RNA synthesis Flashcards
describe the centromere and telomere
Centromere (primary constriction) keeps chromosomes attached to the mitotic spindle so each daughter cell has a copy of the chromosome.
- -Telomere at the double stranded end of chromosomes is formed from repetitive DNA and is part of the repair process. Protects genes at the ends of chromosomes from being lost so that they are transcribed (during condensation/transport). Also prevents chromosomes from joining.
what is the majority of dna made up of ?
intergenic regions -98%
describe the structure of a gene encoding for a protein
-DNA is composed of genes than can code for proteins or non-coding genes. An
example of non-coding genes would be microRNAs.
-A gene has 5’UTR ( part of the first exon) , the 3’ UTR (part of the last exon) and the coding region that contains
exons and introns.
-exons are transcribed and translated and introns are transcribed but not translated.
-Promotor region is upstream of the 5’utr and is within 300bp upstream of the start site of the genes transcription start zone. it ensures rna polymerase binds and determines whether a gene is turned on or off. it is important for the regulation of gene expression and contains specific motifs for transcription that differ in eukaryotes and prokaryotes.
- TATA box in the promoter is recognised by tbp and can be bound by general transcription factors.
-promoter also contains other upstream regions such as Gata, caat box and gc rich box. These are bound by specific transcription factors.
- enhancers are also present that can be upstream of the
gene, downstream of it or with exons and introns. These enhancers facilitate the
binding of transcription factors to regulate transcription.
- Genes differ in size ,differ in number of exons and introns, cluster into families. Some genes can have shared regulation of expression. Sometimes they can evolve from another which is why they can be located next to each other.
what is the function of 5’utr
regulates translation
what is the function of 3’utr
maintains mrna stability and miRNA binding
what is translation
Translation is the conversion of the messager RNA into amino acids forming an elongating polypeptide and ultimately a protein.Translation occurs via the binding of the mRNA to the translational machinery, the comprising of the ribosomes, after it is transcribed and exported from the nucleus to the cytoplasm of the cell.
give 4 key points about rna polymerase II
- Rna polymerase II function is to cayalyse the synthesis of the phosphodiester backbone of RNA.
- The ribonucleotides are being added according to the Watson-crick base pairings, but instead of T there is Uridine-triphosphate in the resulting single stranded RNA
- It incorporates the triphosphate nucleotides one by one in the 5’ to 3’ direction.
- It unwinds small portion of DN A
give the 3 types of rna polymerases and the gene transcribed
rna polymerase i - rRNA
rna polymerase ii - protein coding, microRNA, non-coding RNA
rna polymerase iii - tRNA, 5s rRNAs, other small RNAs
why do dozens of polymerases work on the same gene at the same time ?
to make many transcripts from a gene simultaneously. this allows the cell to respond quickly to the enviroment.
what are the requirments for rna synthesis ?
- Dna template
- Rna polymerase II
- Ribonucleotides
- Buffer
- Transcription factors
what are transcription factors
- Proteins required to initiate or regulate transcription in eukaryotes.
- Assemble on the promotor to position and help RNA pol II to inititate transcription.
- Pull apart DNA helix and expose template strands
describe the initiatiation or RNA synthesis
- B,D,E,F,G – these transcription factors form the large pre initiaton complex and is required to initiate transcription.
- Part of initiation is the transition from a closed promoter complex, where the dna is still double stranded, to an open promoter complex where approx. 15 base pairs of the promoter is unwind and forms a transcription bubble.
- Following this, the transcription start sites are selected and RNA synthesis begins.
describe the steps of transcription
A. TATA box recognised by TATA-binding protein (TBP) subunit of TFIID.
B. TFIIA and TFIIB bind; TFIIA stabilises the complex.
C. Other general transcription factors
(E &H) bind, then RNA polymerase II assembles at the promoter, forming the transcription initiation complex.
D. TFIIH pulls apart the DNA helix and phosphorylates RNA Pol II.
E. Phosphorylated RNA Pol II is released from the complex and begins transcription in a 5’ to 3; direction.
F. The binding of the RNA polymerase to the DNA is asymmetrical and as a consequence, elongation is unidirectional
what is meant by gene-specific regulation
- Not all genes in our cells are active, 20000 genes , only a subset of those are being transcribed at any one time.
- General transcription factors and RNA polymerase II are not sufficient on their own for a gene to be transcribed
- This is because they are found in all cells, but not all cells transcribe all of their genes all the time
- Therefore additional upstream sequences are needed for gene-specific regulation of transcription.
e. g proximal control elements , distant enhancer , tata box
describe rna processing.
Capping – created by capping enzyme complex.its recruited by the phosphorated form of RNA polymerase II. The cap acts as a marker for RNA polymerase II TRSNCRIBED RNA, IT CAN STIMULATE SPLICING AND IS USED for the recognition OF MRNA BY the protein translation machinery. Co-transcriptional modification ( capping reaction is happening whilst mrna is still being transcribed) . The 5′ cap protects the nascent mRNA from degradation and assists in ribosome binding during translation.
- Occurs at the 5’end
- Guanine nucleotide added and 5’ to 5; triphosphate bridge is created
- Methylated at the 7th position
Polyadenylation. - Cleavage by specific endonuclease at the cleavage signal.
- Addition of poly(A) tail by poly (A) polymerase, using ATP. It adds around 200 As to the end of the RNA chain.
- Forms polyadenylated mRNA precursor.
Splicing- removal of introns and joining of exons to make the mature mRNA. Spliceosome complex of snRNAs and 80 proteins remove non-coding sequences within genes
1) The GU is recognsied by an RNA in the splicesome , and the pyrimidine rich region also gets binding of protiens
2) The proteins undergo conformation changes resulting in Cleavage at 5’ splice site GU by splicosomes.
3) The ends join up with the A site to forma lariat-like intermediate.
4) Cleavage at 3’ splice site AG
5) Ligation of exons
6) Degradation and recycling of untranslated regions by mRNAses.
