B2.7. Cell Division and Genetics

DNA, Genes and Chromosomes
1. Chromosomes are made from DNA. Genes are short sections of DNA. Genetically identical cells are produced by a type of cell division called mitosis. In sexual reproduction, a male gamete fuses with a female gamete to produce a new cell. This is called fertilisation. Gametes are produced by a type of cell division called meiosis. They contain a single set of chromosomes, whereas body cells contain two sets of chromosomes.
2. DNA DNA (deoxyribose nucleic acid) molecules are large and complex. They carry the genetic code that determines the characteristics of a living thing. Except for identical twins, each person’s DNA is unique. This is why people can be identified using DNA fingerprinting. DNA can be cut up and separated, forming a sort of 'bar code' that is different from one person to the next.
3. Genes A gene is a short section of DNA. Each gene codes for a specific protein by specifying the order in which amino acids must be joined together.
4. Chromosomes The cell’s nucleus contains chromosomes made from long DNA molecules. The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes.
Mitosis
1. Mitosis is the type of cell division that leads to growth or repair. When a cell divides by mitosis: two new cells form each cell is identical to the other one, and the cell they were formed from
2. Stages of Mitosis
  1. 1) Interphase: The chromosomes cannot be seen but they duplicate the DNA and double in mass when it starts to copy its own DNA.
  2. 2) Prophase: During prophase, chromosomes in the nucleus condense, pairs of centrioles move to opposite sides of the nucleus, spindle fibres form a bridge between the ends of the cell, and the nuclear envelope breaks down and Nucleus begins to disappear.
  3. 3) Metaphase: During metaphase, the chromosomes are pulled by microtubules called spindle fibers into place. The chromosomes line up on the cell's equator, or center line, and are prepared for division.
  4. 4) Anaphase: The chromosomes move from the cell's equator to their respective poles of the cell. The cell begins to stretch out as the opposite ends are pushed apart.
  5. 5) Telophase: The final stage in mitosis, as the cell itself is ready to divide. One complete set of chromosomes is now at each pole of the cell. The spindle fibers begin to disappear, and a nuclear membrane forms around each set of chromosomes,
3. Mitosis is different to meiosis as mitosis produces 2 daughter cells whereas meiosis produces 4, meiosis is only in the gonads whereas mitosis is in body cells, meiosis is reduction and mitosis is multiplication, meiosis is for gametes, also in mitosis the chromatids split whereas in meiosis the chromosomes split.
Meiosis
1. Gametes are formed from cells in the reproductive organs by a type of cell division called meiosis.The cells that are formed by meiosis have half as many chromosomes as the cell that formed them. Human body cells contain 23 pairs of chromosomes, while human gametes contain 23 single chromosomes.
2. In meiosis, the chromosomes are copied and the cell divides 2, forming 4 gametes. This means meiosis is a reduction division.
3. Stages of Meiosis
  1. Meiosis I
    1. 1) Prophase I: The chromosomes become visible, the nuclear envelope disappears and the centrioles (located at the top and bottom of the nucleus) begin forming spindle fibres that envelope the chromosomes.The chromosomes pair up and some DNA is swapped.
    2. 2) Metaphase I: The chromosomes form a line along the equator.
    3. 3) Anaphase I: The chromosomes are divided so that there are equal amounts on either side of the cell. As there are 46 chromosomes in a human cell, 23 end up on either side.
    4. 4) Telophase I: Nuclear membrane reforms, the two daughter cells are completely divided. There are 23 chromosomes in each of these cells.
  2. Meiosis II
    1. 1) Prophase II: The chromosomes become visible, the nuclear membrane disappears and the centrioles form the spindle fibres. Chromosomes pair up and some DNA is swapped.
    2. 2) Metaphase II: The chromosomes line up along the middle line on the spindle fibres.
    3. 3) Anaphase II: The chromosomes get split into its two chromatids.
    4. 4) Telophase II: The nuclear membrane reforms and four new cells with different DNA are created.
4. In males, all four cells become sperm. In females, only one becomes a mature egg, while the remaining three become re-absorbed into the body.
Gametes and Fertilisation
1. Gender
  1. Human body cells have 23 pairs of chromosomes in the nucleus. One of these pairs controls the inheritance of gender - whether offspring are male or female: In males, the two sex chromosomes are different. They are XY. In females, the two sex chromosomes are the same. They are XX.
2. Gametes are sex cells. The male gametes are the sperm, and the female gametes are the eggs. Gametes contain one set of genetic information, while body cells contain two sets of genetic information.
3. Gametes are sex cells. The male gametes are the sperm, and the female gametes are the eggs. Gametes contain one set of genetic information, while body cells contain two sets of genetic information.
4. Fertilisation is the joining or fusion of a male gamete and a female gamete. The new cell that is formed divides over and over again by mitosis. This creates the many cells that eventually form a new individual.
Alleles
1. Alleles are different forms of a gene. They can be dominant or recessive. Genetic diagrams help us to understand the possible outcomes when parents produce offspring. Huntington’s disease is a disorder of the nervous system that is caused by a dominant allele. Cystic fibrosis is a disorder of the cell membranes caused by a recessive allele.
2. Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene. These genes may have different forms. Different forms of the same gene are called alleles (pronounced al-eels). The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.
3. Alleles are dominant or recessive: the characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair the characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair
  1. For example, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. An individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will only have blue eyes if they inherit two copies of the allele for blue eyes.
Genetic Diagrams
1. In a genetic diagram, you show all of the possible alleles for a particular characteristic. There will be two alleles from one parent, and two from the other parent, making four altogether. You then draw lines to show all the possible ways that these alleles could be paired in the offspring. There will be four possible ways, but some or all of them could be repeated. In genetic diagrams, the dominant allele is shown as a capital letter, while the recessive allele is shown as a lower-case letter.
2. Another Genetic diagram is a Punnett Square
Inherited Disorders
1. Recessive
  1. Cystic fibrosis is an inherited disorder that affects the cell membranes, causing the production of thick and sticky mucus. It is caused by a recessive allele. This means that it must be inherited from both parents.
2. Dominant
  1. Huntington’s disease is an inherited disorder that affects the nervous system. It is caused by a dominant allele. This means it can be passed on by just one parent if they have the disorder.

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