3.4.2 - DNA and protein synthesis

Question 1

Genome

Answer 1

The complete set of genes in a cell (including those in mitochondria and /or chloroplasts)

Question 2

Proteome

Answer 2

The full range of proteins that a cell can produce (coded for by the cell’s DNA / genome)

Question 3

Describe the two stages of protein synthesis

Answer 3

1) Transcription
> Production of messenger RNA (mRNA) from DNA, in the nucleus

2) Translation
> Production of polypeptides from the sequence of codons carried by mRNA, at ribosomes

Question 4

Compare and contrast the structure of tRNA and mRNA

Answer 4

Comparison (similarities):
● Both single polynucleotide strand

Contrast (differences):
● tRNA is folded into a ‘clover leaf shape’, whereas mRNA is linear / straight
● tRNA has hydrogen bonds between paired bases, mRNA doesn’t
● tRNA is a shorter, fixed length, whereas mRNA is a
longer, variable length (more nucleotides)
● tRNA has an anticodon, mRNA has codons
● tRNA has an amino acid binding site, mRNA doesn’t

Question 5

Describe how mRNA is formed by transcription in eukaryotic cells

Answer 5

1. DNA helicase catalyses the break down of Hydrogen bonds between complementary DNA bases of base pairs in the double helix sttructure of DNA
2. Only one DNA strand acts as a template
3. Free RNA nucleotides align next to their complementary bases on the template strand
   ○ In RNA, uracil is used in place of thymine (pairing with adenine in DNA)
4. RNA polymerase joins adjacent RNA nucleotides
5. This forms phosphodiester bonds via condensation reactions
6. Pre-mRNA is formed and this is spliced to remove introns, forming (mature) mRNA

Question 6

Describe how production of messenger RNA (mRNA) in a eukaryotic cell is different from the production of mRNA in a prokaryotic cell

Answer 6

● Pre-mRNA produced in eukaryotic cells whereas mRNA is produced directly in prokaryotic cells
● Because genes in prokaryotic cells don’t contain introns so no splicing in prokaryotic cells

Question 7

Describe how translation leads to the production of a polypeptide

Answer 7

1. mRNA attaches to a ribosome and the ribosome moves to a start codon
2. tRNA brings a specific amino acid
3. tRNA anticodon binds to complementary mRNA
   codon
4. Ribosome moves along to next codon and another
   tRNA binds so 2 amino acids can be joined by a
   condensation reaction forming a peptide bond
   ○ Using energy from hydrolysis of ATP
5. tRNA released after amino acid joined polypeptide
6. Ribosome moves along mRNA to form the
   polypeptide, until a stop codon is reached

Question 8

Describe the role of ATP in translation

Answer 8

● Hydrolysis of ATP to ADP + Pi releases energy

● So amino acids join to tRNAs and peptide bonds form between amino acids

Question 9

Describe the role of tRNA in translation

Answer 9

● Attaches to / transports a specific amino acid, in relation to its anticodon
● tRNA anticodon is complementary to mRNA codon, forming base pairs with hydrogen bonds between them
● 2 tRNAs bring amino acids together so peptide bond can form

Question 10

Describe the role of ribosomes in translation

Answer 10

● mRNA binds to ribosome, with space for 2 codons
● Allows tRNA with anticodons to bind
● Catalyses formation of peptide bond between amino acids (held by tRNA molecules)
● Moves along (mRNA to the next codon) / translocation