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

Where does transcription occur in a eukaryotic cell? (1)

nucleus

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

What name is given to a group of three bases on mRNA that codes for an amino acid? (1)

codon

An enzyme is a polypeptide 465 amino acids long.

What is the minimum number of bases in the gene coding for this polypeptide? (1)

1395

Accept 1398 and 1401 for those that include start and/or stop codons

Explain the difference between pre-mRNA and mRNA. (1)

pre-mRNA contains introns / non-coding regions

mRNA contains only exons

Describe how mRNA is obtained from pre-mRNA. (1)

Introns removed and exons joined

The diagram shows part of a pre-mRNA molecule.

a. Name the two substances that make up part X. (1)

Phosphate and ribose

b. Give the sequence of bases on the DNA strand from which this pre-mRNA has been transcribed. (1)

TAGGCA

Name the process that removes base sequences from pre-mRNA to form mRNA. (1)

splicing

The diagram shows the bases on one strand of a piece of DNA.

Give the sequence of bases on the pre-mRNA transcribed from this strand. (1)

UGU CAU GAA UGC UAG

Give the sequence of bases on the mRNA produced by splicing this piece of pre-mRNA. (1)

UGU UGC UAG

Describe how the production of messenger RNA (mRNA) in a eukaryote cell is different from the production of mRNA in a prokaryote cell. (2)

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

In a eukaryotic cell, transcription results in a molecule of pre-mRNA that is modified to produce mRNA. In a prokaryotic cell transcription produces mRNA directly.

Explain this difference. (2)

1. DNA of eukaryotic cell has non-coding regions / introns within gene

But a prokaryotic cell does not have non-coding regions in DNA

pre-mRNA contains non-coding regions

2. After transcription these regions are removed from pre-mRNA

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.

What is the role of RNA polymerase during transcription? (1)

To join nucleotides together to form mRNA / premRNA / RNA

Reject forming base pairs

Describe how RNA polymerase joins RNA nucleotides. (2)

Phosphodiester bonds

by condensation reaction.

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

mRNA can be converted to cDNA.

Name the enzyme used in this process. (1)

reverse transcriptase

Translation

The diagram below represents one process that occurs during protein synthesis.

a. Name the process shown. (1)

translation

b. Identify the molecule labelled Q. (1)

rRNA / transfer RNA

Name the organelle involved in translation. (1)

ribosome

Give the two types of molecule from which a ribosome is made. (1)

One of RNA (ribosomal RNA) and one of ribosomal protein

Reject DNA, tRNA, mRNA.

Starting with mRNA, describe how the process of translation leads to the production of a polypeptide. (4)

mRNA attaches to a ribosome.

Codons on mRNA bind to complementary anticodons on tRNA.

Each tRNA brings a specific amino acid.

Amino acids are joined by peptide bonds in condensation reactions, forming a polypeptide in the order determined by the codons.

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

Describe the role of a ribosome in the production of a polypeptide.

Do not include transcription in your answer. (3)

1.The ribosome attaches to the mRNA molecule at the start codon.

2.The ribosome holds two mRNA codons at a time within its binding sites.

3.This structure allows tRNA molecules with complementary anticodons to bind to the mRNA via hydrogen bonding.

4.The ribosome’s enzyme catalyses the formation of a peptide bond between the two amino acids carried by the tRNA.

5.The ribosome moves along the mRNA to the next codon to continue the chain.

Describe the roles of ribosomes, tRNA and ATP in the process of translation. (6)

1.Ribosome attaches to mRNA

2.The ribosome holds two mRNA codons at a time within its binding sites.

3.tRNA anticodons bind to complementary codons on the mRNA.

4.Each tRNA brings a specific amino acid to the ribosome.

5.ATP provides energy for the attachment of amino acids to tRNA.

ATP is also used to form peptide bonds between amino acids.

6.The ribosome moves along the mRNA, forming a polypeptide.

Ribosome moves along mRNA to stop codon

mRNA tRNA rRNA

The table below looks at the differences between DNA, mRNA and tRNA.

Complete the table to show the differences between DNA, mRNA and tRNA.

DNA has hydrogen bonds and 2 strands

mRNA does not have hydrogen bonds, has 1 strand

tRNA has hydrogen bonds, has 1 strand

The diagram shows a tRNA molecule.

Name the structures labelled W and X in the diagram. (1)

W = amino acid binding site and X = anticodon

Give two differences between the structure of mRNA and the structure of tRNA. (2)

1. mRNA is longer OR has more nucleotides than tRNA

2. mRNA is a straight molecule but tRNA is a folded molecule / clover-leaf shaped molecule

3. mRNA contains no paired bases / hydrogen bonds but tRNA has some paired bases / hydrogen bonds.

Application

Scientists studied the effect of one form, KIR2DS1, of the human KIR gene on mass at birth.

In the following passage the numbered spaces can be filled with biological terms.Write the correct biological term beside each number, that matches the space in the passage.

KIR2DS1 is an ____(1)____ of the KIR gene, found at a ____(2)____ on chromosome 19. KIR2DS1 is 14 021 bases long and is ____(3)____ into mRNA that is 1101 bases long. This mRNA is then ____(4)____ into a polypeptide 304 amino acids long. The polypeptide is then modified in the organelle, ___(5)___, before forming its functional ____(6)____ protein structure.

1. Allele

2. Locus/loci

3. Transcribed

Ignore spliced.

4. Translated

5. Golgi apparatus/Rough endoplasmic reticulum

Ignore RER/ER.

6. Tertiary

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