When is mRNA made?
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?
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?
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?
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
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
What does transcription make in eukaryotes?
What is pre-mRNA?
mRNA strands containing introns and exons