Meiosis Flashcards
Background:
Chromosomes in cells that are not dividing.
Chromosomes:
- Exist as single-stranded structures intertwined to form the chromatic network.
- Occur in homologous pairs called a bivalent. One chromosome of each pair comes from the mother and one comes from the father.
- Homologous - chromosomes that are similar in many ways such as position and structure
- Replicate during the interphase of a cell cycle, ie between cell divisions, to form two identical chromatids (sister chromatids). These are joined by a centromere. This takes place before cells divide by either mitosis or meisos.
- Centromere - a specialized constricted region of a choromosome where the two chromatids are held together and to which spindle fibers attach during cell division.
- Only become visible when the chromosomes shorten and thicken as a cell divides.
Meiosis
Meiosis is a special type of cell division that takes place in the reproductive organs of plants and animals to produce gametes (sex cells) in animals and spores in plants.
Cells divide by mitosis. Mitosis enables organisms to grow and to replace tissue. In mitosis replication is followed by one division. In meiosis, replication is followed by two divisions.
- Meiosis 1 is a reduction division which results in two cells being formed each with half the number of chromosomes of the parent cell, ie the haploid (n) number.
- Meiosis 2 is a copying division which involves two haploid cells formed, each dividing again by mitosis to form four daughter haploid cells.
Meiosis 1: Reduction Division
The first division separates the homologous chromosomes into two intermediate cells.
Meiosis 1: Early Prophase 1
As in mitosis, the chromosomes condense, becoming short and fat. They are visible as two chromatids joined by a centromere. From here on the behaviour of the chromosomes in meiosis 1 differ from that in mitosis.
Meiosis 1: Late Prophase 1
- The chromosomes of homologuous pairs lie alongside one another in a process called synapsis to form bivalents.
- The centrioles move to opposite poles.
- A spindle, made up of protein threads, develops across the cell to form the two centrioles. Centrioles are minute rod-shaped bodies formed from the centrosome.
- The nuclear membrane breaks down.
- Parts of the chromatids can become entangled and exchange segments of chorosomes in a process called crossing over.
Meiosis 1: Metaphase 1
- the bivalents (not single chromosomes) move to the middle of the cell and line up on the equator in a way that is completely random.
- The centromeres become attached to the spindle treads.
- The centromeres do not split.
Meiosis 1: Anaphase 1
- The bivalents separate and the homologues are pulled away from each other by contracting spindle threads. This results in independent assortment of maternal and paternal chromosomes. One chromosome of each pair moves to the opposite pole.
Meiosis 1: Telophase 1
The cytoplasm then divides (cytokineses) to form two haploid cells, both of which have only one of each homologous pair of chromosomes.
Meiosis 2: A Copying Division
Each of the haploid cells will divide again by mitosis so that the chromatids are pulled apart but the number of chromosomes remains the same. This produces a large number of gametes.
Meiosis 2: Late Prophase 2
Each chromosome is made up of two chromatids joined by a centromere. The centriole divides, forming two centrioles. A spindle develops between the centrioles which move to opposite poles. The nuclear membranes dissapears.
Meiosis 2: Metaphase 2
The chromosomes move to the middle of the cell where they line up at the equator. The centromeres become linked to the spindle threads.
Meiosis 2: Anaphase 2
The centromeres split, allowing each chromosome to separate into two chromatids. Spindle threads contract and pull the chromatids apart. The chromatids, which are now called daughter chromosomes, move to opposite ends (poles) of the cell.
Meiosis 2: Telophase 2
Daughter chromosomes group together at the poles. A new nuclear membrane starts to form around each set of daughter chromosomes.
Meiosis 2: Cytokinesis
The cytoplasm starts to divide forming two new daughter cells, each with the haploid numbers of chromosomes. This process is called cytokineses. A new nucleus forms and the daughter chromosomes form the chromatin network.
At the end of meisos, four new, non-identical, haploid gametes are formed from one parent cell, each with half the original number of chromosomes. The gametes are not identical to the parent cell.