Meiosis Subtopic 3.3

The discovery of Meiosis Meiosis was discovered by microscope examinations of dividing germ-line cells. Improved microscopes in the 19th century provided detailed images of cell structures. Around that same time, it was discovered that some dyes specifically stain the nucleus of cells. The process of determining the steps of cell division started in 1880s by a group of German scientists.

The discovery of Meiosis A key observation was that in the horse threadworm (Parascaris equorum) there are 2 chromosomes in the nuclei of an egg and sperm cells, whereas the fertilized egg contains four. This led to the hypothesis that there must be a special nuclear division in every generation that halves the chromosome number. The sequence of events in meiosis was worked out by careful observation of cells taken from the ovaries of rabbits (Oryctolagus cuniculus) between 0 and 28 days old. Why rabbits? in females meiosis begins at birth and occurs slowly over many days.

Meiosis produces gametes

(for growth and repair) Meiosis Cellular Division Mitosis Makes Body Cells (for growth and repair) Meiosis Makes Gametes (sex cells) Process in which cells divide to produce more cells

Meiosis Outline One diploid nucleus divides by meiosis to produce four haploid nuclei. Meiosis is two nuclear divisions Meiosis I: produces 2 nuclei Meiosis II: produces 4 nuclei

Homologous chromosomes Chromosomes of the same type.

Chromatin vs. Chromatid Chromatin = DNA wrapped around histone proteins. Chromosomes at this point are diffused in the nucleus, and are not visible with a light microscope. Interphase. Chromatid = One of the usually paired duplicated chromosome. Chromosomes at this point have condensed, and are visible with a light microscope. Cell division. Sister chromatids = chromatids that have been replicated during S-phase of the cell cycle.

Chromosomes

Haploid vs Diploid Diploid = two copies of each chromosome type. One copy inherited from the mother, and one inherited from the father. Haploid = one copy of each chromosome type.

Meiosis I – chromosome numbers are halved. Each of the four nuclei produced by meiosis has just one chromosome of each type. Halving of the chromosomes happens in the first division, Meiosis I. The two nuclei have only one copy of each type of chromosome, but each chromosome still consists of two chromatids.

Meiosis II Sister chromatids separate. Produces haploid cells, each containing a single chromatid.

Meiosis and sexual life cycles Life cycles can be sexual and asexual Sexual life cycles – have genetic diversity. There are differences between the chromosomes of the offspring and the parents. Asexual life cycles Asexual reproduction is one organism dividing into two organisms without shuffling its genes, so the offspring has the same version of genes as did the parent. In this process, organisms arise from a single organism. There is no exchange of genetic material between two parents.

fertilization Union of gametes (sex cells) usually from two different parents. It doubles the chromosome numbers. Meiosis can happen at any stage during a sexual life cycle, but in animals it happens during the process of creating gametes.

Life cycle questions: The image shows the life cycle of humans and mosses (n = Haploid, 2n = diploid). Outline five similarities between the life cycle of a moss and of a human. Distinguish between the life cycles of a moss and a human by giving five differences.

replication of DNA before Meiosis Once DNA has been replicated, each chromosome consists of two sister-chromatids. DNA replication happens during S-phase of interphase. Chromosomes become visible when they gradually shorter and condense by supercoiling. DNA does not replicate before Meiosis II.

Bivalents (tetrads) formation and crossing over At the start of meiosis: Synapsis: Homologous chromosomes pair up with each other. Remember DNA replication already took place, so each chromosome has two chromatids. When the homologous chromosomes pair up, they form a tetrad or bivalent. Crossing over: Happens soon after synapsis. One chromatid in each of the homologous chromosomes breaks and rejoins with the other chromatid. Happens at random positions anywhere along the chromosomes. At least one crossover occurs in each bivalent and there can be several. It happens at precisely the same position on the two chromatids involved. The genes are the same, but the alleles may be different.

