Protein Synthesis

Question 1

Describe transcription

Answer 1

• 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

Question 2

Describe splicing

Answer 2

• 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

Question 3

What is a splicozome?

Answer 3

Small nuclear ribonucleoproteins (SnRNPs)

Question 4

Describe translation

Answer 4

• 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)

Question 5

What is a ribosome made out of?

Answer 5

• rRNA

* protein

Question 6

Describe gene mutations

Answer 6

• 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

Question 7

Why do gene mutations occur?

Answer 7

• can occur spontaneously during replication

* can be caused by mutagenic agents

Question 8

Why do chromosome mutations occur?

Answer 8

• can occur spontaneously by chromosome non-disjunction during meiosis/crossing-over errors

Question 9

Describe substitution

Answer 9

change of nucleotides

Question 10

Describe addition/insertion

Answer 10

Addition of an extra nucleotide

Question 11

Describe deletion

Answer 11

Loss of nucleotide(s)

Question 12

Describe inversion

Answer 12

Region breaks off, rotates 180° and then rejoins

Question 13

Describe duplication

Answer 13

• 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

Question 14

Define missense

Answer 14

Change in amino acid residue

Question 15

Define nonsense

Answer 15

Change to stop codon

Question 16

Describe frameshift

Answer 16

• occurs because DNA is non-overlapping

* caused by addition and deletion

Question 17

How can a mutation be neutral

Answer 17

• 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

Question 18

Beneficial mutations

Answer 18

Natural selection