2 - ICH - Protein synthesis with nucleic acids

Question 1

Why is DNA called the genetic code?

Answer 1

DNA codes for the sequence of amino acids in a polypeptide chain

Question 2

List 5 characteristics of the genetic code

Answer 2

Triplet code

Code is universal

It’s a degenerate code

It’s non-overlapping

It has nonsebse triplets

Question 3

Why does it mean?

Genetic code:

It’s a triplet code

Answer 3

• Each amino acid is coded for by a sequence of 3 DNA bases.
• Sequence is always read in ONE direction.
• Sequence is always read from one of the 2 strands of DNA (coding strand/reference strand)
• Non coding strand is important, makes DNA very stable and allow accurate copying by semi-conservative replication.
• Must code for all 20 amino acids. (4⁴ = 64 combinations)

Question 4

Why does it mean?

Genetic code:

Universal

+ example?

Answer 4

The same DNA triplets code for the same amino acids in nearly all organisms

E.g.

CGT codes for arginine in ALL living organisms

Question 5

Why does it mean?

Genetic code:

Degenerate code

+ example?

Answer 5

• There’s 64 combinations of the 4 bases (4⁴ = 64) but there’s only 20 amino acids.
• Some amino acids are coded for by more than one triplet code. Only the first 2 bases are important!

E.g.

The 4 codes in the picture all code for Arginine but all begin with CG

Question 6

Why does it mean?

Genetic code:

Non-overlapping

Answer 6

• Each sequence of 3 base codes for a seperate amino acid.
• There’s NO overlap in the base sequence

Question 7

Why does it mean?

Genetic code:

It has nonsense triplets

Answer 7

• DNA triplet sequences that don’t code for any amino acids.
• These act as START (initiation) and STOP (termination) signals in protein synthesis.

Question 8

Name of:

Coding DNA

Non-coding DNA

Answer 8

Coding DNA = Exon DNA

Non-coding DNA = Intron DNA

Question 9

What is:

Transcription

Translation

Answer 9

Transcription - DNA base sequence of a gene is used to make a molecule of mRNA. (Copying)

Translation - The code on the mRNA is used to control the production of a polypeptide chain at a ribosome. (Bases → Amino acids)

Question 10

How does transcription work?

Answer 10

It’s the 1st stage of protein synthesis.

1. DNA helicase breaks the hydrogen bonds which hold the 2 DNA strands together break along the length of the gene.
2. One of the DNA strands act as a template (sense strand) for the formation of a mRNA molecule.
3. RNA polymerase moves along the DNA template strand to join the nucleotides in the RNA strand to form the mRNA molecule. The bases line up as complementary base pairings but with U instead of T.
4. Hydrogen bonds reform and DNA resumes double helix shape one that part of mRNA is formed.
5. RNA polymerase will reach a STOP codon. Causes the 2 to break off and seperate. mRNA moves out of the nucleus through nulear pore, attaches to ribosome in cytoplasm for translation.

Question 11

How does translation work?

Answer 11

1. A mRNA molecule arrives at a ribosome.
2. The ribosome binds to START codon on the mRNA moelcule and then moves along, one codon at a time.
3. The ribosome provides 2 binding sites that allow 2 tRNA molecules to attach to the mRNA at the same time.
4. tRNA molecules can only attach with their anticodons are complementary to the mRNA codons e.g. ACU with UGA.
5. The 2 amino acids brought by the 2 tRNA molecules can form a peptide bond between them. rRNA = catalyst for this.
6. Ribosomes move along one codon, releases the first tRNA. This tRNa returns to cytoplasm to pick up another amino acid.
7. The next tRNA molecule can now join the mRNA on the ribosome and its amino acid can be joined to the others.
8. This process repeats till the ribosome reaches the STOP codon. Polypeptide chain is released.
9. Amino acids of the polypeptide interact: secondary ► tertiary ► quaternary structures…