BCH3031 Lecture Notes - Lecture 2: Nuclear Lamina, Nuclear Pore, Nuclear Dna

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25 May 2018

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Lecture 2 – Genome Structure: Organisation of Eukaryote Genome

Packing of DNA into chromatin → into chromosome

Genetic Information in DNA Molecule

• DNA – where genetic information is found

o Double helix structure formed from 2

complementary strands of nucleotides held

together by hydrogen bonds between GC and

AT base pairs

• Nucleus – storage sight

o DNA stored within nucleus, associated with

proteins

▪ Interphase nucleus: DNA loose not

compact

▪ Can organise into chromosomes

o RNA in presence

o Complex DNA and proteins – chromatin

o Nuclear membrains connected by nuclear

pores

o Nuclear lamina - a network of filaments that

provide mechanical support

▪ DNA region not transcribed (silence) – repetitive sequences in

genome (can be repeated many times) – heterochromatin

• Function of Chromosomal DNA

o Genomes of eukaryotes are divided up into chromosomes

o Each chromosome has unique composition of DNA sequences – label

with probe and can see pattern

o Chromosome painting by DNA hybridization with specific DNA

probes conjugated with combination of fluorescent dyes

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Chromosome Banding Pattern

• Pattern of bands on each chromosome – unique

o Allow each chromosome to be identified and numbered

• Principle bands:

o G and C bands – late replicating in S phase and corresponds to

heterochromatin (bands that are dark, not transcribed, compact,

repetitive DNA sequences)

o R bands that replicate earlier – represent euchromatin (highly

accessible for transcription factors)

• Display of 46 human chromosomes – human karyotypes

Aberrant Chromosome (in Liposarcomas)

• Large rearrangement in chromosome numbers and DNA composition in

chromosomes

o Result of instability of genome that can reduce cancers

Chromosomes in Two Related Species

• Humans and chimpanzee

o Same chromosome number and similar chromosome banding pattern

• Chinese muntjac and indian muntjac

o Very different chromosome number and banding pattern

o But similar gene number

• No simple rule between chromosome number, species complexity and genome

size

Organisation of genes on Chromosome

• Short arm

o Consists of heterochromatin (repeated DNA sequences), not encoding

for any proteins, takes up majority of chromosome

• Most genes found on Q arm (long arm)

o Exon (codes for protein), intron (noncoding regions)

Gene Content

• Long interspersed elements (LINEs) and short interspersed elements (SINEs)

o Can be found in between genes

• Transposable elements are mobile genetic elements

o Assigned into 2 classes according to their mechanism of transportation

▪ Copy and paste

• Involves RNA transcription step

▪ Cut and paste

o Transposon elements can move around in genome unless they are

silenced (complex)

o Must be maintained in the state of inert: transcriptionally inactive,

compact, cannot be transcribed

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Importance of Packaging of Chromosomal DNA

