Lecture 2 - Nucleic Acids (Part II)

simplist chromatin is in

non-dividing eukaryotic cell

simple chromatin is seen by microscope

False

In electron microscope, chromatin resembles a

regularly beaded thread

Each Chromatid Consists of:

1- A single DNA molecule. 2- Histone proteins

Histone proteins

They are basic proteins rich in lysine and arginine (positively charged

DNA which is

acidic (negatively charged due to the presence of phosphate groups)

histones are in contact with

minor groove of DNA

major types of histones

(H1, H2A, H2B, H3 and H4)

lysin rich histones

H1, H2A, H2B

Arginine rich histones

H3 and H4

role of histones

DNA supercoiling and regulation of gene expression.

A. Core Histones may become

Acetylated during Replication

Acetylated and Methylated during Transcription

ADP-ribosylated during DNA Repai

H2A may become covalently linked t

Ubiquitin

H1 histone may become

phosphorylated

Nucleosome

formed of eight histones (histone octamer or histone core) or two copies of each of H2A, H2B, H3 and H4

round the histone octamer is wrapped

ne and three quarters of toroidal supercoiled DNA segment (contains average 140 bp).

histone proteins represent

disc shaped core protein of Nucleosomes.

Linker or spacer DNA

ontains average 60 bp) connects the different nucleosomes like beads on a string. H1 is attached to linker DNA.

a histone attached to the linker DNA

H1 histone

Non-histone proteins

They interact with the major groove, important for regulation of gene expression and are responsible for replication and transcription

types of non-histone proteins

a) Nucleoplasmin b) Scaffold proteins c) Enzymes d) Regulatory proteins

Nucleoplasmin

Anionic nuclear protein that interacts with histones. It probably helps the proper assembly of nucleosomes by preventing nonspecific binding of histones and DNA.

Centromere

rich in A===T bp

Kinetochores

specific proteins bound to centromere, which is connected to the mitotic spindle

telomeres

characterized by the presence of variable repeat number of specific sequence of several kilobases long, which is (TTAGGG)n in humans.

Supercoiling of DNA

important for its packing within the small nucleus. A typical human cell contains 46 chromosomes, whose DNA is approximately 2 m long

importance of having supercoiled DNA

efficiently replicated, and its genetic information expressed.

The first level of supercoiling

packing ratio of 10) around histone octamer in the form of nucleosomes produces a ten-fold shortening of the length of DNA to form the 10-nm fibril (10 nm in diameter)

second level of supercoiling

(packing ratio of 50) requires the presence of H1, this leads to 50-fold shortening of the DNA and it looks like a solenoid (cylindrical coil), each turn contains 6 nucleosomes that form the 30-nm fiber (30 nm in diameter).

third level of supercoiling

(packing ratio of 8000) is the coiling of the 30-nm fiber into twisted-looped structure attached to a protein scaffold in the form of rosettes. Each rosette contains 6 loops. Additional levels of supercoiling lead to chromosomal structure and increase the packing ratio to about 8000

Genes

a unit of the DNA that encodes a particular protein or RNA molecule

Genes

sequence of nucleotides along a DNA molecule

Most eukaryotic genes are

discontinuous

Coding sequences

unique and non repetitive

Non-coding sequences

Repetitive

locus

The position of a gene along a chromosome

Genome

The total genetic information presented by the group of chromosomes in any cell.

genotype

Set of genes in our DNA which is responsible for a particular trait

phenotype

physical or biochemical expression of that trait. e.g. skin color and hair color.

Most complex traits are influenced by

many genes and by environment

three major types of RNA t

ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA)

RNA are composed of

nucleoside monophosphate joined together by phosphodiester bonds

RNAs exist as

Single strands that are capable of folding into complex structures.