Gene Mutations

A gene mutation is a change in the sequence of DNA bases in a gene. The base sequence determines the sequence of amino acids encoded by the gene, so a mutation can change that sequence.

Mutations can arise spontaneously when DNA is copied during DNA replication. They can also occur more frequently in the presence of mutagenic agents, such as certain chemicals and radiation.

Base substitution

A base substitution replaces one base with another. Bases are read in groups of three called codons. Each codon specifies an amino acid or signals where synthesis should stop.

The genetic code is degenerate: more than one codon can specify the same amino acid. A substitution can therefore produce a different codon without changing the amino acid sequence. Alternatively, it can produce a codon that specifies a different amino acid, or a stop codon that causes the amino acid sequence to end early.

Base deletion and the reading frame

A base deletion removes a base from the sequence. Because bases are read in groups of three, deleting one base shifts the grouping, or reading frame, from the point of the deletion onwards.

For example, consider the DNA sequence GAA CCT GGA, grouped into triplets. If the second base is deleted, the remaining sequence is grouped as GAC CTG GA…. The bases after the deletion are now grouped differently, and the final shown group is incomplete.

This frameshift changes the codons read after the deletion and can therefore alter the amino acid sequence encoded by the gene.

Chromosome Non-disjunction

During cell division, chromosomes must separate so that each new cell receives the correct number. Non-disjunction occurs when chromosomes fail to separate normally. In meiosis, this can produce cells with an extra chromosome or a missing chromosome.

Meiosis has two divisions. Before they separate, each chromosome consists of two identical sister chromatids joined together. The two chromosomes of a homologous pair carry the same types of genes, but one comes from each parent.

Non-disjunction during meiosis

In the first division, homologous chromosomes normally separate from each other. If a homologous pair fails to separate, both chromosomes move into the same cell, while the other cell receives neither chromosome from that pair.

In the second division, sister chromatids normally separate. If they fail to separate, both move into the same cell, while the other cell receives neither. Once separated, each chromatid is counted as a chromosome.

In either case, non-disjunction can produce haploid cells that have one chromosome more or one chromosome less than the usual haploid number.

Effect on chromosome number after fertilisation

If an affected cell forms a gamete and that gamete joins with a normal gamete during fertilisation, the resulting cell will have an altered chromosome number. It may have an extra chromosome or be missing a chromosome, depending on which affected gamete took part.

Meiotic Divisions

Meiosis is a sequence of two nuclear divisions that reduces the number of chromosome sets in a cell. It usually produces four haploid daughter cells from one diploid parent cell.

A diploid cell has two sets of chromosomes: its chromosomes occur in homologous pairs. A haploid cell has one set, normally containing one chromosome from each homologous pair.

Before meiosis begins, the cell replicates its DNA. Each chromosome then consists of two joined copies called sister chromatids. Replication does not change the chromosome number: each joined pair of sister chromatids is still counted as one chromosome.

The two meiotic divisions

In the first division, the two chromosomes in each homologous pair separate into different cells. Each resulting cell therefore receives one chromosome from each pair. This reduces the chromosome number from diploid to haploid, even though each chromosome still consists of two sister chromatids.

In the second division, the sister chromatids separate. Each separated chromatid is now a chromosome in a daughter cell. No further DNA replication occurs between the two divisions. The usual result is four haploid cells, each with one chromosome from every original homologous pair.

Tracking chromosomes through meiosis

A chromosome-content diagram can follow the members of a homologous pair through both divisions. For example, call the two homologous chromosomes $$A$$ and $$B$$. After DNA replication, $$A$$ has sister chromatids $$A_1$$ and $$A_2$$, while $$B$$ has sister chromatids $$B_1$$ and $$B_2$$.

1. In the first division, chromosome $$A$$ enters one cell and chromosome $$B$$ enters the other. The sister chromatids are still joined.
2. In the second division, $$A_1$$ and $$A_2$$ separate into different cells. Likewise, $$B_1$$ and $$B_2$$ separate into different cells.

The four products thus have one chromosome each from this homologous pair. If the parent cell has more pairs, the diagram must track each pair in the same way: one homologue enters each cell after the first division, and its sister chromatids separate after the second.

Meiosis, mitosis and life cycles

Unlike meiosis, mitosis involves one nuclear division and normally produces two genetically identical daughter cells. They have the same chromosome number as the parent cell, rather than half that number.

In a life cycle, meiosis is identified by the reduction from a diploid cell or stage to haploid cells or a haploid stage. The haploid products are not necessarily gametes in every life cycle.

Independent Segregation of Homologous Chromosomes

In a cell undergoing meiosis, chromosomes occur in homologous pairs. The two chromosomes in a pair carry the same genes: one was inherited from the individual’s mother and the other from their father.

Before the homologous chromosomes separate in meiosis I, each pair lines up so that one homologue faces each side of the cell. The orientation of one pair is independent of the orientation of the other pairs. When the homologues separate, each daughter cell receives one chromosome from each pair, but the chromosomes it receives can be a mixture of maternal and paternal homologues.

Independent segregation with two chromosome pairs

Imagine a cell with two homologous pairs. One daughter cell could receive the maternal homologue from both pairs, while the other receives the paternal homologue from both. Alternatively, the pairs could orient differently: the first daughter cell could receive the maternal homologue from one pair and the paternal homologue from the other. The second daughter cell would receive the remaining homologues.

After meiosis I, a second division separates the chromosome copies. The resulting haploid products have one chromosome from each homologous pair. Because the pairs orient independently, meiosis can produce haploid products with different combinations of maternal and paternal chromosomes.

Crossing Over

During meiosis, matching chromosomes pair up and can exchange corresponding sections of DNA. This exchange, called crossing over, rearranges alleles between the chromosomes.

The matching chromosomes are homologous chromosomes: one came from each parent. They carry the same genes at corresponding positions, although they may carry different alleles of those genes. Each chromosome has been copied and consists of two sister chromatids.

Crossing over occurs between non-sister chromatids: one chromatid from each homologous chromosome. Corresponding sections are exchanged, so the chromatids can contain new combinations of alleles. Crossing over rearranges existing alleles; it does not create new ones.

As meiosis continues, the chromatids are distributed to daughter cells. The new allele combinations contribute to genetic variation among the haploid products, which have one set of chromosomes.

Random Fertilisation

During fertilisation, two gametes fuse to form a new cell. Each gamete is haploid: it contains one set of chromosomes. The resulting cell is diploid, with two sets of chromosomes, one from each gamete.

Gametes can be genetically different because they can carry different alleles, or versions of genes. Fertilisation is random in the sense that any one gamete may fuse with any compatible gamete from the other parent. Different pairings can therefore bring together different combinations of alleles in the resulting cell.

For example, if one gamete carries an allele for a gene and two possible partner gametes carry different alleles of that gene, the resulting combination depends on which partner gamete fuses with it. Across many genes, the many possible pairings of genetically different gametes further increase genetic variation within a species.