Organisation of Genomes Flashcards
What is a genome?
The genetic material of an organism or virus
What exists in each “chromosome” for dividing and non-dividing cells?
One DNA double helix, which exists as decondensed chromatin
Compare euchromatin and heterochromatin
Euchromatin is decondensed and thus transcriptionally active, but heterochromatin does not fully decondensed and is thus transcriptionally active.
What happens to the DNA after S phase for dividing cells?
Each chromosome comprises two DNA double helices, which exists as
- Decondensed chromatin (after S phase and before prophase)
- Two identical sister chromatids (during nuclear division)
Function of Centromere
Responsible for attaching chromosomes to the kinetochore microtubules during cell division
How many pairs of chromosomes does a human genome have?
23 pairs
How many sister chromatids does each chromosome have?
Two sister chromatids
How many levels of condensation are there for packing of DNA in eukaryotics chromosomes?
3 levels
What happens to the DNA during the FIRST level of condensation?
- DNA is packed into nucleosomes to produce the nucleosome fibre (10-nm interphase chromatin fibre)
- dsDNA coils 1 3/4 turns around the histone core, forming a nucleosome core.
- Chromatin fibre = Nuclesomes + Linker DNA - The nucleosome consisting the DNA wound twice around a protein core composed of an octamer of 4 histone molecules, two of each histones.
- Histones leave the DNA only briefly during DNA replication and gene expression.
- Histones are small proteins with a high concentration of basic residues (postively-charged R groups), which interact with the negatively-charged sugar-phosphate backbone of DNA.
What happens to the DNA during the SECOND level of condensation?
- DNA is further folded or coiled to produce the solenoid (30-nm chromatin fibre).
- Histone H1 and linker DNA is involved in the coiling of the 10-nm nucleosome fibre to produce the solenoid.
What happens to the DNA during the THIRD level of condensation?
- Non-histone chromosomal proteins form a scaffold that is involved in condensing the solenoid into looped domains, forming the 300-nm fibre.
- In mitotic and meiotic chromosomes, the looped domains themselves coil and fold, further compacting all the chromatin to produce the characteristic metaphase chromosome.
- Width of one chromatid: 700nm - Particular genes always end up located at the same places, indicating that the packing steps are highly specific and precise.
What are the roles of condensation?
- To organise and pack the giant DNA molecules into a more compact structure
[Packing into chromatin]
- To fit within the cell’s nucleus
- To prevent the long DNA molecules from getting tangled
[Packing into chromosome during cell division]
- To facilitate their segregation into daughter nuclei
- DNA molecules of different chromosome will not be entangled and break during separation at anaphase.
How many percent of the DNA sequences in eukaryotic genome are coding regions?
3% (Code for proteins/RNA)
What are coding sequences?
[Exons] DNA sequences that carry information correspond to specfic genes (e.g. protein-coding/RNA-coding genes), that is ultimately translated into a functional polypeptide
How are heavily used gene products encoded
By multiple copies of genes arranged in repeats, either interspersed or in tandem
What are examples of non-coding sequences?
Introns 5' & 3' Untranslated regions (UTRs) Promoters Enhancer and Silencer Replication Origin and Termination Centromeres and Telomeres
What are tandemly repetitive DNA?
DNA sequences in which multiple copies are arranged next to each other
Transcriptionally inactive and may play a structural role in the chromosome
E.g. Centromeres and Telomeres
What are moderately repetitive DNA?
DNA sequences that are scattered about the genome
E.g. Promoters, Enhancers, Origins of Replication
Structure of Centromere
Repetitive DNA sequence, which is an A-T rich region
Function of Centromere
Binds several proteins with high affinity to form the kinetochore, which provides the anchor for the spindle fibres
Essential structural for chromosome segregation during cell division
Structure of Telomere
Repetitive DNA sequence, which are typically short, repeated T-G rich sequences
Function of Telomere
- To prevent fusion of the ends of the chromosomes with other DNA molecules
- To prevent exonucleases from degrading the ends of the linear DNA molecules
- To prevent the loss of genetic information by acting as a disposable buffer
- To facilitate replication of the ends of the linear DNA molecules, without loss of the termini
- To prevent cell cycle arrest/cell death
Why do telomeres shorten?
- Since DNA polymerases assemble DNA only in the 5’ to 3’ direction and need a primer, they cannot complete DNA replication to the 5’ ends of both daughter DNA strands.
- After the removal of the primer, DNA polymerase cannot fill the gap because there is no 3’ -OH group present as a starting point.
- After many rounds of cell division and hence replication, chromosomes would become progressively shorter because of the gap left after removal of the primer on the 5’ end of the lagging strand.
Roles of Telomere
In germ cells, the end replication problem is overcome by the presence of the telomere, a terminal sequence consiting of repeated DNA sequences. These repeats are added on to the ends of chromosomes by telomerase. The template for telomerase is provided by an RNA molecule within the RNA complex.