Random orientation of bivalents Orientation of homologous chromosomes prior to separation is random. After the nuclear membrane has broken down, the spindle microtubules attach to the centromeres of the chromosomes, following these principles: Each chromosome is attached to one pole only, not to both. The two homologous chromosomes in a bivalent are attached to different poles. The pole to which a chromosome is attached depends on which way the pair of chromosomes is facing. This is called random orientation. The orientation is random, so each chromosome has an equal chance of attaching to each pole, and eventually of being pulled to it. The orientation of one bivalent does not affect other bivalents.

Halving the chromosome numbers The first division, when homologous chromosomes separate, halves the chromosome number.

The movement of chromosomes is not the same in the first division of meiosis and mitosis! Centromere divides and the two chromatids that make up the chromosome move to opposite poles. Meiosis: - Centromere does not divide and whole chromosomes move to the poles. Comparison of attachment of chromosomes to spindle microtubules in mitosis and meiosis.

Disjunction Disjunction Separation of homologous chromosomes during meiosis I. The separation of pairs of homologous chromosomes to opposite poles of the cell halves the number of the cell. Chiasmata A point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands. The chiasmata slide to the end of the chromosomes and then the chromosomes can separate.

Chorionic villus sampling & amniocentesis Both are procedures for obtaining cells containing fetal chromosomes for producing a karyotype. Amniocentesis Draws a sample of amniotic fluid containing fetal cells from the amniotic sac. 1% chance of miscarriage. Chorionic villus sampling Obtains cells from the chorion. The chorion is one of the membranes from which the placenta develops. Can be done earlier in the pregnancy than amniocentesis 2% chance of miscarriage.

Amniocentesis https://youtu.be/bZcGpjyOXt0

Chorionic villus sampling

The stages of meiosis

Meiosis I – Prophase 1 Chromosomes become visible due to supercoiling. The paired and replicated homologous chromosomes are called tetrads (bivalents ), which have two chromosomes and four chromatids.  At this stage, non-sister chromatids may cross over at points called chiasmata. Centrioles, if present, migrate to opposite poles and spindle fibers start to form. The nucleolus and nuclear membrane disintegrate.

Meiosis I – metaphase I Homologous pairs (bivalents) move together along the metaphase plate, which lies halfway between the two poles. The spindle fibers attach to the centromeres of each chromosome and gentle pulling from them aligns the homologous chromosomes along the equatorial plane.  Spindle fibers connect each centromere to one pole only.

Meiosis I – Anaphase i Spindle microtubules shorten, pulling homologous chromosomes apart towards opposite poles.

Meiosis I – Telophase I The first meiotic division effectively ends when the chromosomes arrive at the poles. The chromatids uncoil and a nuclear membrane then reforms around each nucleus formed. Telophase I is usually followed by cell division which gives rise to two daughter cells with half the number of chromosomes as the parent cell; though each chromosome consists of a pair of sister chromatids.

Meiosis ii – Prophase ii Chromosomes (two sister chromatids) condense again. Centrioles, if present, migrate to opposite poles and spindle fibers start to form. The nucleolus and nuclear membrane disintegrate.

Meiosis ii – Metaphase ii The spindle fibers attach to the centromere of each pair of sister chromatids and gentle pulling from them aligns the sister chromatids at the equator. Spindle fibers connect each centromere to both poles.

Meiosis ii – anaphase ii Centromeres divide and chromatids are moved to opposite poles by spindle fibers.  Once sister chromatids are separated, they are called chromosomes.

Meiosis ii – telophase ii Chromosomes reach opposite poles and uncoil. This is followed by nuclear envelope formation and cytokinesis. Meiosis is now complete and ends up with four new daughter cells.

The four main stages in meiosis Prophase: chromosomes condense (crossing over occurs here!). Metaphase: Spindle microtubules attach and align at the equatorial plate. Anaphase: The chromosomes or chromatids move to the opposite poles. Telophase: Chromosomes uncoil.