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Explain why regeneration of telomeres is not normally required in human somatic cells.
Human somatic cells divide only a limited number of times (large numbers of cells present), after which the cell undergoes apoptosis / programmed cell death when the telomeres shorten to a critical length.
Describe how DNA can be organised to form chromosomes.
- -DNA is bonded to histones via ionic attraction to form a nucleosome.
- -The nucleosome is coils around itself to form a 30nm chromatin fiber/solenoid
- -Looped domains are coupled with scaffold proteins to form chromatin
- - Chromatin condenses to form chromosomes
Telomeres are repetitive sequences found at the ends of eukaryotic chromosomes but not in prokaryotic genome.
Suggest a reason why prokaryotic genomes have no need for telomeres.
Prokaryotes have circular DNA which does not have ends so there is no degradation of chromosome ends at each replication of DNA.
The TATA-binding protein binds to the TATA box found in the promoters of eukaryotic genes.
Suggest how this protein associates with DNA.
- It fits into the major grooves of the DNA where it binds via
- 1) ionic interactions between the negatively charged DNA and the positively-charged R groups of amino acids of proteins
- 2) hydrophobic interactions between hydrophobic R-groups and bases
Explain the modifications done to the DNA structure before transcription can take place.
The DNA molecule has to be acetylated so as to make heterochromatin less compact and loosens up the DNA structure so that DNA polymerase can gain access to the promoter.
Explain why changes to the organization of DNA are required before transcription can take place.
- -This is due to the tight packing of DNA where RNA polymerase is unable to access the promoter for transcription.
- -Thus chromatin modification such as histone acetylation and chromatin remodelling are required to unpack the DNA.
Explain the end-replication problem.
- -During DNA replication, RNA primers are added to the parental strand for the elongation process
- - When the last primers are removed, DNA polymerase cannot fill the resulting gap as there is no free 3' OH group available to add nucleotides to
- - Thus, each rounds of replication generates shorter DNA molecules
Explain how activators control the rate of gene expression.
- -Activators are specific transcription factors that bind to enhancers
- -This causes the bending of the DNA
- -so as to contact and position the proteins of the transcription initiation complex at the promoter
- -thus increasing the rate of gene transcription
Explain the process of RNA splicing.
- RNA splicing is a post-transcriptional processing where
- - small nuclear ribonucleoproteins (snRNPs) and other proteins form a molecular complex called spliceosome
- -which interacts with the splice sites at the ends of the intron
- - thus excising out the introns and joining the two ends of the exons together
Protein degradation is an example of a post-translational processing. Explain this process.
- Proteins intended for degradation are marked by the attachment of ubiquitin proteins.
- Proteosomes will recognise the ubiquitin and thus degrade the tagged protein.
Explain the role of two distal control elements that are involved in influencing eukaryotic transcription.
1) Enhancers -specific sequences of DNA that are a few thousands of nucleotides away from the promoter where activators bind to to increase the rate of transcription
2) Silencers- specific sequences of DNA that are a few thousands of nucleotides away from the promoter where repressor proteins bind to to inhibit transcription.
Explain how the Taq polymerase makes Polymerase Chain reaction(PCR) possible to carry out.
- -PCR is made up of 3 stages: heating, cooling and replications
- -Taq polymerase comes form a hot-spring bacteria and hence is able to withstand high temperatures needed to denature the DNA strand without itself being denatured.
Explain the following terms:
i) Multigene family
ii) Interspersed repetitive DNA
i) Multigene family is a collection of identical genes which are clustered in tandem or genes which are non-identical but similar in nucleotide sequences.
ii) Interspersed repetitive DNA are repetitive DNA sequences appearing at multiple sites in the genome.
State the two functions of the 5' cap at the 5' end of the mRNA strand.
- 1) The 5' cap protects the mRNA from degradation by hydrolytic enzymes.
- 2) It serves as an attachment site for ribosomes.
Describe how translation of mRNA can be controlled.
1) Translation of specific mRNAs can be blocked by regulatory proteins which bind to specific sequences within the 5' leader region which prevents the attachment of ribosomes.
2) Ribosomal translational initiation factors needs to be phosphorylated in order to be activated so that translation can take place.