section 2 - lecture 4 - jane farrar

evidence that genes are located in the chromosone

  • the x chromosone determines the sex in some insects

  • chromosones behave as the unit factors mendel predicted

  • - extracted a phosphorus-rich polymer called nuclein - this was actually DNA

  • The genetic material from the Type III S contains the information for making the polysaccharide coat required for virulence – an example of horizontal gene transfer.

  • Transformation involves uptake of short fragments of naked DNA by naturally transformable bacteria. Transduction involves transfer of DNA from one bacterium into another via bacteriophages (virus). Conjugation involves transfer of DNA via sexual pilus and requires cell-to-cell contact.

  • virus is made of 50% DNA and 50% protein

Two findings became known as Chargaff’s rules: 1. The base composition of DNA varies between species 2. In any species the number of A and T bases is equal and the number of G and C bases is equal

The bacterial chromosome is a double-stranded, circular DNA molecule associated

with a small amount of protein. In a bacterium, the DNA is “supercoiled” and found

in a region of the cell called the nucleoid.

n Eukaryotic chromosomes have linear DNA molecules associated with a large

amount of protein. In the eukaryotic cell, DNA is precisely combined with proteins

in a complex called chromatin.

n Chromosomes fit into the nucleus through an elaborate, multilevel system of

packing. Proteins called histones are responsible for the first level of packing in

chromatin.

n Unfolded chromatin resembles beads on a string, with each “bead” being a

nucleosome, the basic unit of DNA packaging. They are composed of two each of

the four basic histone types, with DNA wrapped twice around the core of the eight

histones.

n The N-termini (“tails”) of the histones protrude from the nucleosome.

Nucleosomes, and especially their histone tails, are involved in the regulation of

gene expression. Chemical modification to histones can change how tightly or

loosely the DNA is packed and therefore can aid in controlling whether a gene will

be expressed.

- loosely packed chromatin is called euchromatin.

During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into heterochromatin

‘Epigenetics involves heritable changes in gene expression (active vs inactive genes) that are NOT due to changes to the DNA sequence - a change in phenotype without a change in genotype - which affects how cells read the genes’.

When epigenetic modifications are dysregulated, it can result in disease, e.g., cancers, aging disorders etc