha Chase’s work on E. coli infected with bacteriophage T2.

2. In one part of the experiment, T2 proteins were labeled with 35S, and in the other part, T2 DNA was labeled with 32P.

3. The 35S-labeled protein was found outside the infected cells, while the 32P-labeled DNA was inside the E. coli, indicating that DNA carried the information needed for viral infection. This provided additional support for the idea that genetic inheritance occurs via DNA.

COMPOSITION OF NUCLEIC ACIDS

Nucleic acids are one of several macromolecules in the body in addition to fats, proteins and carbohydrates.

Nucleic acids are polymers made up of four nucleotides linked together in long chains known as polynucleotides.

A nucleotide can itself be further broken down to yield three components:

- a pentose sugar, - a nitrogenous base, and - Phosphate group

When a sugar bonds together with a Nitrogenous base, it forms a structure known as a nucleoside.

There are two types of nucleic acids: DNA and RNA. DNA stores genetic information, and RNA allows that information to be used in the cell.

Both DNA and RNA contain nucleotides with similar components. In RNA, the sugar component is ribose, as indicated by the name "ribonucleic acid". In DNA, or deoxyribonucleic acid, the sugar component is deoxyribose. The prefix deoxy means that an oxygen atom is missing from one of the ribose Carbon atoms.

Purines - Adenine and Guanine Pyrimidines- Thymine (only in DNA),

Cytosine, and Uracil (only in RNA).

o These bases are divided into two categories (purines and pyrimidines) based on their molecular structure.

There are Four Nitrogen bases that are found in DNA: Adenine, Guanine, Thymine and Cytosine.

The same bases are also found in RNA, except that there is Uracil instead of Thymine.

THE STRUCTURE OF NUCLEIC ACID CHAINS To form polynucleotides of either DNA or RNA ,

nucleotides are linked together by covalent bond between the phosphate groups. These phosphate linkage are called phosphodiester bonds.

Nucleotides are joined together in DNA and RNA by phosphodiester bonds between the phosphate component of one nucleotide and the hydroxyl component in the sugar molecule of the next nucleotide.

But no matter how long a polynucleotide chain is, one end of the nucleic acid molecule always has a free -OH group on the sugar at the Carbon known as C3' (called the 3' end) and the other end of the molecule always has a phosphate group at C5' (the 5' end).

In 1953 James Watson and Francis Crick proposed a model for the physical and chemical structure of the DNA molecule.

According to the Watson-Crick model, a DNA molecule consists of two polynucleotide strands coiled around each other in a helical manner - "twisted ladder" structure.

The sugar-phosphate backbone is on the outside of the double helix, and the bases are on the inside, so that a base on one strand points directly toward a base on the second strand.

The two strands of the DNA double helix run in opposite directions, one in the 5' to 3' direction, the other in the 3' to 5' direction. The term that describes how the two strands relate to each other is known as antiparallel.

THE TWO STRANDS OF DNA ARE ANTIPARALLEL

THE GENERALIZED STRUCTURE OF DNA

The specific pairings observed are A with T and G with C. The specific A – T and G – C pairs are called complementary

base pairs

The two strands are held together by hydrogen bonds between the nitrogenous bases. In the double helix, adenine and thymine form two hydrogen bonds to each other but not to cytosine or guanine. Similarly, cytosine and guanine form three hydrogen bonds to each other in the double helix, but not to adenine or thymine.

THERE ARE TWO HYDROGEN BONDS BETWEEN ADENINE AND THYMINE

THERE ARE THREE HYDROGEN BONDS BETWEEN GUANINE AND CYTOSINE

o The expression of genetic information is that the flow of information is from DNA to RNA to polypeptide which is known as Central Dogma Of Biology.