4.2.1 State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei
Meiosis is the process by which sex cells (gametes) are made in the reproductive organs:
- Most sexually reproducing animals are diploid - meaning they have two copies of every chromosome (one of maternal origin, one of paternal origin)
- In order to reproduce, these organisms need to make gametes that are haploid (have only one copy of each chromosome)
- Fertilisation of two haploid gametes (egg + sperm) will result in the formation of a diploid zygote that will grow into a new organism
Meiosis consists of two cell divisions:
- The first division is a reduction division of the diploid nucleus to form haploid nuclei
- The second division separates sister chromatids (this division is necessary because meiosis is preceded by interphase, wherein DNA is replicated)
4.2.2 Define homologous chromosomes
Homologous chromosomes are chromosomes that share:
- The same structural features (e.g. same size, same banding pattern, same centromere position)
- The same genes at the same loci positions (while genes are the same, alleles may be different)
4.2.3 Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells
The process of meiosis involves two divisions, both of which follow the same basic stages as mitosis (prophase, metaphase, anaphase and telophase)
Meiosis is preceded by interphase, which includes the replication of DNA (S phase) to create chromosomes with genetically identical sister chromatids
Meiosis I
Homologous chromosomes must first pair up in order to be sorted into separate haploid daughter cells
In prophase I, homologous chromosomes undergo a process called synapsis, whereby homologous chromosomes pair up to form a bivalent (or tetrad)
- The homologous chromosomes are held together at points called chiasma (singular: chiasmata)
- Crossing over of genetic material between non-sister chromatids can occur at these points, resulting in new gene combinations (recombination)
The remainder of meiosis I involves separating the homologous chromosomes into separate daughter cells
- In metaphase I, the homologous pairs line up along the equator of the cell
- In anaphase I, the homologous chromosomes split apart and move to opposite poles
- In telophase I, the cell splits into two haploid daughter cells as cytokinesis happens concurrently
Meiosis II
In meiosis II, the sister chromatids are divided into separate cells
- In prophase II, spindle fibres reform and reconnect to the chromosomes
- In metaphase II, the chromosomes line up along the equator of the cell
- In anaphase II, the sister chromatids split apart and move to opposite poles
- In telophase II, the cell splits in two as cytokinesis happens concurrently
Because sister chromatids may no longer be genetically identical as a result of potential recombination, the process of meiosis results in the formation of four genetically distinct haploid daughter cells
4.2.4 Explain that non-disjunction can lead to a change in chromosome number, illustrated by reference to Down syndrome (trisomy 21)
Non-disjunction refers to the chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy)
The failure of the chromosomes to separate may either occur via:
- Failure of homologues to separate during Anaphase I (resulting in four affected daughter cells)
- Failure of sister chromatids to separate during Anaphase II (resulting in two affected daughter cells)
Non-Disjunction
Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21)
- One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction
- The other parental gamete was normal and had a single copy of chromosome 21
- When the two gametes fused during fertilisation, the resulting zygote had three copies of chromosome 21, leading to Down syndrome
4.2.5 State that, in karyotyping, chromosomes are arranged in pairs according to their structure
A karyotype is a visual profile of all the chromosomes in a cell
The chromosomes are arranged into homologous pairs and displayed according to their structural characteristics
Human Male Karyotype
Karyotyping involves:
- Harvesting cells (usually from foetus or white blood cells of adults)
- Chemically inducing cell division, then halting it during mitosis when chromosomes are condensed and thus visible
- The stage during which mitosis is halted will determine whether chromosomes appear with sister chromatids
- Staining and photographing chromosomes, before arranging them according to 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
Pre-natal karyotyping is often used to:
- Determine the gender of an unborn child (via identification of sex chromosomes)
- Test for chromosomal abnormalities (e.g. aneuploidies resulting from non-disjunction)
Amniocentesis
- A needle is inserted through the abdominal wall, into the amniotic cavity in the uterus, and a sample of amniotic fluid containing foetal cells is taken
- It can be done at ~ 16th week of pregnancy, with a slight chance of miscarriage (~0.5%)
Chorionic Villus Sampling
- A tube is inserted through the cervix and a tiny sample of the chorionic villi (contains foetal cells) from the placenta is taken
- It can be done at ~ 11th week of pregnancy, with a slight risk of inducing miscarriage (~1%)
Amniocentesis Chorionic Villus Sampling
4.2.7 Analyse a human karyotype to determine gender and whether non-disjunction has occurred
Every cell in the human body has 46 chromosomes (except anucleate red blood cells and haploid gametes)
Males (X,Y) and females (X,X) can be differentiated on the basis of their sex chromosomes
Non-disjunction during gamete formation can lead to individuals with an abnormal number of chromosomes (aneuploidy)
These disorders can be classified according to the chromosome number affected and the number of chromosomes present
Analysing Karyotypes