Gene to Genome: DNA to protein

Question 1

What are the properties in translation and describe them (mRNA and tRNA)

Answer 1

•mRNA carried generic code in triplets of bases
- codons: START and STOP codons
- reading frame: continuous, non overlapping set of triplet codons
- specify the sequence of amino acids in proteins produced by translation

• tRnA have complementary triplets (anticodon) to position amino acids during translation
- takes place in ribosomes
- specialised structures composed of rRNA and assessory proteins

Question 2

What are the three stages of translation?

Answer 2

• Initiation- ribosome attached to mRNA and the first tRNA (start codon methionine)

• Elongation- amino acids are brought to the ribosome by tRNAs and linked together to form a polypeptide chain

• Termination- the finished polypeptide is released

Question 3

What is degeneracy?

Answer 3

When amino acids are coded for by more than one codon- minimises effect of mutations

Question 4

What is wobble in genetic code?

Answer 4

A wobble base pair:
- pairing between two RNA nucleotides that does not follow watson crick rules
- only first two bases of the codon have a precise pairing
- the third bases may ‘wobble’

this allows fewer tRNA’s to cover all the codons on the genetic code and minimises risk from misreading code

Question 5

What is epigenetics and how do we carry this out?

Answer 5

• Modifications to the DNA to affect gene expression without altering the DNA sequence

• DNA methylation
• Modification of the histone protein
• Chromatin remodelling
• Non- coding RNA
• real life implications ie codeine

Question 6

(Epigenetics) What is DNA methylation?

Answer 6

> addition of methyl group
often occurs at cytosine bases
can silence genes by preventing transcription

Question 7

(Epigenetics) What is Modification of histone proteins?

Answer 7

> eg acylation, phosphorylation
condense or relax chromatin structure regulating gene expression

Question 8

(Epigenetics) What is chromatin remodelling?

Answer 8

Changes in the structure of chromatin impacts transcription

Question 9

(Epigenetics) What is non coding RNA

Answer 9

> may directly degrade mRNAs or inhibit translation
some are involved in directing other epigenetic modifications eg methylation

Question 10

(Epigenetics) What is real life implications (codeine)

Answer 10

> prodrug, conversion to active form requires CYP2D6
epigenetic modifications, eg DNA methylation, can affect CYP2D6 expression
influences patient response

Question 11

Is epigenetic modifications reversible?

Answer 11

Potentially

Question 12

What is genetic imprinting?

Answer 12

• Some genes retain their epigenetic modifications
• genes ‘silenced’ in the reproductive cells
• only one active copy of the gene in the embryo
• allow for phenotypic changes to be passed to offspring

Question 13

Why is replication not perfect?

Answer 13

• mutations can alter phenotypes
• imprinted genes typically have a higher mutation rate

Question 14

What are the types of mutations and why are they beneficial?

Answer 14

They are the raw material for evolution- shapes both past and current diversity

• somatic vs germ line mutations in cell division

types:
• point mutations, insertions, deletions, and chromosomal mutations
• transitional eg purine<->purine
• transversion eg purine<->pyrimidine

Question 15

Describe the different types of point mutation

Answer 15

• Substitution: One base is incorrectly added during replication
• Insertion: One or more extra nucleotides are inserted
• Deletion: One or more nucleotides is ‘skipped’

Question 16

Describe the different types of Chromosomal mutation

Answer 16

• Inversion: One region of a chromosome is flipped and reinserted
• Deletion: A region of a chromosome is lost
• Duplication: A region of a chromosome is repeated
• Translocation: A region from one chromosome is aberrantly attached to another chromosome

Question 17

Describe the different types of Copy number variation (mutation)

Answer 17

• Gene amplification: The number of tandem copies of a locus is increased
• Expanding trinucleotide repeat: the normal number of repeated sequence is expanded

Question 18

What happens when there is no mutation?

Answer 18

Wild type protein produced

Question 19

What happens when there is a missense mutation

Answer 19

The new codon encodes a different amino acid, there is a change in amino acid sequence

Question 20

What happens when there is a nonsense mutation

Answer 20

The new codon is a stop codon, there is premature termination of translation

Question 21

What happens when there is a silent mutation?

Answer 21

The new codon encodes the same amino acid, there is no change in amino acid sequence

Question 22

Give examples of mutagenesis induced by chemicals

Answer 22

• Base analogues
- resemble normal bases, cause mis pairing

• Alkylating agents
- damage bases, alter base pairing

• Intercalating agents
- insert between base pairs
- cause insertions, deletions and unwinding

Question 23

What are the steps towards DNA repair?

Answer 23

• Detect and recognise damage
• Remove the damage by excising part of one strand
- Base excision: incorrect base removed followed by cutting of DNA backbone, lyase removes the deoxyribose
- Nucleotide excision: double excision removes damage as part of an oligonucleotide

• Polymerisation and resynthesis: DNA polymerase fills gap using info from other strand
• Ligation: DNA ligase seals nicks and so old and new DNA is joined together to restore continuity

Question 24

Which type of cell replication and division happens in somatic and germ line cells

Answer 24

• Mitosis: somatic cells, leads to genetically identical daughter cells with 2n chromosomes (diploid)
• Meiosis: germ line cells, leads to genetically non identical daughter cells with 1n chromosomes (haploid)

•variation arises from independent assortment and recombination events
•segregation of alleles is random