Nucleoid

The nucleoid is a region within prokaryotic cells, like bacteria, where the genetic material (DNA) is located.

Unlike eukaryotic cells, which have a membrane-bound nucleus, prokaryotic cells have their DNA freely distributed in the nucleoid region.

The nucleoid is essential for organizing and protecting the bacterial genome.

Experiment Design:

Griffith injected mice with the S strain, leading to their death due to pneumonia.

Mice injected with the R strain survived, as the non-virulent strain did not cause disease.

Streptococcus Pneumoniae

Streptococcus pneumoniae is a bacterium responsible for pneumonia and other infections.

It was used in Griffith’s experiment to study the transforming principle.

Bacterial Strains:

Smooth (S) Strain: Virulent strain of Streptococcus pneumoniae with a protective polysaccharide capsule.

Rough (R) Strain: Non-virulent strain lacking the capsule.

Transformation Observation:

Griffith heat-killed the S strain bacteria and injected them into mice. Surprisingly, the mice survived.

Conclusion - Transformation:

Something in the heat-killed S strain had transformed the R strain into a virulent form.

The transforming substance was later identified as DNA.

Terms Introduced:

Transforming Principle: The substance responsible for transforming the R strain into a virulent form.

Transformation: The process by which genetic material is transferred and accepted by a cell.

Avery, MacLeod, and McCarty Experiment

Objective:

The primary aim was to identify the macromolecule responsible for genetic transformation in bacteria.

Background:

Building upon Griffith’s transformation experiment, which demonstrated that a substance could transform non-virulent bacteria into virulent ones.

Experimental Process:

Avery, MacLeod, and McCarty isolated cellular components (DNA, RNA, proteins) from the S strain of Streptococcus pneumoniae bacteria and treated it with DNase, RNases and Protease separately.

This cell extract was then mixed with the non-virulent R strain

This extract was injected in mice

Observations:

The experiment revealed that when the cell extract was treated with protease, the virulent S-strain developed. Conversely, when the cell extract was treated with DNase, the virulent S-strain did not develop.

Conclusion

This experiment confirmed that DNA was the transforming principle by using enzymes to selectively degrade DNA, RNA, and proteins in heat-killed bacteria.

Only the destruction of DNA prevented transformation.

The Genetic Material in Viruses

Viruses can have DNA or RNA as their genetic material.

The type of genetic material varies among different viruses.

Life Cycle of Bacteriophage

Bacteriophages are viruses that infect bacteria.

Their life cycle typically involves attachment, injection of genetic material, replication, assembly, and release of new viral particles.

Why Radioactive Phosphorus and Sulfur

Hershey and Chase used radioactive phosphorus (32P) to label DNA and radioactive sulfur (35S) to label proteins in bacteriophages.

This allowed them to track which component (DNA or protein) entered the bacterial cells during infection.

Hershey-Chase Experiment

The Hershey-Chase experiment provided conclusive evidence that DNA, labeled with radioactive phosphorus, was the genetic material transferred from bacteriophages into bacterial cells.

Differences Between Positive and Negative Supercoiling

Positive Supercoiling: In positive supercoiling, the DNA is overwound or twisted in the same direction as its natural helical structure. It results in increased tension in the DNA molecule.

Negative Supercoiling: Negative supercoiling involves the DNA being underwound or twisted in the opposite direction of its natural helix. It results in reduced tension in the DNA molecule.

Both types of supercoiling can affect gene expression and DNA packaging.

Why DNA Packaging is Needed

DNA packaging is essential to fit the long DNA molecules into the confined space of a cell’s nucleus.

Efficient packaging prevents tangling and damage to the genetic material.

It also ensures that specific regions of DNA are accessible for various cellular processes, such as gene expression and replication.

Proteins Involved in Nucleosome Model

In the nucleosome model of DNA packaging, histone proteins play a central role.

The core histone octamer consists of H2A, H2B, H3, and H4 proteins.

These histones form the core around which DNA is wrapped.

Histone H1 binds to the linker DNA regions, contributing to higher-order chromatin structure.

Different Types of Chromosomes Depending on Their Centromere Position

Metacentric Chromosomes: The centromere is positioned near the middle, creating arms of roughly equal length.

Submetacentric Chromosomes: The centromere is slightly off-center, resulting in one long arm and one short arm.

Acrocentric Chromosomes: The centromere is located closer to one end, leading to a very short arm and a long arm.

Telocentric Chromosomes: The centromere is at the very end of the chromosome, resulting in a single long arm.

Search for Genetic Material

Early scientists sought to identify the substance responsible for inheritance.

This search led to key experiments to determine whether DNA or proteins were the genetic material.

Contribution of Scientists in the Search for Genetic Material

Gregor Mendel’s work with pea plants laid the foundation for understanding heredity.

Frederick Griffith’s experiment with Streptococcus pneumoniae showed that a “transforming principle” could transfer genetic information.

Avery, MacLeod, and McCarty’s experiment confirmed that DNA was the transforming principle.

Hershey and Chase’s experiment with bacteriophages provided further evidence that DNA, not protein, was the genetic material.

Features of Genetic Material

Genetic material should be stable, capable of replication, and contain information for an organism’s traits.

It should be subject to mutations, allowing for genetic diversity.

DNA fulfills these criteria and serves as the genetic material in most organisms.

Griffith Experiment

Griffith’s experiment involved injecting mice with live disease-causing bacteria (virulent) and heat-killed non-virulent bacteria.

He observed that mice injected with the heat-killed bacteria became infected when combined with live virulent bacteria.

This suggested that a transforming principle in the heat-killed bacteria could change the virulent bacteria.