• Each human cell contains ~2 m of DNA if stretched

• Packaging DNA accomplished by specialized proteins that bind to and fold

DNA → generating series of coils and loops

o Provide increasingly higher levels of organisation, preventing DNA

from becoming unmanageable tangle

• DNA compacted so it is easily available to many enzymes in the cell that

replicate it, repair it and use its genes to produce proteins

• Packaging of DNA can control how DNA is transcribed by transcription

factors – each cell type: DNA packaged differently

Chromsomes exist in Different States during Progression of Cell Cycle

• DNA comes in form of nucleus

• Chromosomes become visible under microscope when they become compact

sausage like structures

• Interphase chromosome → mitosis → segratation into two cells

o Gene expression can occur in interphase

o S phase: DNA can be replicated

o Organisation of chromosome is important

DNA Elements in Stable Chromosomes

• Telomere

o End of chromosome

o Structure where repetitive DNA sequences

▪ Ensures complete replication of

chromosomes ends

o Stabilises linear DNA

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Document Summary

Lecture 2 genome structure: organisation of eukaryote genome. Packing of dna into chromatin into chromosome. Genetic information in dna molecule: dna where genetic information is found, double helix structure formed from 2 complementary strands of nucleotides held together by hydrogen bonds between gc and. At base pairs: nucleus storage sight, dna stored within nucleus, associated with proteins. Aberrant chromosome (in liposarcomas: large rearrangement in chromosome numbers and dna composition in chromosomes, result of instability of genome that can reduce cancers. Organisation of genes on chromosome: short arm, consists of heterochromatin (repeated dna sequences), not encoding for any proteins, takes up majority of chromosome, most genes found on q arm (long arm, exon (codes for protein), intron (noncoding regions) Gene content: long interspersed elements (lines) and short interspersed elements (sines, can be found in between genes, transposable elements are mobile genetic elements, assigned into 2 classes according to their mechanism of transportation, copy and paste.

Related Questions

The bacterium E. coli, which is a major model system for the study of genes and DNA, has a singular circular chromosome of about 4x106 bp. In contrast, human diploid cells have __1__ chromosomes, of __2__ different types (donât forget that the X Â¹ Y!). The total amount of DNA found in a haploid cell such as a sperm or egg is called the organismâs âC-valueâ; the human C-value is __3__.

Also in contrast to the E. coli circular chromosome is the fact that eukaryotic chromosomes are not circular, but instead are __4__. Eukaryotic chromosomes are complexed heavily with protein to form a DNA-protein complex generically called __5__. The proteins found can be considered to fall into two general classes: __6__ and __7__. The first class is made up of 5 major proteins called __8__, __9__, __10__, __11__, and __12__. On a weight to weight basis, these proteins make up about __13__% of the total mass of a chromosome. Since these proteins must associate relatively non-specifically with DNA, they are rich in the basic amino acids __13__ and __14__, which gives the proteins a net __15__ charge.

One goal of packaging the DNA into chromatin is to compact the genome so it will fit into a nucleus. The basic unit of chromatin structure is the __16__, which has __17__ base pairs of DNA wrapped around a âcore particleâ composed of __18__ copies each of the core histones. Each nucleosome is separated by a stretch of âlinkerâ DNA so that the repeating unit is approximately 200 bp. Lets do a little arithmetic: If we consider that the nucleosome is approximately __19__ nm in diameter, that there is about 200 bp in this structure, and that 200 bp of DNA would normally occupy __20__ nm if stretched out straight (recall that each base is spaced about 0.34 nm from the next), we can see that the nucleosome alone allows for a __21__-fold compaction ratio.

The nucleosome structure is further compacted by histone __22__, which is not a part of the core particle. Instead, this histone __23__ (does what?) to cause the nucleosomal 10 nm fiber to condense into a higher-order structure referred to as the __24__ nm fiber. In the nucleus, chromosomes do not just float around freely: they are tethered to a proteinaceous structure called the __25__ at specific DNA regions called âSARsâ, which stands for __26__. SARs are rich in topoisomerases, to allow the winding and unwinding of the DNA that accompanies replication and transcription to occur, despite this constraint.

The 30 nm fiber discussed above is the form in which transcriptionally active DNA is believed to exist in the nucleus; however, further condensation leads to a highly condensed inactive form called __27__; in this terminology, the active form is referred to as __28__. The inactive condensed form can be divided into two types: one type is essentially invariant between cell types, and since it is âon all t timeâ, it is referred to as __29__. The other type comes and goes, depending on the expression of the genes present in that region of the chromosome; it is referred to as __30__.

Eukaryotic chromosomes carry the genes, of course, but also have other functional regions. One such region is the place where the spindle apparatus attaches during mitosis and meiosis; this region of the chromosome is called the __31__. Another important part of the linear eukaryotic chromosome is the ends; these are called __32__. In humans, the end of each chromosome is composed of many repeats of the short sequence __33__. One more type of functional region on each chromosome is the place where DNA replication starts; in humans, each chromosome has several of these __34__. Thus, eukaryotic chromosomes carry several different types of functional sequences, which together allow for their __35__ during S phase, __36__ during mitosis and meiosis, and the __37__ of their genes

BCH3031 Lecture Notes - Lecture 2: Nuclear Lamina, Nuclear Pore, Nuclear Dna

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