4.2 DNA and protein synthesis

Define ‘genome’ and ‘proteome’

Genome - the complete sets of genes in a cell (including those in mitochondria / chloroplasts)

Proteome - the full range of proteins that a cell is able to produce (coded for by the cell’s DNA / genome)

Describe the two stages of protein synthesis

Transcription - production of messenger RNA (mRNA) from DNA, in the nucleus

Translation - production of polypeptides from the sequence of codons carried by mRNA, at ribosomes

Compare and contrast the structure of tRNA and mRNA

Similarities:

• both single polynucleotide strand

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

Describe how mRNA is formed by transcription in eukaryotic cells

1. Hydrogen bonds between DNA bases break

2. Only one DNA strand acts as a template

3. Free RNA nucleotides align next to their complementary bases on the template strand

4. - in RNA, uracil is used in place of thymine (pairing with adenine in DNA

5. RNA polymerase joins adjacent RNA nucleotides

6. This forms phosphodiester bonds via condensation reactions

7. Pre-mRNA is formed and this is spliced to remove introns, forming (mature) mRNA

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

• 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

Describe how translation leads to the production of a polypeptide

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 a complementary mRNA codon

4. Ribosome moves along to next codon, and another tRNA binds so 2 amino acids can be joined by 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

Describe the role of ATP, tRNA and ribosomes in translation

ATP:

• hydrolysis of ATP to ADP + Pi releases energy

• So amino acids joins to tRNAs and peptide bonds form between amino acids

tRNA:

• attaches to / transports a specific amino acid, in relation to its anticodon

• tRNA anticodon complementary base pairs to mRNA codon, forming hydrogen bonds

• 2 tRNAs bring amino acids together so peptide bonds can form

Ribosomes:

• mRNA binds to ribosomes, 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

Describe how the base sequence of nucleic acids can be related to the amino acid sequence of polypeptides when provided with suitable data

• you may be provided with a genetic code to identify which triplets / codons produce which amino acids

• tRNA anticodons are complementary to mRNA codons

• - e.g. mRNA codon = ACG → tRNA anticodon = uGC

• Sequence of codons on mRNA are complementary to sequence of triplets on DNA triplets on DNA template strand

• - e.g. mRNA base sequence = ACG UAG AAC → DNA base sequence = TGC ATC TTG

• In RNA, uracil replaces thymine