Chromosomes and Chromatin Flashcards
Prokaryotic chromosomes are ?
Circular and typically have one origin of replication
The E. coli chromosome is about ?
4.6 million bases long and encodes 4300 different proteins
Eukaryotes genomes are?
Larger and more complex
Explain Eukaryotes further?
- More genes which contain introns
- More regulation eg cell type specific expression
- More ‘junk’ DNA
- No correlation between genome size and evolution
- Organised in linear chromosomes
- Packaging problem - 2m DNA into 6μm human cell nucleus
Eukaryotic chromosomes are ?
Linear
In eukaryotes, the genome is broken up into ?
Long, linear, double-stranded structures called chromosomes
How many chromosomes does the human genome have ?
The human genome has 24 chromosomes
22 autosomes and 2 sex chromosomes
Human cells are ?
Diploid (2n = 46)
Even closely related species can have ?
Different numbers of chromosomes
Chromosomes change during ?
The cell cycle
In Interphase nuclei, the chromosomes are ?
Distributed throughout the nucleoplasm
At mitosis the chromosomes?
Condense - individual chromosomes can be seen with the light microscope
What are telomeres ?
Stabilise ends
What is the function of the centromere ?
To segregate the sister chromosome
What is the use of replication origins ?
To duplicate the DNA
A chromosome consists of?
Two sister chromatids
After replication there are?
Two identical sister chromatids
Each chromatid contains ?
One old and one new strand (semi-conservative replication)
Sisters are held together until ?
Chromosomes segregation during mitosis
In some cells, chromosomes adopt the Rabl configuration with ?
The centromeres clustered at one end, and the telomeres abutting the nuclear envelope at the opposite pole reflecting the arrangement of the chromosomes at anaphase
During Interphase, individual chromosomes occupy ?
Particular regions of the nucleus
This arrangement minimises?
Tangles between the chromosomes and helps condensation and segregation during mitosis
Chromosomal DNA is ?
Highly condensed
DNA is compacted by association with proteins -
Chromatin
Individual nucleosomes are arranged on ?
The DNA like beads on a string
DNA isolated from an interphase nucleus appears as ?
A 30 nm thick fibre
The nucleosome core particle consists of ?
147 base pairs of DNA wound around a protein core
Explain Histones ?
The protein core is made of a small highly basic proteins
The octomeric histone core contains ?
Two molecules of each of four histones, H2A, H2B, H3 and H4
DNA is wound around?
Histones
An additional histone, Histone H1, arranges ?
Nucleosomes into the 30nm fibre
During mitosis, this 30 nm fibre is further ?
Condensed at
two additional levels to achieve the packing that is observed in mitotic chromosomes
By TEM, the chromatin in the nucleus appears in two forms:
A diffuse open configuration, called euchromatin and a condensed electron dense form called heterochromatin
Euchromatin is ?
Transcriptionally active
Heterochromatin is ?
Transcriptionally inactive
Decondensed or open chromatin (= euchromatin) is associated ?
With transcriptionally active regions of the genome
Closed Condensed chromatin (=heterochromatin) is associated ?
With transcriptionally repressed regions of the genome
Chromatin condensation can vary along ?
The length of the chromosome
Telomeres and centromeres are typically?
Heterochromatic. These regions are also gene poor
The gene-rich regions of the chromosomes are typically?
Euchromatic. Regional modifications to produce blocks of different degrees of extension or condensation
Each core histone has a ?
‘tail’ that extends out of the nucleosome core
These tails can be modified by ?
Methylation and acetylation to influence chromatin structure and thus gene expression
DNA + histones =
Chromatin
