RNA Synthesis Flashcards
What is a gene?
A unit of hereditary that contains the instructions to make an organisms phenotypes. They can differ in the number of exons and introns.
*similar genes can cluster into different families
What are the differences between RNA and DNA?
- DNA is much more stable than RNA due to:
- In DNA there is a deoxyribose sugar whereas RNA has only a ribose sugar.
- In RNA, thymine is replaced by Uracil
- RNA is single stranded whereas DNA is a double stranded
- RNA is used as a temporary copy of storing genetic information whereas DNA is used as a permanent store.
- RNA can act as a temporary messenger allowing the cell to regulate gene expression in a short term manner.
What are the differences between the types of RNA present in eukaryotic cells?
- mRNA = transmits information from the genes to ribosomes where it’s translated into protein.
- tRNA = ensures the correct amino acids matches the codon specified by mRNA
- rRNA = forms a part of the ribosomal structure
- small nuclear RNA (snRNA) = helps align introns to be spliced out of the primary structure.
What is transcription?
Transcription is the synthesis of a RNA molecule which is complementary to the antisense strand in DNA.
Enzymes are need to synthesise (RNA polymerase) and also use nucleotide triphosphates to synthesise RNA.
What are the different types of RNA polymerases?
- RNA polymerase I = transcribes tRNA genes
- RNA polymerase II = transcribes mRNA genes
- RNA polymerase III = transcribes tRNA and other smaller RNAs
- these are complex enzymes and contain many subunits
What are the steps for translation?
1) At the 5’ end, RNA polymerase is recruited by the promoter region (BATATA box)
2) A local region of DNA will be unwound, so there is always an open strand for RNA polymerase to copy
3) RNA synthesis occurs by RNA polymerase in the 5’-3’ direction
4) Elongation occurs afterwards with RNA polymerase moving along the DNA, synthesis RNA, a strand coming out from the end.
5) Termination occurs when it reaches the end and the RNA dissociates- the primary transcript is created.
What does capping mean?
Capping is a modification that occurs after the transcript is made and is when a guanosine nucleotide is added to the 5’ end of the mRNA transcript.
It is used to indicate the mRNA is ready to be transported from the nucleus.
It is also used to protect the mRNA end from phosphates and other small molecules.
*capping is processed by capping enzymes- guanyl transferase.
What does polyadenylation mean?
Polyadenylation is a post-transcriptional modification where at the 3’ end a poly-A-tail is added (a long chain of adenine nucleotides that can be 100-250 residues long).
The tail is used to increase stability of the mRNA molecule and prevents degradation ( the mRNA is able to stay within the cytoplasm for a longer time).
It also indicates that the mRNA molecule is ready to leave the nucleus.
How does the poly-A-tail shorten?
Certain enzymes found in the cytosol such as poly(A) nuclease can get rid of parts and shorten the poly(A) tail.
However if it gets too short, the mRNA cap will get removed and it will eventually become degraded and so it will not transcribed.
What is splicing?
Splicing is a post-transcriptional modification where introns are removed.
There are protein complexes that recognise DNA sequences at the ends of the exons, snRNPs, which bind to these regions on either side of the intron, forming a spliceosome, removing the intron and leave the exons side by side.
What can happen to a mutate pre-mRNA during splicing?
If a primary transcript is mutated, it may be spliced in the wrong way, leading to an improperly functioning protein.
How much of the human genome encodes proteins?
Only 1% is transcribed
What is tissue-specific gene expression?
Human cells all contain the same genes, the type of gene expressed will depend on the type of cell.
There are some genes that code for proteins needed by all cells so are therefore expressed.
Some genes are only inducible so will have to be activated when needed.
Some genes are only tissue specific e.g. alpha and beta globin genes are only expressed in red cells precursors.
What is alternative splicing?
It is when exons are also removed to give different combinations of sequences and therefore, more than one protein can be synthesised from one single gene.
Why are transcriptional factors needed?
Due to RNA polymerase not being able to bind properly to the DNA alone, so uses transcription factors to help aid it.
