Mitosis and Meiosis - 2.1.6 Cell division, cell diversity and cellular organisation

Stages of Mitosis

• Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus (they have the same number of chromosomes as the parent cell)

• Although mitosis is, in reality, one continuous process, it can be divided into four main stages

• These stages are:

  • Prophase

  • Metaphase

  • Anaphase

  • Telophase

• Most organisms contain many chromosomes in the nuclei of their cells (eg. humans have 46) but the diagrams below show mitosis of an animal cell with only four chromosomes, for simplicity

• The different colours of the chromosomes are just to show that half are from the female parent and half from the male parent

Prophase

• Chromosomes condense and are now visible when stained

• The chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere

• The two centrosomes (replicated in the G₂ phase just before prophase) move towards opposite poles

• Spindle fibres (protein microtubules) begin to emerge from the centrosomes (consists of two centrioles in animal cells)

• The nuclear envelope (nuclear membrane) breaks down into small vesicles

• The nucleolus disappears

Prophase

Metaphase

• Centrosomes reach opposite poles

• Spindle fibres (protein microtubules) continue to extend from centrosomes

• Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles

• Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres

  • This attachment involves specific proteins called kinetochores

• Each sister chromatid is attached to a spindle fibre originating from opposite poles

Metaphase

Anaphase

• The sister chromatids separate at the centromere (the centromere divides in two)

• Spindle fibres (protein microtubules) begin to shorten

• The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)

Anaphase

Telophase

• Chromosomes arrive at opposite poles and begin to decondense

• Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes

• The spindle fibres break down

• New nucleoli form within each nucleus

Telophase

Meiosis Stages

• Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells

• It produces gametes in plants and animals that are used in sexual reproduction

• It has many similarities to mitosis however it has two divisions: meiosis I and meiosis II

• Within each division there are the following stages: prophase, metaphase, anaphase and telophase

Prophase I

• DNA condenses and becomes visible as chromosomes

• DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere

• The chromosomes are arranged side by side in homologous pairs

  • A pair of homologous chromosomes is called a bivalent

• As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)

• In this stage centrioles migrate to opposite poles and the spindle is formed

• The nuclear envelope breaks down and the nucleolus disintegrates

Metaphase I

• The bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres

• The maternal and paternal chromosomes in each pair position themselves independently of the others; this is independent assortment

  • This means that the proportion of paternal or maternal chromosomes that end up on each side of the equator is due to chance

Anaphase I

• The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle

• The centromeres do not divide

Telophase I

• The chromosomes arrive at opposite poles

• Spindle fibres start to break down

• Nuclear envelopes form around the two groups of chromosomes and nucleoli reform

• Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I

Cytokinesis

• This is when the division of the cytoplasm occurs

• Cell organelles also get distributed between the two developing cells

• In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half

• In plant cells, vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane and also secrete a layer of calcium pectate which becomes the middle lamella. Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell

• The end product of cytokinesis in meiosis I is two haploid cells

Second division of Meiosis : Meiosis II

• There is no interphase between meiosis I and meiosis II so the DNA is not replicated

• The second division of meiosis is almost identical to the stages of mitosis

• Prophase II

  • The nuclear envelope breaks down and chromosomes condense

  • A spindle forms at a right angle to the old one

• Metaphase II

  • Chromosomes line up in a single file along the equator of the spindle

• Anaphase II

  • Centromeres divide and individual chromatids are pulled to opposite poles

  • This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell

• Telophase II

  • Nuclear membranes form around each group of chromosomes

• Cytokinesis

  • Cytoplasm divides as new cell surface membranes are formed creating four haploid cells