Session 13 ILOs - Gene Expression

Question 1

Describe in broad terms ‘What is a gene?’

Answer 1

A gene is a DNA sequence located on a specific chromosomal location, which includes the regulatory sequences around it

Question 2

Explain how ‘the code carries the function’

Answer 2

The DNA code needs to be ready and then transformed into something functional an dit does this using the Central Dogma - DNA is transcribed into mRNA, which is then translated into the protein which then carries out the function from the code that was in the DNA

Question 3

Describe the main differences between DNA and RNA

Answer 3

RNA is ribose nucleic acid and contains an C2 OH hydroxyl group, whereas DNA is deoxyribose nucleic acid and only has a H on the C2.

RNA contains ‘Uracil’ bases, whereas DNA contains ‘Tyrosine’ bases

RNA is single stranded, while DNA is double stranded

RNA can form stem loops: complementary, antiparallel strands which form a mini helix, whereas DNA can only form double strands

Question 4

Explain how RNA can form complex 3D structures

Answer 4

RNA can form complex 3D structures due to non-standard base pairing e.g. 3 base pairs can pair together

Question 5

Describe the three main types of RNA: ribosomal RNA, transfer RNA and messenger RNA

Answer 5

Ribosomal RNA: most abundant (>80%)
- Few kinds, many copies
- Small and large RNA
Function: rRNA associates with a set of proteins to form ribosomes

Transfer RNA: (<15%)
- 100's kinds, many copies
- Can form Clover shapes
- Small
Function: Each tRNA is dedicated to one of the 20 amino acids and carries the correct amino acid to the site of protein synthesis in the ribosome

Messenger RNA: least abundant (<2-5%)
- 10,000’s kinds, few copies
- mRNA transcripts
Function: Serves as intermediary between DNA and protein

Question 6

Describe the general structure of ribosomes in both prokaryotes and eukaryotes

Answer 6

Generally, ribosomes are composed of a small and a large subunit and also rRNA and proteins. The 2 subunits combine to form an overall ribosomal unit.

A prokaryotes has 3 rRNA, whereas a eukaryote has 4 rRNAs (both have many proteins)

Question 7

Describe the general structure of transfer RNA and explain how it functions as the genetic adaptor molecule

Answer 7

Transfer RNA tends to be in a ‘clover shape’, with an amino acid attachment site and an anticodon region which recognises the codon on the mRNA

tRNA molecule acts as an adaptor because it converts mRNA into proteins by the process of translation, in 3 steps of initiation, elongation and termination

Question 8

Explain the principles of the genetic code

Answer 8

Genetic code has the following characteristics:

1. Goes from a 4 letter DNA language to a 20 letter protein language which requires an adaptor molecule (tRNA)
2. Triplet codes
3. Degenerate
4. Non-overlapping and has no gaps

Question 9

Describe the general outline of the mechanism of translation initiation, elongation and termination

Answer 9

Initiation:

1. 5’ Cap recognised by cap binding protein initiation factors
2. tRNA with amino acid bound, plus the small ribosomal subunit, bind to the cap region
3. Energy is used to bind to the AUG start codon
4. Large ribosomal subunit is recruited

Elongation:

1. Binding of aminoacyl-tRNA to the A site
2. Peptide bond formation between the 2 amino acids
3. Translocation occurs and the ribosome moves along the mRNA and the A site is empty again

Termination:

1. When the A site reaches a STOP codon, it’s recognised by a release factor
2. The release factor brings 1 H2O which causes a hydrolysis reaction and the tRNA is now uncharged
3. The polypeptide chain is now released and can be pushed through the exit tunnel

Question 10

Explain the principles of the genetic code, and be confident using the genetic code

Answer 10

a

Question 11

Describe the general outline of the mechanism of translation initiation, elongation and termination

Answer 11

a