Lecture 5

Question 1

What are functions of proteins?

Answer 1

Enzymatic
Transport
Defence roles
Regulation
Structural Support
Receptor molecules
Hormones
Cell signalling
Amino Acid storage
Movements within cells

Question 2

What are proteins? What makes their structure?

Answer 2

Proteins are linear chains of amino acids
arranged in a 3-D structural hierarchy

Diversity of structures, resulting in a wide range of
a functions
▪ functions depend on the 3-D protein shape
▪ 3-D protein shape is determined by its
amino acid sequence

Question 3

What is protein synthesis?

Answer 3

Protein synthesis (or gene expression) consists in two phases - transcription
and translation (separated by the RNA maturation in eukaryotic cells)

Question 4

What are the brief details of DNA transcription?

Answer 4

DNA transcription – DNA to mRNA
* DNA transcription consists of 3 phases
➢Initiation
➢Elongation
➢Termination
* RNA polymerase uses one of DNA strands as a template to produce pre-messenger
RNA (pre-mRNA) with a complementary sequence to DNA.
* It adds RNA nucleotides (complementary to DNA) in the 5′→3′ direction

Question 5

What happens during initiation?

Answer 5

RNA polymerase, with the help of transcription factors, binds to a specific sequence of
DNA (promoter – just before the start site of a gene)

• RNA polymerase separates the double
  strands and use a single strand as a template
• Begin the complementary RNA strand
  synthesis

Question 6

What happens during elongation?

Answer 6

RNA polymerase moves along the DNA
coding sequence in 5ʹ-to-3ʹ direction.
* Melting DNA and adding RNA nucleotides

Question 7

What happens during termination?

Answer 7

The RNA polymerase encounters a specific termination sequence (stop site).
This releases the RNA strand

Question 8

What are the three RNA processing steps?

Answer 8

Capping at 5’ end
➢Polyadenylation at 3’ end
➢and splicing

Question 9

What is capping at 5’ end?

Answer 9

A modified nucleotide 5′ G (cap), is added to the 5’ of the pre-mRNA
▪ Allow the attachment to the ribosome

Question 10

What is polyadenylation at 3’ end?

Answer 10

A repetition of poly-A (100–250 bp of ribonucleotide A) is added to the 3’ end
▪ increase the stability of mRNA molecules (preventing its degradation),
▪ facilitate its migration to the cytosol

Question 11

What is splicing?

Answer 11

introns are cut out and all the exons are joined together

Question 12

What is translation?

Answer 12

Translation – conversion of the mRNA code (4
nucleotides) into protein (20 amino acids)
* In the cytoplasm on the ribosomes

Question 13

How does translation happen?

Answer 13

• mRNA carries the genetic code in groups of three consecutive
  nucleotides → Codons
• Each codon specifies one amino acid (Universal Genetic
  Code), except 3 stop codons

Question 14

What happens during initiation of translation?

Answer 14

• Small Ribosome binds to
  the mRNA (starting codon)
  and tRNA carrying
  methionine (linked to the
  start codon)
• Large subunit assembles
  on top

Question 15

What happens during elongation of translation?

Answer 15

1. A charged tRNA carrying the next
   amino acid interacts with the
   respective mRNA codon
2. The two amino acids are linked by a
   peptide bond
3. The ribosome slides along the mRNA
   and another tRNA can bind the next
   codon (5’ → 3’)
   4 mRNA moves in 5ʹ-to-3ʹ
   * the protein is growing (N-term to C-term

Question 16

what happens during termination of translation?

Answer 16

1. Translation halts at a stop codon (UAA, UAG, and UGA)
2. Release factors (not tRNA) bind to any stop codons inducing the ribosome
3. The ribosome releases the mRNA and dissociates into its two subunits