TOPIC E: ORGANISATION OF EU. GENOME Flashcards
What is the structure of an Eukaryotic Chromosome?
The nucleosome is the basic level of the eukaryotic chromosome organisation. Double stranded DNA wrapped around a core of histone proteins. High levels of postively charged lysine and arginine residues, which associate closely with negatively-charged sugar phosphate backbone of DNA. Nucleosomes are joined by linker DNA, giving rise to beads-on-string structure.
Further folding and coiling of nucleosomes and linker DNA results in the solenoid structure which appears as 30nm chromatin fibre.
Looped Domains are approx 300nm thick and are formed by folding of the 30nm fibres into loops which are attached to non-histone proteins/scaffolding proteins.
What does the core promoter region do?
It includes the TATA box and the transcription start site. They are found immediately upstream of coding sequences. TATA binding protein recognises and binds to the TATA box and recruits other transcription factors and RNA pol. to initiate transcription by facilitating binding of RNA polymerase. It results in a basal rate of transcription.
On eukaryotes
What are enhancers?
Enhancers are control elements on DNA which activators bind to.
Binding of activators to enhancers increase interxn between promoter and RNA pol, hence increasing the rate of transcription.
It occurs by looping the DNA, which further stabilises the Transcription initiation complex through protein protein interaction.
What do silencers do?
Silencers are control elements on DNA which repressors bind to. Repressors are proteins with DNA binding domains.
However, when repressors are bound to silencers, rate of transcription is decreased.
What is the structure of the centromere?
Centromere is DNA in nature which consists of highly repetitive DNA sequences. Centromere is a constricted region of mitotic chromosome which holds 2 sister chromatids tgt. Wrapped around the centromere is the kinetochore, which is made up of proteins and the interface between the microtubules of spindle fibre and DNA of the centromere. In higher organisms, kinetochore contains proteins and some RNA in a trilaminar structure.
What is the structure of the telomere?
Telomere is DNA in nature and does not contain genes but consists of multiple repetitions of one short nucleotide sequence.
3’ single stranded overhang folds back and forms a T-loop, hiding the single stranded DNA overhang. Hybridises with an earlier repeat of the same complementary seq in the opp. strand.
Telomere capping proteins bind at the T-loop junc to maintain stability of this structure prevents exonuclease degradation
What is the function of telomere?
Prevents the end of chromosomes from being degraded by exonucleases. Due to T loop.
Protects important genes near telomeres. Telomeres do not prevent shortening of DNA molecules due to successive rounds of rep, but delays gene degradation.
Telomeres prevent ends of different chromosomes from accidentally fusing with each other.
Telomeres provide a counting mechanism for the number of times cell division a cell has undergone.
What is histone modification?
Histone acetylation involves the covalent addition of acetyl groups to +vely charged lysine residues in the tails of histone proteins. It does not bind to negatively charged DNA.
Reduces interaction between histones and DNA, making it less compact and more accessible to transcription machinery for transcription initiation.
Histone acetyltransferase catalyse addition of acetyl groups and promote transcription.
Histone deacetylases catalyse removal of an acetyl group and inhibit transcription.
What is DNA modification?
Addition of methyl groups on certain bases of DNA is a common method of gene silencing.
Transfer of a methyl group to C5 of cytosine is catalysed by DNA methytransferases.
Methylated DNA attracts other proteins which and in turn recruits histone deacetylation enzymes.
Makes DNA more compact and inhibits transcription.
What is RNA processing?
What is alternative splicing?
RNA splicing variation mechanism in which the exons of the pre-mRNA are separated and ligated so as to produce alternative mature mRNA arrangements.
Many different mRNA molqs and polypeptides can be made with a single gene. It increases the number of proteins without increasing the number the genes in a genome. It regulates differential gene expression such as tissue specific alternative splicing.
What is translation initiation?
Regulation of gene expression at a translational stage occurs at translation initiation. It reqs translation initiation factors, Phosphorylation and dephosphorylation can activate or inactivate translation initiation factors needed to initiate ribosomal binding.
How is the stability of mRNA modified?
The more stable an mRNA molecule is, the longer it is translated and more polypeptides are produced. It depends on the length of the poly-A-tail, subseq removal of 5’ cap followed by rapid degradation by nucleases.
What is cytoplasmic polyadenylation?
Some mRNA are synthesised to deliberately contain a short poly-A-tail.
These mRNA molecules accumulate in the cytoplasm of unfertilised oocytes but are untranslated because translation cannot be initiated due to short poly-A-tails.
Only at specific times the oocyte mutation and post-fertilisation, cytoplasmic polymerase catalyses the cytoplasmic polyadenylation of the 3’ end.
It allows temporal control of when exactly a polypeptide to be translated to be regulated.
What is localisation of mRNA?
RNA localisation has been observed in many organisms and this likely is a common mechanism for cells to concerntrate high mRNA levels to ensure high levels of polypeptide to be synthesised at a specific location. (Spatial control)
