3.4.2 DNA and Protein synthesis

Content:

The concept of the genome as the complete set of genes in a cell and of the proteome as the full range of proteins that a cell is able to produce.

The structure of molecules of messenger RNA (mRNA) and of transfer RNA (tRNA).

Transcription as the production of mRNA from DNA. The role of RNA polymerase in joining mRNA nucleotides.

In prokaryotes, transcription results directly in the production of mRNA from DNA.

In eukaryotes, transcription results in the production of pre-mRNA; this is then spliced to form mRNA.

Translation as the production of polypeptides from the sequence of codons carried by mRNA.

The roles of ribosomes, tRNA and ATP.

Students should be able to:

relate the base sequence of nucleic acids to the amino acid sequence of polypeptides, when provided with suitable data about the genetic code
interpret data from experimental work investigating the role of nucleic acids.

Genome and proteome

Transcription

Describe how a gene is a code for the production of a polypeptide.

Do not include information about transcription or translation in your answer. (3)

1. Because base / nucleotide sequence

2. In triplets

3. Determines order / sequence of amino acid sequence / primary structure in polypeptide

Describe how the production of messenger RNA (mRNA) in a eukaryote cell is different from the production of mRNA in a prokaryote cell.

1. Pre-mRNA only produced in eukaryote cell

2. Splicing only occurs in eukaryote cell

3. Introns removed in eukaryote cell

OR Introns not present in prokaryote cell

Describe how mRNA is produced from an exposed template strand of DNA.

Do not include DNA helicase or splicing in your answer. (3)

1.Free RNA nucleotides form complementary base pairs with the exposed DNA template strand (A–U, G–C).

2.Phosphodiester bonds form between adjacent nucleotides to build the mRNA strand.

3.This process is catalysed by RNA polymerase.

Describe how mRNA is formed by transcription in eukaryotes. (6)

1.Hydrogen bonds between DNA bases break.

Ignore DNA helicase.

Reject hydrolysing hydrogen bonds.

2.Only one DNA strand acts as a template.

3.Free RNA nucleotides align by complementary base pairing.

accept ‘align to complementary bases’ or ‘align by base pairing’

4.In RNA, uracil base pairs with adenine on DNA.

OR In RNA Uracil is used in place of thymine

Do not credit use of letters alone for bases.

5.RNA polymerase joins adjacent RNA nucleotides.
Reject suggestions that RNA polymerase forms hydrogen bonds or joins complementary bases.

6.Phosphodiester bonds form between adjacent nucleotides.

7.Introns are removed to form mRNA.

OR pre-mRNA is spliced to form mRNA

OR Introns are removed to form mRNA

Translation

Describe how a polypeptide is formed by translation of mRNA. (6)

1.mRNA attaches to ribosomes or to rough endoplasmic reticulum

2.tRNA anticodons bind to complementary mRNA codons.

3.tRNA brings a specific amino acid.

4.Amino acids join by peptide bonds.
5.Amino acids join together with the use of ATP.
6.tRNA is released after the amino acid is joined to the polypeptide.
7.The ribosome moves along the mRNA to form the polypeptide.

Describe the role of ATP in the process of translation in protein synthesis. (2)

1. Releases / provides energy

Reject ‘produce energy’

2. So, peptide bonds form between amino acids

OR so, amino acid joins to tRNA

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