A-level Biology - 3.1.8 Protein Synthesis
mRNA is made during transcription, the first stage of protein synthesis. In this process, a complementary mRNA strand is formed using a DNA template, allowing genetic information to be carried from the nucleus to the ribosome for translation.
When is mRNA made?
During transcription
Key Terms
When is mRNA made?
During transcription
Describe the structure of mRNA
It’s a single polynucleotide strand (& has groups of 3 adjacent bases)
What are 3 adjacent bases called?
Codons (or sometimes called triplets or base triplets)
What does mRNA do?
Carries genetic code from DNA to ribosomes & is then used to make a protein during translation
What does tRNA do?
Carries amino acids that are used to make proteins to ribosomes
(involved in translation)
Describe the structure of tRNA
It’s a single polynucleotide strand that’s folded into clover shape
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| Term | Definition |
|---|---|
When is mRNA made? | During transcription |
Describe the structure of mRNA | It’s a single polynucleotide strand (& has groups of 3 adjacent bases) |
What are 3 adjacent bases called? | Codons (or sometimes called triplets or base triplets) |
What does mRNA do? | Carries genetic code from DNA to ribosomes & is then used to make a protein during translation |
What does tRNA do? | Carries amino acids that are used to make proteins to ribosomes (involved in translation) |
Describe the structure of tRNA | It’s a single polynucleotide strand that’s folded into clover shape |
How does tRNA stay in a clover shape? | H-bonds between specific base pairs hold it in shape |
What does every tRNA molecule have? (2x) | Anticodon at one end - specific sequence of 3 bases An amino acid binding site at other end |
What is the main thing that happens in transcription? | mRNA copy of gene is made from DNA |
In eukaryotic cells, where does transcription takes place? | Nucleus |
Describe the stages in transcription | DNA helicase (in eukaryotes) breaks H-bonds between 2 DNA strands (in beginning of a gene) Only one DNA strand acts as a template (to make mRNA copy) Free bases in RNA nucleotides are attracted to exposed bases Attraction occurs according to complementary base pairing (∴ mRNA strand becomes a complementary copy of DNA template strand) RNA nucleotides are joined together by RNA polymerase (In eukaryotes) mRNA moves out through nuclear pore |
Where does transcription take place in prokaryotes? | Cytoplasm |
When does the RNA polymerase stop making mRNA and detachs from DNA in transcription? | When RNA polymerase reaches a particular sequence of DNA called stop signal |
Where does translation occur in both eukaryotes and prokaryotes? | At ribosomes in cytoplasm |
What is the main thing occuring during translation? | Amino acids are joined together to make polypeptide chain, following the sequence of codons (triplets) carried by mRNA |
Describe the stages in translation | mRNA moves into cytoplasm & attaches to ribosome tRNA carry amino acids to it Specific tRNA molecule for specific amino acid Anitcodon of tRNA binds to complementary codon on mRNA (attaches by specific base pairing) Peptide bond forms between amino acids tRNA detaches and collects another amino acid Ribosome moves along mRNA = forming a long polypeptide chain |
What determines how the folded protein structure of protein will be? | Position of R groups in polypeptide chain |
What issue can happen as polypeptide chains are formed on a ribosome? | Regions where amino acids have hydrophobic R groups can to clump together or join with other hydrophobic molecules spontaneously Leads to forming a non-functional, ‘mis-folded’ protein & many of them = disease |
What do cells produce to ensure the polypeptide chains made on ribosomes are folded correctly? | Chaperone proteins |
Describe the role of chaperone proteins (3) | Chaperone proteins bind to hydrophobic groups (on the secretory proteins) Prevent incorrect hydrophobic interactions Allows protein to be correctly folded |
Where are chaperone proteins are found? | In endoplasmic reticulum |
What is the genetic code? | Sequence of base triplets (codons) in mRNA which code for specific amino acids |
Name 3 qualities of the genetic code | Non-overlapping Degenerate Universal |
Describe how genetic code is non-overlapping | Each base triplet is read in sequence only once |
Describe how genetic code is degenerate | Some amino acids are coded by more than one base triplet e.g. tyrosine can be coded for by UAU or UAC |
Describe how genetic code is universal | Same specific base triplets code for same amino acids in all living things e.g. UAU codes for tyrosine in all organisms |
What are start/stop signals (or codons)? | Triplets that tell the cell when to start/stop production of a protein |
Transcription Makes _ Products in Eukaryotes and Prokaryotes | Different |
What does transcription make in eukaryotes? | pre-mRNA |
What is pre-mRNA? | mRNA strands containing introns and exons |
Why does pre-mRNA contain introns and exons? | Introns and exons are both copied into mRNA during transcription |
Name and describe the process how pre-mRNA is made into mRNA | Process called splicing occurs: introns are removed and exons join together = forming mRNA strands |
Where does splicing occur? | In nucleus |
What does transcription make in prokaryotes? | mRNA - it's produced directly from DNA without splicing |
Transcription What enzyme separates the strands of DNA in prokaryotes? | RNA polymerase |
Explain why the percentage of bases from the middle part of the chromosome and end part are different (2) | Different genes Have different base sequences Codes for different proteins |
Different sequences of bases code for different proteins. Explain how. (2) | Protein made up of (chain of) amino acids Each amino acid has its own base code |
Explain how copying bases more than once may give rise to differences in the protein (2) | Changes base sequence of later triplets/amino acid codes |