4.2 DNA and protein synthesis

Define ‘genome’ and ‘proteome’

Genome

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

Proteome

The full range of proteins that a cell can 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

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

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

○ 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

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 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

Describe the role of ATP, tRNA and ribosomes in translation

ATP

● Hydrolysis of ATP to ADP + Pi releases energy

● So amino acids join 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 bond can form

Ribosomes

● 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

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