Organisation of Prok Euk Flashcards
Role of telomeres
Non-coding, tandem repeat sequences of DNA on both ends of linear chromosomes
1) Each round of DNA replication will result in the shortening of daughter molecules at the telomeres because DNA polymerase is unable to replace the RNA primers with DNA at the ends of the strand. Hence telomeres prevent erosion of vital genetic information after each round of DNA replication
2) Protect and stabilise the ends of chromosomes by forming a loop with 3’overhang. This prevents ends of chromosomes from fusing with other chromosomes and prevents DNA repair machinery from recognising it as DNA breaks, hence preventing cell cycle arrest and apoptosis
3) The 3’overhang of telomeres allow their own extension by providing an attachment point for correct positioning of enzyme telomerase in certain cells, e.g. germ cells
Telomeres, telomerase and telomerase reverse transcriptase
Telomerase is an enzyme with an active site complementary in shape and charge to specific telomeric DNA sequence.
Nucleotides of telomerase RNA anneal and forms complementary base pairing with the single-stranded overhang at 3’end of the telomere.
Using telomerase RNA as a template, TRT forms complementary DNA sequence through CBP. Catalyses formation of phosphodiester bonds between deoxyribonucleotides and translocates in 5’ to 3’ direction to produce series of tandem repeats, elongating telomere.
Germ cells (gametes) (vs somatic cells)
Germ cells undergo continuous cell division while somatic cells only divide a limited number of times.
Somatic cells undergo apoptosis while germ cells need to pass on intact genomes to its daughter cells over many generations.
Germ cells require telomerase to catalyse regeneration of telomeres so they can undergo many rounds of DNA replication without loss of vital genetic information through erosion and chromosome ends
Prok vs Euk genome
Multiple, linear molecules vs single, circular molecule
Nucleus vs Nucleoid region
Large amounts of histones and scaffold proteins vs few histone-like proteins
Introns and no introns
Multiple ori, single ori
Many genes, fewer genes
Both: Double-helix DNA
Genome packaging
Negatively-charged DNA is wound around positively-charged histone proteins by electrostatic interactions to form nucleosomes. Linker DNA joins adjacent nucleosomes and fibre coils around itself to form a solenoid. Solenoid associates with scaffold proteins and forms looped domains and then undergoes supercoiling to form a chromosome.
Why package?
To make long DNA molecule more compact to fit inside the nucleus.
To prevent entanglement which may cause DNA breaks and damage
To regulate transcription when DNA wound around histones and prevent access of RNA polymerase to genes not meant to be expressed
Bind to DNA?
DNA-binding site complementary in shape and charge which comprises a specific sequence of nucleotide bases with complementary base pair with those on the DNA strand
Genomic control
DNA methyltransferase:
Addition of methyl groups to selected cytosine nucleotides on DNA in CG sequence.
Proteins bind to methylated histones, causing chromatin to be further compacted to form tighter nucleosomes, preventing binding of general transcription factors and RNA polymerase to promoter to assemble transcription initiation complex, preventing transcription.
Why methylate large areas of differentiated cells?
Genome is of large size, consisting of both non-coding and coding regions of DNA. A large percentage of the DNA is non-coding so histone methylation occurs on these regions.
Most of the chromatin comprises many genes not expressed in differentiated cells so condensation of DNA in these regions prevents expression of these genes.
Histone acetyltransferase:
Addition of acetyl groups to lysine residues removes positive charge on histones, decreasing electrostatic interactions between DNA and histones causing chromatin to decondense and allow access.
Transcriptional
Proximal control
Promoter:
Serve as recognition site for binding of RNA polymerase and GTFs. Contains critical elements TATA box that determines precise starting location of transcription, and CAAT and GC boxes which help to recruit RNApol and GTFs to promoter
Distal control
Enhancers:
When bound with specific transcription factors known as activators, increase rate of transcription by bending spacer DNA to allow easier access of RNA polymerase and GTFs AT THE PROMOTER to assemble transcription initiation complex. (Recruits HAT, CRCs to decondense chrtin)
Silencers:
When bound with STFs known as repressors, decrease rate of transcription by condensing chromatin to block access of R and G from assembling TIC at promoter. Recruit HDAT, repressive CRCs
Post-transcriptional
1) Anti-sense RNA that binds to mRNA transcripts to prevent translation of the proteins
Alternative splicing of pre-mRNA involves excising introns and joining some exons where introns are non-coding regions and exons code for the sequence of amino acids in a protein.
This gives rise to different combinations of exons, so one gene produces mature mRNA with diff combis of exons, giving different proteins.
snRNA: sequence of nucleotides targets intron-exon boundaries
Post-translational
Proteins selected for degradation are tagged by ubiquitin molecules. Target proteins enter the proteasome where enzymes hydrolyse peptide bonds of the protein to produce small peptides which are released into the cytosol where they are further hydrolysed into amino acids by enzymes. Ubiquitin molecules are released and reused.
Molecular inhibitors that bind directly to the proteins and prevent their activity
Gain-in-function vs Loss
Gain-in-function mutation results in a gene product that is excessively produced or hyperactive while loss-in-function mutations results in gene product that is not produced or defective.
GIF only requires a single mutation to take effect but LIF requires both alleles to be mutated to take effect.
GIF mutation is usually dominant, while LIF is usually recessive.
Cancer triggers
Exposure to ultraviolet light, ionising radiation, carcinogens such as tar, viruses such as HPV
Increase chances of DNA damage and mutations
1) Loss-in-function mutation of tumour suppressor genes, resulting in inability to inhibit cell cycle, repair damaged DNA and trigger apoptosis.
2) Gain-in-function mutation of proto-oncogenes to become oncogenes, resulting in overexpression of growth factors or production of hyperactive/degradation-resistant growth factors which promote progression of cell cycle.
Disruption of these regulatory genes leads to uncontrolled cell division to form a tumour.
3) Loss of contact inhibition allows cells to grow into a tumour
4) Activation of genes coding for telomerase allows tumour cells to divide indefinitely
5) Angiogenesis occurs and blood vessels transport oxygen and nutrients to the cells allowing their growth
6) Resulting in formation of malignant tumour capable of metastasing to other parts of the body
Cancer is a multi-step process
The development of cancer requires an accumulation of mutations in a single cell.
Examples: 1) ras proto-oncogene and 2) p53 tumour suppressor
3) Chromosomal aberration and other events eventually lead to metastasis.
