Objectives:compare mitosis and meiosis, know the stages of meiosis, understand when halving of chromosome number occurs, how meiosis contributes to variation
- Crossing over- the entanglement and subsequent exchange of genetic material between homologous chromosomes. It occurs in prophase 1.
- Independent assortment – Mendel’s second law. That homologous chromosomes separate randomly and independently from each other.
- Prophase, Metaphase, Anaphase, Telophase, Cytokinesis (all stages of cell division. They happen twice in meiosis).
- Mitosis and Meiosis: two different kinds of cell division.
- Gametes: sex cells (eg. egg and sperm)
- Somatic cells: Body cells i.e. non gametic cells.
Comparing Mitosis and Meiosis
Meiosis is a kind of cell division that produces sex cells. This is not to be confused with mitosis, which is a kind of cell division that produces somatic cells (normal cells).
Cell division Kind of cells Number of chromosomes Number of cells Mitosis Somatic diploid 2 Meiosis Gametic Haploid 4
Task Q) How many chromosome do human somatic and gametic cells contain? Where do mitosis and meiosis occur in the human body?
History: The discovery of meiosis was an accident!
German scientists discovered that in the horse threadworm (Parascaris equorum) in 1876, there are two chromosomes in the nuclei of egg and sperm cells, and four in the adult cells. Something must have been causing the number of chromosomes to half during gametogenesis (making sperm and egg cells).
Scientists were studying this terrible parasite of horses, in order to try and learn to interrupt its life cycle.
Figure. Horse Threadworm
The stages of meiosis
Meiosis involves two divisions, meiosis 1 and meiosis 2:
- Meiosis 1 involves separating homologous chromosomes
- Meiosis 2 involves separating sister chromatids.
Figure: Overview Stages of Meiosis
In prophase 1, homologous chromosome pairs become entangled with each other when pairing up with each other.
The surprising result of this ‘crossing over’ is that pieces of the chromosomes are ‘exchanged’. This results in new combinations of genes. The junctions between the chromosomes can happen at any random part of the chromosomes.
- Chromosomes supercoil and condense during prophase 1
- Homologous chromosomes pair up with each other, this is called synapsis and the pair can be called a bivalent
- Entanglement occurs between the homologous chromosomes at random places, these are called chiasmata
- DNA is exchanged between the homologous chromosomes at the chiasmata
- Q)During which stage does the chromosome number halve; change from diploid to haploid?
- Q) What will be the end result of crossing over?
The separation of the homologous chromosome pairs in anaphase 1 is random, in that a particular homologous chromosome could go to either pole. This is important, as it means that we have an equal chance of inheriting a chromosome from our mother, or from our father, for each of the 23 homologous chromosome pairs.
This is called the law of independent assortment, and it helps to increase variation (differences between living organisms).
Activity: Using the plasticine, make a model of the stages of mitosis, and take a photograph of each stage. Use pages 162-163 in the textbook. Also make notes, following the instructions.
NOTES: Summary of events occurring at each stage of meiosis
Meiosis 1: Separating homologous chromosomes
Prophase 1: Nuclear membrane disintegrates, chromosomes form, centrosomes move to opposite poles. Spindle fibres grow, and attach to chromosomes (this really happens in an extra stage called prometaphase 1). Synapsis (pairing of homologous chromosome pairs) occurs, and Crossing over / recombination.
Metaphase 1: Homologous chromosome pairs line up on equatorial plate. Crossing over occurs.
Anaphase 1: Separation of bivalents (homologous chromosomes), by contraction of spindle fibres
Telophase 1: Chromosomes uncoil, nuclear membranes reform.
There is no interphase after meiosis 1!
Cytokinesis: the cell divides into two daughter cells.
These two daughter cells are strictly haploid, because only one of each homologous chromosome pair is present (although there is a double copy of each homologous chromosome i.e. the sister chromatids).
Meiosis 2: Separation of sister chromatids
Prophase 2: Nuclear membranes disintegrate, chromosomes form. Centrosomes move to opposite poles, spindle fibres attach (this really happens in an extra stage called prometaphase 2).
Metaphase 2: Chromosomes line up along equatorial plate, NO CROSSING OVER
Anaphase 2: Sister chromatids separate, and travel to opposite poles
Telophase 2: Chromosomes uncoil. Nuclear membranes reform.
Cytokinesis: the two cells divide into four haploid cells.
Task: the meiosis picture challenge https://www.sporcle.com/games/Tolkienite/stage-of-meiosis-images
Support material – watch the video!