Protein synthesis Flashcards
Whats the Central dogma?
How can information be passed on? and also not passed on?
- Use to describe flow of genetic information
- Not just a one-way system
- Special information transfer can also go back
- RNA can replicate making use of RNA polymerase enzyme
- Can occur in eukaryotes but also going backwards is common in viruses
*** Nucleic acids can pass on information to other nucleic acids and proteins - Transfer of information from protein to protein or protein to nucelic acid is not possible **
What type of functions does protein synthesis help with?
o Structure
o Transport
o Enzymes (catalyse reactions)
o Cell signalling
What are proteins consisted of?
- Proteins consist of one or more polypeptide chains and are coded for by gene (portion of DNA)
o DNA is therefore critical for cell function, and must be protected- (kept in nucleus in eukaryotes)
What is used as intermediates durind the process of protein synthesis?
RNA molecules
What are the stages of protein synthesis?
o DNA transcription
o mRNA translation
What is transcription?
Transcription (DNA –> mRNA)
- Transcription is the process by which the genetic code within genes is transcribed from DNA to mRNA
- Occurs in the nucleus of eukaryotes and the cytoplasm of prokaryotes
- Bacterial mRNA is polycistronic (can code for many genes) whereas eukaryotic mRNA is monocistronic- each mRNA codes for only one gene
- Which genes are “read” and therefore expressed is controlled by** gene regulation **
What are the stages of transcription?
o Initiation
o Elongation
o Termination
Why is it important to regulate which genes are expressed?
o To ensure that cell-type specific proteins are only synthesised where they need to be
o To ensure that proteins aren’t produced wastefully when not needed
Expression of genes can be controlled with regulatory proteins. name some?
o Positive transcription factors
o Negative transcription factors
What does the RNA polymerase need to bind to?
A promotor(primer equivalent)
Tell me about the E.coli- Lac Operon?
o Has an operator next to the promotor to which a repressor can bind- stops mRNA synthesis
o Operon is a group of genes that are normally linked in function and controlled by a single promotor
o If lactose is present, it binds to the repressor and stops it binding to operator- allowing for the transcription of the lactase gene
What are the stages of initiation of transcription?
- The DNA double helix must unwind to allow for mRNA synthesis
o This region is known as transcription bubble - Transcription factors bind to a promotor sequence
o Usually upstream of the gene it is regulating
o Transcription factors will then recruit RNA polymerase - The sequence of the promotor will determine if the corresponding gene is transcribed; all the time, some of the time, or hardly at all
What are the stages of elongation of transcription?
- Transcription always proceeds from the** template strand** of DNA
- mRNA product is complementary to the template strand
o uracil’s complement adenines - RNA polymerase adds nucleotides in the 5’ to 3’ direction
- RNA strand does not remain bound to the DNA template
- DNA is continuously unwound (and dissociated from histones) ahead of the core enzyme and then rewound after it is read
What are the stages of termination of transcription?
- Once the gene has been transcribed, the polymerase must be instructed to dissociate from the DNA and liberate the newly formed mRNA
- The termination signal depends on the types of RNA polymerase
o RNA polymerase II elongates 1000-2000 ntds beyond end of gene (pre-mRNA tail) which is later cleaved
o RNA polymerase I and III recognise a termination sequence in the DNA after the gene - In eukaryotes, the mRNA transcript is produced in the nucleus and must be transferred to the cytoplasm for translation into proteins
- Therefore, the mRNA transcripts must undergo modifications to prevent degradation:
o A special nucleotide ”cap” is added to the 5’ end of the growing transcript- aid in the initiation of translation by ribosomes
o An enzyme adds a strong of roughly 200 adenine residues (poly-A tail) to the e’ end of the mRNA after elongation- signals that the transcript needs to be exported to the cytoplasm
Tell me about RNA splicing
Eukaryotic genes are composed of:
o** Exons**- expressed protein-coding sequences
o Introns- intervening sequences
- Introns must be removed before translation to ensure the mRNA codes for the correct AA
- The removal of introns and re-joining of exons is known as splicing
- If this does not happen or there are errors, then the resulting proteins would be non-functional
- Finished mRNA can then be exported from the nucleus to the cytoplasm
What is translation?
**Translation (mRNA –> Protein) **
- Translation is the process by which the newly transcribed mRNA is “translated” into the corresponding specific sequence of AA to form a polypeptide chain
- During transcription one DNA nucleotide corresponded to one mRNA nucleotide
- In translation, every three mRNA nucleotides (called a codon) translated to one AA
o This translation is enabled by transfer RNA molecules (tRNA)
Where does translation takeplace and what are the stages?
- Translation takes place in the ribosomes (made of ribosomal RNA and polypeptides) and occurs in 3 stages:
**o Initiation
o Elongation
o Termination **
Tell me about transfer RNA (tRNA)
Translation- transfer RNA (tRNA)
* tRNAs “translate” RNA language to protein language
How many amino acids does a triplet codon code?
Each triplet codon codes 1 AA
What sites do tRNA molecules have?
tRNA molecules have an anti-codon sequence and an **amino acid attachment site **
Are some amino acids coded by multiple codons?
Yes
Give the start and stop codons?
- AUG is a start codon- codes for **methionine **
- There are **three stop **codons: **UAA, UAG and UGA **
What are the stages of translation?
- Initiation complex of proteins recognises 5’ cap of mRNA and tracks along to find AUG start codon
- mRNA template provides binding specifically for tRNA molecules carrying AA
- mRNA is read in 5’ to 3’ direction and the polypeptide chain is formed
- termination of translation occurs at a stop (or nonsense) codon and complex dissociates
Summary
- Genes in the DNA are transcribed to mRNA by RNA polymerase in the nucleus
o mRNA is modified with a 5’ cap and poly-A tail to prevent degradation - introns are spliced out of pre-mRNA leaving only the exons
- transcripts are transported to ribosomes in the cytoplasm where they are “translated” into polypeptide chains
o tRNA molecules with AA attached have complimentary anti-codons to mRNA codons
