3.10 Protein synthesis Flashcards
Describe where DNA is in a eukaryotic cell?
Contained within nuclear envelope (double membrane) that encloses the nucleus
What does the nuclear envelope protect DNA from?
Damage in the cytoplasm
What problem occurs with protein synthesis at normal ribosomes?
Chromosomal DNA molecule is too large to fit through the nucleus to provide the coding info needed to determine the protein’s amino acid sequence?
How can the problem of chromosomal DNA being too large to fit through a nucleus be solved?
What is this process called and what does it produce?
The base sequence of genes has to be copied and transported to a ribosome (site of protein synthesis)
Transcription, produces shorter RNA molecules
How is transcription similar to DNA replication?
Section of DNA containing gene unwinds + unzips under control of DNA helicase, beginning at a start codon.
(This involves breaking of H bonds between bases.)
What is the sense strand?
The one of two strands which contains the code for the protein to be synthesised.
It runs from 5’ to 3’
What is the anti-sense strand?
- The other strand (3’ to 5’), which is a complementary copy of the sense strand and doesn’t code for a protein
- Acts as a template strand during transcription, so the complementary RNA strand formed carries the same base sequence as the sense strand
How does transcription work?
• Free RNA nucleotides will base pair with complementary bases on antisense strand when DNA unzips
• The T base in RNA nucleotides is replaced with U base (so U binds to A)
• Phosphodiester bonds are formed between the RNA nucleotides by RNA polymerase (enzyme)
• Transcription stops at the end of the gene
- The completed short strand of RNA is called messenger(m)RNA + has the same base sequence the base sequence of the gene on the DNA making up the gene on DNA (but has U instead of T)
-The mRNA molecule travels to a ribosome for the next step in protein synthesis
In eukaryotic cells, what are ribosomes composed of?
What is that composed of?
2 subunits, 1 large 1 small
Nearly equal amounts of protein + rRNA
What is the role of rRNA?
Maintain structural stability of protein synthesis sequence + act as catalyst
What does mRNA do after leaving the nucleus?
What is the process of translation?
Binds to specific site on the small subunit of a ribosome
Ribosome holds the mRNA in place while it’s being coded into a sequence of amino acids. (This process is called translation)
What is tRNA?
Transfer (t) RNA, necessary for translation of mRNA
What is tRNA composed of? What is the function of the composition?
A strand of RNA folded in a way that 3 bases (called the anticodon) are at one end of the molecule.
The anticodon will bind to a complementary codon on mRNA (normal base pairing rules apply). The tRNA molecule carries an amino acid equivalent to that of the codon
When the tRNA anticodons bind to complementary codons along the mRNA, the amino acids are brought together in the correct sequence to form the primary structure of the protein coded for by the mRNA
Describe the 5 stage process of translation.
Elaborate on it.
- mRNA binds to small subunit of the ribosome at its start codon AUG
- A tRNA with the complementary anticodon (UAC) binds to the mRNA start codon. This tRNA carries the amino acid methionine
- Another tRNA with the anticodon UGC + carrying corresponding amino acid threonine, binds to the next codon on the mRNA, ACG. Max. of 2 tRNAs can be bound at the same time
- The first amino acid, methionine, is transferred to the amino acid threonine, on the second tRNA by forming a peptide bond. This is catalysed by peptidyl transferase, an rRna component of the ribosome
- The ribosome moves along mRNA, releasing the first tRNA. The second tRNA becomes the first.
Stages 3-5 are repeated w/ another amino acid added to the chain each time. The process keeps repeating until the ribosome reaches the end of the mRNA at a stop codon + polypeptide is released
What happens to protein structures as amino acids join?
As amino acids are joined together forming the primary structure of the protein, they fold into secondary and tertiary structures.
This folding + bonds formed are determined by the sequence of amino acids in the primary structure.
The protein may undergo modification at the Golgi apparatus before it’s fully functional + ready to carry out the specific role for which it’s been synthesised.
Many ribosomes can follow on the mRNA behind the first so multiple identical polypeptides can be synthesised simultaneously
