6. Controles de Seguridad Técnica
6.1. Generación e instalación del par de claves
Mitosis
When cells such as those in your skin reproduce, they duplicate their chromosomes. When each cell divides, the resulting daughter cells each receive a copy of
the parent cell chromosomes. This type of cell division is called mitosis. Mitosis is an organised series of steps that ensures that each daughter cell is an exact copy of the parent cell. The major steps in mitosis are shown in Figure 4.1.5.
Prac 1 p. 104
Inheritance
Inheritance
Mitosis—cell division to produce new cells identical to the parent cell
Membranes form to produce two daughter cells.
a skin cell
two skin cells
Two pairs of chromosomes are visible.
Chromosomes are doubled but attached at a point called the centromere.
Chromosomes line up along the ‘equator’ of the cell.
Chromosomes separate and move to the ends of the cell.
Fig 4.1.5
Fig 4.1.6 Mitosis—chromosomes separate at opposite
ends of the cell. Human chromosomes treated with stain, then
UNIT
UNIT
Meiosis
A different type of cell division, called meiosis, occurs in the cells in the ovaries and testes, which produce eggs and sperm. Each gamete contains only one of each type of chromosome. When a sperm meets an egg, the resulting cell will have the correct number of chromosomes. During meiosis the chromosomes are duplicated, as for mitosis. This is followed by two divisions.
• In the first division, the
individual chromosomes of each homologous pair separate to form
two cells, each containing only one copy of each kind of chromosome.
• In the second division, the duplicated chromosomes separate to produce a total of four daughter cells.
The major steps in meiosis are shown in Figure 4.1.7.
Meiosis, and the subsequent joining of
gametes, allows for the passing of chromosomes from two parents to an offspring. In this way you have acquired chromosomes, and therefore genes, from both your parents. But you do not simply have half
your father’s characteristics and half your mother’s characteristics. A closer look at genes and how they interact is needed to give you an understanding of
how this happens.
Four types of daughter cells are possible due to the random way in which pairs separate during meiosis. Homologous pair of chromosomes —one inherited from each parent
Cell divides by meiosis.
Fig 4.1.8
During meiosis, homologous chromosomes separate randomly to produce different types of gametes.
Worksheet 4.1 Cell division
Prac 2 p. 104
Will there ever be another me?
Homologous chromosomes randomly
separate during the first division of
meiosis. Hence a cell with only two pairs
of chromosomes will produce four different
possible gamete types (shown in Figure 4.1.8). For three pairs of chromosomes,
eight gamete types are possible. This in
turn means that there are 64 possible
combinations when two gametes join.
Humans have 23 pairs of chromosomes. The number of possible combinations of chromosomes in offspring of the same two parents is 70 million million! It is therefore extremely unlikely that there will ever be
another you!
Meiosis—cell division to produce gametes with half the chromosome number of the parent cell
Membranes form to produce four daughter cells. an ovary cell four egg cells (ova) Two pairs of chromosomes are visible. Chromosomes are doubled but attached at a point called the centromere. Homologous chromosomes line up along the ‘equator’ of the cell. One of each pair of chromosomes moves to the ends of the cell.
Chromosomes line up along the ‘equator’ of each cell. Chromosomes separate and move to the ends of each cell. Fig 4.1.7
4.1
4.1
>>>
Inheritance of pod colour in Mendel’s peas
G G G g g g G G g g g g g First cross parent cells Meiosis produces gametes. Fertilisation produces a zygote. F1 generation homozygous green pods (GG) homozygous yellow pods (gg)
(all heterozygous green pods)
Second cross heterozygous green pods (Gg) heterozygous green pods (Gg) parent cells Meiosis produces gametes. Fertilisation produces a zygote (four possibilities). (homozygous green pods) (heterozygous green pods) (homozygous yellow pods) Gg Gg Gg Gg × × G g G GG Gg gG gg G G g G G g G g G g g g g F2 generation Fig 4.1.10
Simple inheritance
The gene that controls pod colour in pea plants comes in two forms: one codes for green pods, the other for yellow pods. Different forms of the same gene are called alleles. In his experiments, Mendel observed that green pods were more numerous or dominant, suggesting that:
• the allele for green pods is a dominant gene. We can represent the allele for green pods as G. A capital letter is used to indicate dominance. • the allele for yellow pods is a recessive gene. The
allele for yellow pods can be shown as g. A lower case letter is used to indicate that it is recessive. Each pea plant contains two genes for pod colour, one received from the female, the other from the male. The different combinations of the parents’ genes are known as the genotype of the plant. For pea pods, the possible genotypes are:
• GG (called homozygous as both alleles are the same) • Gg (called heterozygous as the two alleles are
different)
• gg (also homozygous).
The appearance produced by a genotype is called the phenotype of the organism. The genotypes GG and Gg would both be green since G is a dominant allele, while gg would be yellow. Hence there are two possible phenotypes: green (GG and Gg) and yellow (gg).
With these definitions we can explain Mendel’s observations in terms of genes. The diagram shows the inheritance of pod colour
in Mendel’s pea plants. Prac 3p. 105
Inheritance
Inheritance
Cells in the testes divide by meiosis. mother’s cell Cells in ovary divide by meiosis. egg cell (ovum) diploid cells with two pairs of chromosomes haploid cells with two chromosomes Gametes join.first cell of new organism
sperm cell
father’s cell
Fig 4.1.9