13491_eukaryotic Dna Org

8/8/2019 13491_eukaryotic Dna Org

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The Organization andThe Organization andControl of EukaryoticControl of Eukaryotic

GenomesGenomes

The Structure of Chromosomes


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Key Concepts1. P

rokaryotic and eukaryotic cells both contain double-stranded DNA, but their genomes are organized

differently

2. Eukaryotic chromosomes contain an enormous

amount of DNA, which requires an elaborate systemof DNA packing to fit all of the cells DNA into the

nucleus

3. Changes in the physical arrangement of DNA, such as

level of DNA packing, affect gene expression

4. Gene make up only a tiny portion of the genomes of

most multicellular eukaryotes


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The Structure of ChromatinThe Structure of Chromatin

Prokaryotic and eukaryotic cells

Gene expression in eukaryotes has two maindifferences from the same process in prokaryotes.

First, the typical multicellular eukaryotic genomeis much larger than that of a bacterium.

Second, cell specialization limits the expressionof many genes to specific cells.

The estimated 35,000 genes in the human genomeincludes an enormous amount of DNA that doesnot program the synthesis of RNA or protein.


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In eukaryotes, the DNA molecules are tightly complexed

with protiens to form chromatin, which is nucleoprotien

fibres with a beaded appearance. The key to the bead like organisation of chromatin is its

histone protiens, which are divided in to five main

classes: H1, H2A, H2B,H3 and H4.

Out of these histones, H2A, H2B, H3 and H4 are

involved in formation of the fundamental sub-unit of

chromatin called a nucleosome. Nucleosome core

consists of an 8 histone complex containing two

molecules each of H2A, H2B, H3 and H4.


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DNA is packed inside the nucleus in association with a

number of proteins, which are extensively coiled and

folded forming nucleosomes. Each nucleosome is madeup a histone octamer mainly made up of histones H2A,

H2B, H3 and H4. Histones consists of large amounts of

positively charged amino acids mainly lysine and

arginine, that binds electro statically to the negatively

charged phosphate groups of the DNA backbone. The

DNA turns in a 1.65 left handed orientation around

each histone octamer covering a total of 146 bp of

double stranded DNA. The next 50 bp links one

nucleosome to another also interacting with another

histone (H1) forming a thicker fibre consisting of six

nucleosomes, known as the solenoid


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STRUCTURE OF NUCLEOSOME


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HistoneHistone withwith

its functionits function


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Eukaryotic chromosomes contain an enormous amountof DNA relative to their condensed length.

Each human chromosome averages about 2 x

108 nucleotide pairs.

If extended, each DNA molecule would be

about 6 cm long, thousands of times longer than

the cell diameter.

This chromosome and 45 other human

chromosomes fit into the nucleus.

This occurs through an elaborate, multilevel

system of DNA packing.


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Histoneproteins are responsible for the first levelof DNA packaging.

Their positively charged amino acids bind

tightly to negatively charged DNA.

The five types of histones are very similar from

one eukaryote to another and are even present in

bacteria.

Unfolded chromatin has the appearance of beads

on a string, a nucleosome, in which DNA winds

around a core of histone proteins.


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Histones leave the DNA only transiently duringDNA replication.

They stay with the DNA during transcription.

By changing shape and position, nucleosomes

allow RNA-synthesizing polymerases to move

along the DNA.

The beaded string coils to form the 30-nm

chromatin fiber.

This fiber forms looped domains attached to a

scaffold of nonhistone proteins.


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In a mitoticchromosome,the looped

domainscoil and fold to

produce thecharacteristic

metaphasechromosome.

These packingsteps are highly

specific andprecise withparticular geneslocated in thesame places.


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Interphase chromatin is generally much less

condensed than the chromatin of mitosis.

The chromatin of each chromosome occupies

a restricted area within the interphase

nucleus.

Interphase chromosomes have areas thatremain highly condensed, heterochromatin,

and less compacted areas, euchromatin.


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Genome Organization at the DNA levelGenome Organization at the DNA level

1. Prokaryotes and eukaryotes

Prokaryotes

Eukaryotes

2. R

epetitive DNAa. Tandemly repetitive DNA

b.Interspresed repetitive DNA


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Genome Organization at the DNA levelGenome Organization at the DNA level

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13491_eukaryotic Dna Org