Chromosomes, genes, alleles and mutations.

4.1.1: State that eukaryote chromosomes are made of DNA and proteins.

Eukaryote chromosomes are made of DNA and proteins. DNA is a huge molecule consisting of two paired strands in the form of a double helix. Supported by protein, about 50% of the chromosomes is built of protein.  

TN: The names of the proteins (histones) are not required, nor is the structural relationship between DNA and the proteins.

4.1.2: Define gene, allele and genome.

Gene - A heritable factor that controls a specific characteristic and consists of a length of DNA occupying a position on a chromosome known as a locus.

Allele - one specific form of a gene, differing from other alleles by one or a few bases only and occupying the same gene locus as other alleles of the gene.

Genome - the whole of the genetic information of an organism.

4.1.3: Define gene mutation.

Gene mutation is a change in the base sequence of the gene.

4.1.4: Explain the consequence of a base substitution mutation in relation to the processes of transcription and translation, using the example of sickle-cell anaemia.

GAG has mutated to GTG causing glutamic acid to be replaced by valine, and hence sickle-cell anaemia.

  • Substitution: the replacement of one (or more) base with another.
  • Because of the redundancy of the genetic code, some substitutions will have no effect on the amino acid sequence coded for by the gene.
  • Where the substitution does have an effect, it may produce either mis-sense, altering a single amino acid, or nonsense, introducing a new stop codon.
  • Sick cell anaemia: is substitution in gene for ß chain of haemoglobin.


4.2.1: State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei

In meiosis, four daughter cells are produced – each with half the number of chromosomes of the parent cell. Halving of the chromosome number of gametes is essential because at fertilisation the number is doubled. Therefore each of the daughter cells will have half the number of chromosomes of each type is present in the nuclei which is called haploid (n) [mitosis produces two daughter cells that have the full number of chromosomes which is called diploid (2n).]

Meiosis undergoes two divisions of the nucleus – meiosis I and meiosis II.

4.2.2: Define homologous chromosomes

Homologous chromosomes resemble each other in structure. They occur in a diploid cell, contain the same sequence of genes, but have come from different parents.

4.3.4: Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells.


  • Pairing of homologous chromosomes.
  • Crossing over
  • 2 divisions (meiosis I and II)
  • End product of 4 haploid daughter cells.

Limit crossing over to the exchange of genetic material between non-sister chromatids during prophase L Names of the stages are required.

4.2.4: Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).

Non-disjunction is the term for the failure of a pair of chromatids to separate and go to opposite poles during division of the nucleus. In meiosis, this results in gametes with more than and less than the haploid number of chromosomes.

For example, people with Down’s syndrome have an extra chromosome 21, giving them a total of 47 chromosomes.

4.2.5: State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.

Karyotyping, chromosomes are arranged in pairs according to their size and structure.

4.2.6: State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

Karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

Chorionic villus sampling – withdrawal of a sample of the felt tissue part buried in the wall of the uterus in the period 8-10 weeks into the pregnancy; the tiny sample is of cells that are actively dividing and can be analysed quickly.

Amniocentesis – withdrawal of a sample of amniotic fluid in the period 16-30 weeks of gestation; the fluid contains cells from the survey of the embryo.

4.2.7: Analyse a human karyotype to determine gender and whether non-disjunction has occurred.

Karyotyping can be done by using enlarged photographs of chromosomes.


10.1.1: Describe the behaviour of the chromosomes in the phases of meiosis.

Meiosis I

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There is NO interphase between Meiosis I and Meiosis II

10.1.2: Outline the formation of chiasmata in the process of crossing over.

1. Homologous chromosomes commencing pairing to form a bivalent as they continue to shorten and thicken by coiling.

2. Breakages occur in parallel non-sister chromatids at identical points.

3. Rejoining of non-sister chromatids forms chiasmata.

4. Positions of chiasmata become visible later, as tight pairing of homologous chromosomes ends.

5. When homologous chromosomes move apart in anaphase I, crossing over becomes fully apparent.

10.1.3: Explain how meiosis results in an ...

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