Sex determination of the fetus:
- The egg cell from the mother only contains X chromosome, so the sex of a fetus is determined by the father’s sperm cell, which has either the X or the Y chromosome.
- During fertilisation, if the mother’s egg cell receives X chromosome from the sperm cell of the father, the resulting fetus will be a female having XX chromosome pairs.
- And, if the mother’s egg cell receives the Y chromosome from the sperm cell of the father, the resulting fetus will be a male having XY chromosome pairs.

Chromosomes and their numbers in human body cells:
- The human body is composed of trillions of cells.
- Each cell in the human body consists of 23 pairs of chromosomes (46 chromosomes) except the gametes.
- The gametes have only one set of chromosomes (23 chromosomes).
- Gametes are haploid cells which are represented as n, where ‘n’ stands for the number of chromosomes in one set of chromosomes (23 chromosomes).
- Normal body cells are diploid cells and are represented as 2n as they contain two sets of chromosomes (46 chromosomes).
- Among the two sets of chromosomes (23 pairs) in a diploid cell, one set (23 chromosomes) is from the father and the other set (23 chromosomes) is from the mother.
- Each chromosome pair is numbered sequentially in size order from the largest to the smallest. Its image is called a karyogram.
- Karyogram of a human male and a female:
- The pairs of chromosomes which have similar structure and composition are called homologous chromosomes.
- There are 22 pairs of homologous chromosomes and they are called autosomes.
- The two X chromosomes in females are also considered as homologous chromosomes, since they have similar structure.
- The non-matching X and Y chromosomes or the 23^rd pair in males are non-homologous chromosomes.
- The X and Y chromosomes which determine the sex are called sex chromosomes.

Meiosis:
- It is a type of cell division, where a cell divides into four daughter cells.
- Each daughter cell contains half the original number of chromosomes, so it is also called a reductional cell division.
- It occurs in the female’s ovaries and the male’s testes to produce gametes, which contain only one set of chromosomes and include one sex chromosome.
- Necessity of meiosis for sexual reproduction:
  - During fertilisation, haploid male and female gametes fuse together to form a diploid zygote.
  - If the number of chromosomes is not halved, then the number of chromosomes would double every generation.
  - So to keep the number of chromosomes the same in every generation, gametes must be haploid.

Stages of meiosis:
- Before meiosis begins, cells go through an interphase stage. In this stage, DNA replication takes place in a semi conservative manner.
- Unlike mitosis, meiosis involves two divisions:
  - Meiosis I
  - Meiosis II
- Meiosis I:
  - It is a reductional division, resulting in two daughter nuclei with half the number of chromosomes of the parent nucleus.
  - First, the centrosome replicates just before prophase I and the centrosomes move to the opposite ends of the nucleus.
  - There are four main stages under meiosis I:
    - Prophase I
    - Metaphase I
    - Anaphase I
    - Telophase I
  - Prophase I:
    - During the early prophase I stage, chromosomes start to become visible as the chromatin coils up, thus becoming shorter and thicker.
    - Also, the nuclear envelope remains intact.
    - In the middle of the prophase I stage, the homologous chromosomes pair up. This process is called synapsis and each pair is called bivalent.
    - The homologous chromosomes contains to form two identical sister chromatids, and each chromatid consists of one DNA molecule.
    - This is followed by an exchange of sections of DNA between non-sister chromatids. This is known as crossing over.
    - In the late prophase I, the nuclear envelope breaks up and the nucleolus disappears as it does in mitosis.
  - Metaphase I:
    - The cells then enter metaphase I, where the nucleus and nuclear membrane disappear. The centrosomes polarise and help to organise the spindle fibres produced by microtubules.
    - The chromosomes lie on the equator of the cell with the attachment of centromeres to the spindle fibres.
  - Anaphase I:
    - Metaphase I is followed by anaphase I. In this stage, centromeres do not divide as in mitosis.
    - Instead, the spindle fibres contract, and the homologous chromosomes separate and move towards the opposite poles of the cell.
  - Telophase I:
    - It is the last stage of meiosis I where the chromosomes reach the two opposite poles, each with one centrosome.
    - The chromosomes start to uncoil themselves and the spindle fibres disappear.
    - Meanwhile, the nucleolus and nuclear envelope reform.
      - In animal cells, a constriction appears in the middle of the two nuclei, dividing the cytoplasm of the parent cell. This stage is called cytokinesis.
      - Eventually, two daughter cells are formed, with their own plasma membrane, nucleus, and cytoplasm.
      - In plant cells, they directly go into the next stage of meiosis without splitting.
- Meiosis II:
  - In meiosis II, chromosomes behave as in mitosis. Each of the two haploid daughter nuclei divides again resulting in a total number of four haploid nuclei.
  - Like meiosis I, meiosis II also has four major stages:
    - Prophase II
    - Metaphase II
    - Anaphase II
    - Telophase II
  - Prophase II:
    - During prophase II, chromosomes become shorter and thicker and are clearly visible.
    - The nuclear envelope and nucleolus start to disappear.
    - Centrosomes divide into two and move to the opposite poles.
  - Metaphase II:
    - Then the cells enter metaphase II, where the nuclear membrane and nucleolus disappear.
    - Chromosome lies on the equator with the centromeres attached to the spindle fibres.
  - Anaphase II:
    - In anaphase II, the chromosomes split at the centromere, the spindle fibres shrink and the chromatids get pulled apart towards the opposite poles of the cell.
  - Telophase II:
    - In telophase II, the chromatids reach the two opposite poles, each with one centrosome.
    - Telophase II is followed by cytokinesis.
    - In animal cells, constriction appears in the middle of the two nuclei, dividing the cytoplasm of the parent cell.
    - In plant cells, cytokinesis is slightly different. The nuclei in a plant cell are not separated by plasma membrane yet, so the cell splits two times to make four cells.

Photomicrographs of the meiotic cycle of animal cells:
- Meiosis I:
- Meiosis II:

Significance of meiosis:
- Meiosis is essential for the production of haploid gametes.
- Genetic variation:
  - Crossing over and the independent assortment of the homologous chromosomes help to produce genetic variation between the resulting daughter cells.
  - Crossing over results in each chromosome having different combinations of alleles than it did before.
  - It also helps to differentiate prophase I from prophase II as the short piece of exchanged chromosome can be seen in prophase II.
  - Independent assortment results in the shuffling of alleles.
  - During the first division of meiosis (meiosis I), the pairs of homologous chromosomes that line up on the equatorial plane are pulled to opposite ends of the cell.
  - Each pair behaves independently from every other pair, so many different combinations can occur.
  - Further genetic variation is produced when these genetically different gametes produced by meiosis fuse randomly to produce a zygote. This combination of factors makes everyone unique.

16.01 - Meiosis

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