1.6 Mitosis and meiosis

• It is only at the onset of cell division that

  chromosomes become visible.
• Shortly before cell division begins each DNA molecule makes

  a copy of itself. The single thread of DNA becomes two identical threads; these are called

  chromatids. The chromatids are joined

  at the centromere.
• Humans always have

  46 chromosomes
• Chromosomes are found in matching

  pairs, called homologous pairs. Humans have 23 pairs of homologous chromosomes.
• The total number of chromosomes is called the diploid number.
• Gametes (sex cells) have

  half the diploid number, this is called haploid; human gametes have 23 chromosomes.
• Mitosis produces two daughter cells that are genetically identical to the parent cell.
• During interphase the following occurs:

   Replication of DNA.

   Replication of organelles which have their own DNA – mitochondria and

  chloroplasts.

   Making new organelles (replication is not acceptable here, only organelles with DNA can be replicated).

   Synthesis of ribosomal material, ATP and proteins
• In plants mitosis

  only takes place in the meristems – tip of

  the root, tip of the shoot, buds and tree

  rings.
• At any given time most cells

  will be in interphase as it is the longest

  part of the cell cycle.
• Prophase is the first stage of mitosis.

  During prophase the DNA condenses

  (becomes shorter and thicker) forming

  chromosomes. Chromatids become visible.

  In animal cells the centrioles move to

  opposite poles of the cell. Protein

  microtubules form from each centriole and

  the spindle develops, extending from pole

  to pole. Towards the end of prophase the

  nuclear membrane disintegrates and the

  nucleolus disappears. Pairs of chromatids

  can clearly be seen lying free in the

  cytoplasm.
• Metaphase is the 2nd stage of mitosis. The chromosomes

  arrange themselves at the centre or

  equator of the spindle. The chromosomes

  become attached to the spindle fibres at

  the centromere. Contraction of the spindle

  fibres draws the individual chromatids

  apart.
• Anaphase is the 3rd stage of mitosis. Anaphase is a very rapid stage. The

  centromere splits. The spindle fibres

  contract. The chromatids separate and are

  pulled to opposite poles of the cell; the

  centromeres lead the way.
• Telophase is the 4th stage of mitosis. The

  chromatids have now reached the poles of

  the cells and are referred to as

  chromosomes again. The chromosomes

  uncoil and lengthen. The spindle breaks

  down. The nucleolus reappears and the

  nuclear membrane reforms.
• In animal cells cytokinesis (last stage) occurs by the cytoplasm splitting to create two fully formed cells.

  In plant cells, a cell plate forms across the equator of the parent cell from the centre

  outwards and a new cell wall is laid down.
• Mitosis is essential for growth, the repair of tissues and the replacement of dead or worn

  out cells.
• Asexual reproduction takes place by mitosis. Offspring produced asexually are

  genetically identical to the parent. An advantage of asexual reproduction is the ability to

  increase in numbers quickly to take advantage of an ideal environment. The disadvantage is

  the lack of genetic variation, leading to an inability to adapt if the environment changes.
• Mitosis maintains the diploid chromosome

  number. Each parent cell produces

  two new daughter cells.
• Cancers are the result of uncontrolled mitosis. Cancerous cells divide repeatedly, out of

  control, with the formation of a tumour. A tumour is an irregular mass of cells; tumours

  prevent the normal function of body organs. Cancers are thought to be initiated when

  mutations (changes) occur in the genes that control cell division.
• Meiosis takes place in the reproductive organs (gonads) of both plants and animals.
• Meiosis results in the

  formation of gametes with half the normal chromosome number; this is the haploid

  number.
• Meiosis produces cells which have genetic variation and plays an important role

  in bringing about genetic variation in living organisms.
• Interphase in meiosis is

  exactly the same as interphase

  mitosis
• Prophase I is the first stage of meiosis (after interphase)

  During prophase the DNA condenses forming

  chromosomes. Chromatids become visible. In animal cells the centrioles move to

  opposite poles of the cell. Protein microtubules form from each centriole and the spindle

  develops, extending from pole to pole. Paternal and maternal chromosomes associate as

  homologous pairs (this process is called synapsis); each pair is called a bivalent.
• Towards the end of prophase the nuclear

  membrane disintegrates and the

  nucleolus disappears.
• Interphase is not a part of meiosis or mitosis, but plays an essential role in the cell cycle.
• During metaphase I

  the homologous chromosomes

  arrange themselves randomly on

  the equator of the spindle in pairs. Chance

  determines how the homologous

  chromosomes are arranged on the

  equator.
• Anaphase I is a very

  rapid stage. The spindle fibres

  contract

  . The chromosomes in

  each bivalent separate and are

  pulled to opposite poles of the cell.

  The random arrangement of

  homologous pairs at metaphase I

  means that each pole has a random

  mixture of paternal and maternal

  chromosomes
• Anaphase I meiosis - Homologous pairs are

  separated. The centromere does

  not split and chromatids are not

  pulled apart. The chromosomes

  remain intact at this stage.
• Telophase I marks the end of the first meiotic division. The chromosomes

  have reached opposite poles. The nuclear envelope reforms around each group of haploid

  chromosomes. The chromosomes remain in their condensed form. In animal cells

  cytokinesis occurs after telophase I. Meiosis II follows on immediately.
• In meiois prophase I - Each bivalent has 4 strands, consisting of 2 chromosomes, each with 2 chromatids.
• Prophase II

  In animal cells a new spindle develops at right angles to the old spindle.

  Many plant cells do not need to form a new spindle as the old one

  remains.
• Metaphase II

  The chromosomes line up separately on the spindle fibres at the equator.

  Each chromosome is attached to the spindle by its centromere.
• Anaphase II

  Spindle fibres contract. The centromeres split. Chromatids are pulled to opposite poles.
• Telophase II

  On reaching the poles the chromatids lengthen and are indistinct. The

  spindle disappears. The nuclear membrane reforms. At the end of

  telophase II cytokinesis takes place.
• End of meiosis - The result of these two meiotic divisions is that there are 4 haploid daughter cells. Each

  daughter cell has genetic variation; they are genetically different.
• Genetic

  variation is introduced due to

  homologous pairs carrying

  different genetic material,

  crossing over at chiasmata during

  prophase I and due to random

  assortment of maternal and

  paternal chromosomes during

  metaphase I.
• Mitosis

  • One division resulting in 2 daughter cells
  • Number of chromosomes is unchanged
  • Homologous chromosomes do not associate in pairs
  • Crossing over does not occur
  • Daughter cells are genetically identical (no genetic variation)
• Meiosis

  • Two divisions resulting in 4 daughter cells
  • Number of chromosomes is halved
  • Homologous chromosomes pair up to form bivalents
  • Crossing over occurs and chiasmata form
  • Daughter cells are genetically different (genetic variation)
• Genetic variation occurs from:

  • crossing over (forming chiasmata)
  • independent assortment