3.4.1 DNA, genes and chromosomes

Content:

In prokaryotic cells, DNA molecules are short, circular and not associated with proteins.

In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones. Together a DNA molecule and its associated proteins form a chromosome.

The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein.

A gene is a base sequence of DNA that codes for:

the amino acid sequence of a polypeptide

a functional RNA (including ribosomal RNA and tRNAs).

A gene occupies a fixed position, called a locus, on a particular DNA molecule.

A sequence of three DNA bases, called a triplet, codes for a specific amino acid. The genetic code is universal, non-overlapping and degenerate.

In eukaryotes, much of the nuclear DNA does not code for polypeptides. There are, for example, non-coding multiple repeats of base sequences between genes. Even within a gene only some sequences, called exons, code for amino acid sequences. Within the gene, these exons are separated by one or more non-coding sequences, called introns.

What is a gene? (1)

A sequence of DNA nucleotides bases that codes for a polypeptide

Ignore codes for a protein.

Accept ‘codes for a sequence of amino acids/primary structure’

Name the fixed position occupied by a gene on a DNA molecule. (1)

locus (sg) / loci (pl)

Define locus. (1)

Fixed position of a gene on chromosome / DNA molecule

Define exon. (1)

Base sequence coding for sequence of amino acids

Nucleotide sequence coding for sequence of amino acids

Triplet sequence coding for sequence of amino acids

Ignore polypeptide chain/protein

Define universal genetic code. (1)

The same codon / triplet always codes for the same amino acid

Accept 3 bases for triplet/codon

Define non-overlapping genetic code. (1)

Each base is only part of one triplet / codon

OR Adjacent codons / triplets do not overlap

Define degenerate genetic code. (1)

More than one codon / triplet codes for each amino acid

Define ‘non-coding base sequences’ and describe where the non-coding multiple repeats are positioned in the genome. (2)

1.DNA sequences that do not code for proteins or polypeptides.

DNA sequences that do not code for sequences of amino acids.

DNA sequences that do not code for tRNA or rRNA.

Do not credit ‘DNA that does not code for an amino acid’.

Ignore reference to introns.

2.Are positioned between genes or at the ends of chromosomes.

Reject positioned ‘in introns’ or ‘between exons’.

Complete the table below to show whether the feature of DNA is associated with the DNA molecule found in each of these locations.

Tick (✓) the appropriate boxes. (3)

is circular: prokaryotic cell and chloroplast

contains four different types of nucleotide: prokaryotic cell, nucleus and chloroplast

is associated with histones: nucleus

Name the protein associated with DNA in a chromosome. (1)

Histones

Describe one difference between the structure of DNA in a prokaryotic cell and in a eukaryotic cell. (1)

In prokaryotes DNA is circular not linear

OR not associated with proteins / histones

OR no introns

Ignore ‘loop’

Ignore ‘plasmid’

Outline the similarities in the structures of DNA and RNA molecules. (3)

1.Polymers of nucleotides

Accept ‘chain’ for polymer

2.Nucleotide has pentose, nitrogen-containing organic base and a phosphate group

Accept in correct context

‘ribose/deoxyribose’ for pentose

3. Cytosine, guanine and adenine as bases

4. Have phosphodiester bonds

Outline the differences between the structures of DNA and RNA molecules. (3)

1. Pentose: Deoxyribose for DNA v ribose for RNA

2. Nitrogenous base: Thymine for DNA v uracil for RNA

3. Long chain DNA v short chain for RNA

Accept DNA longer

Ignore ‘large’ and ‘small’

4. Double helix /stranded for DNA v single stranded for RNA

Spaced practice

Describe the primary structure of all proteins. (2)

Sequence / order of amino acids

Joined by peptide bonds

Spaced practice

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule. (2)

1.A phosphodiester bond forms by a condensation reaction, releasing water.

2.The phosphodiester bond forms between the phosphate group of one nucleotide and the deoxyribose sugar of another.

3.This reaction is catalysed by DNA polymerase.

In the process of semi-conservative DNA replication, the two strands within a DNA molecule are separated. Each then acts as a template for the formation of a new complementary strand.

Describe how the separation of strands occurs. (2)

1.DNA helicase separates the two strands.

2.By breaking the hydrogen bonds between complementary base pairs (A–T and G–C).

This results in the two polynucleotide strands separating.

Outline three differences between DNA in the nucleus of a plant cell and DNA in a prokaryotic cell. (3)

1.DNA in plant cells is associated with histone proteins, whereas prokaryotic DNA is not.

2.Plant DNA is linear, while prokaryotic DNA is circular.

3.Plant cells do not contain plasmids, whereas prokaryotic cells do.

4.Plant DNA contains introns, whereas prokaryotic DNA does not.

5.Plant DNA molecules are longer, whereas prokaryotic DNA molecules are shorter.

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