Protein Synthesis Flashcards
Describe transcription
- DNA is unwound by helicase by breaking the hydrogen bonds between fomplementary base pairing
- complementary copy of the coding sequence is made from mRNA using one strand of DNA as a temple. Joined by RNA polymerase catalysing the formation of phosphodiester bonds
- always 5’ -> 3’
- splicing occurs
- mRNA leaves nucleus
Describe splicing
- only in eukaryotes- in prokaryotes, transcription directly produces mRNA from DNA
- splicozomes stade either end of an intron and splice it out, resulting in mature mRNA
What is a splicozome?
Small nuclear ribonucleoproteins (SnRNPs)
Describe translation
- mRNA leaves nucleus and attached to non-membrane-bound ribosome in the cytoplasm
- binds to a small subunit
- 1st amino acid always AUG (start codon) = Met
- in cytoplasm there are 64 different molecules of tRNA each with a specific triplet anticodon
- each tRNA has a specific amino acid attached by a specific amino-acyl tRNA synthétase enzyme
- process uses ATP and forms activated molecule to provide energy for peptide bond
- the anticodon of the correct tRNA then pairs with the codon of the mRNA
- this brings two tRNAs together in the ribosome and allows a peptide bond to form between the 2 amino acids by peptidyl transferase
- continues until a stop codon is reached (UAA, UAC, UGA)
What is a ribosome made out of?
- rRNA
* protein
Describe gene mutations
- DNA encodes for proteins and therefore a mutation (change in DNA sequence) may lead to a change in protein sequence
- change in a base pair within a DNA sequence of a gene can result in an altered or mutant gene product (protein)
- such a point mutation can result in a protein of altered function or a protein which lacks function altogether
- the majority of such mutations are disadvantageous or even lethal
- some mag be neutral or beneficial
Why do gene mutations occur?
- can occur spontaneously during replication
* can be caused by mutagenic agents
Why do chromosome mutations occur?
• can occur spontaneously by chromosome non-disjunction during meiosis/crossing-over errors
Describe substitution
change of nucleotides
Describe addition/insertion
Addition of an extra nucleotide
Describe deletion
Loss of nucleotide(s)
Describe inversion
Region breaks off, rotates 180° and then rejoins
Describe duplication
- production of one or more copies of a gene or region of a chromosome
- occurs in all organisms- especially prominent among plants
- important mechanism by which evolution occurs
Define missense
Change in amino acid residue
Define nonsense
Change to stop codon
Describe frameshift
- occurs because DNA is non-overlapping
* caused by addition and deletion
How can a mutation be neutral
- DNA is degenerate; a different codon could fire for the same amino acid, leaving the protein structure unaffected
- a different codon causes a different amino acid, but if it is not at an essential site on the polypeptide chain, it is possible that the change in amino acid could have no effect on protein structure and therefore function
Beneficial mutations
Natural selection
