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Fertilization occurs when the nucleus of a male reproductive cell combines with the nucleus of a female reproductive cell The reproductive cells are called gametes In animals, the male gamete is the sperm cell and the female gamete is the ovum In flowering plants, the male gamete is a cell in the pollen grain and the female gamete is an egg cell in the ovule When the male and female gametes combine, the resulting cell is called a zygote 2 Fertilization

pollen nucleus egg cell nuclei combinecell division (mitosis) embryo formed PLANT sperms ovum nuclei combinecell division (mitosis) embryo formed ANIMAL 3

PLANT This sperm will fertilize the ovum 0.1 mm pollen grain the pollen cell reaches the egg cell through a pollen tube ovary egg cell ANIMAL 4 ovule

As a result of fertilization, the chromosomes from the male and female cells are combined in the same nuclear membrane Do you see a problem with this? 5

The problem is that if the full complement of male and female chromosomes combine, the zygote and embryo would have twice as many chromosomes as its parents If 46 male chromosomes combined with 46 female chromosomes, the offspring would have 92 chromosomes in their cells And the next generation would have 184 chromosomes, and so on In fact, when the gametes are formed, the number of chromosomes is halved so that the zygote ends up with the same number of chromosomes as its parents. 6

The type of cell division which gives rise to gametes is called At cell division, before the chromatids separate, the chromosomes are shared equally between the two daughter cells The following slides describe this process. Only two pairs of chromosomes are shown 7

The chromosomes appear, shorten and thicken just as in mitosis The ‘blue’ chromosomes are from the male parent; the ‘red’ chromosomes are from the female parent The two long chromosomes and the two short chromosomes are called homologous pairs 8 Meiosis 1

Homologous chromosomes come to lie closely alongside each other and behave like a single chromosome 9 Meiosis 2

The nuclear membrane disappears A spindle forms At this stage it is the chromosomes (and not the chromatids) which are separated The cell starts to constrict 10 Meiosis 3

One long chromosome and one short chromosome go to either end of the dividing cell So the cells now contain only two, rather than four chromosomes By this time the chromatids have become clear 11 Meiosis 4

Two new spindles form, at right angles to the original spindle At this stage, it is the chromatids which separate and pass to opposite ends of the cells The cell constricts in the plane at right angles to the first constriction 12 Meiosis 5

Cell division is completed, forming four gametes each with half the number of chromosomes of the parent cell gametes 13 Meiosis 6

The number of chromosomes in the body cells is called the diploid number The number of chromosomes in the gametes is called the haploid number (half the diploid number) The diploid number in human cells is 46. The haploid number in the gametes is 23 14

sperm mother cell ovum mother cell sperms produced by meiosis fertilization zygote ova produced by meiosis but only one develops to maturity 15

46 Cell division continues by mitosis, so all the cells will contain 46 chromosomes early embryo 16

Genes for any one characteristic occupy corresponding positions on homologous chromosomes But they do not necessarily control the characteristic in the same way For example, one of the gene pair responsible for eye colour might determine brown eyes and its partner determine blue eyes* gene for brown eyes gene for blue eyes gene for curly hair gene for straight hair 17 Genes

Usually only one of a gene pair will be expressed in an individual A person inheriting the gene for brown eyes and the gene for blue eyes will have brown eyes The gene for brown eyes is said to be dominant to the gene for blue eyes. The gene for blue eyes is not expressed in this individual The gene for blue eyes is said to be recessive to the gene for brown eyes 18

In the first stage of meiosis, the illustration (slide 10) showed one ‘red’ and one ’blue’ chromosome going to each daughter cell B One gamete will receive the gene combination for brown eyes and curly hair. The other will receive the genes for blue eyes and straight hair B b C c B = gene for brown eyes b = gene for blue eyes C = gene for curly hair c = gene for straight hair 19 Gene combinations

It is just as likely that both ‘blue’ chromosomes will go to one daughter cell and both ‘red’ chromosomes go to the other B b c C One gamete will receive the genes B and c (brown eyes and straight hair) The other gamete will receive genes b and C (blue eyes and curly hair) 20

So, there could be 4 types of gamete with different combinations of the genes BC brown eyes, curly hair bc blue eyes, straight hair Bc brown eyes, straight hair bC blue eyes, curly hair 21

Meiosis not only halves the number of chromosomes but can also rearrange the genes This is one cause of the variations that occur in members of the same species 22 Variation

Rearrangement of genes can also take place at fertilization A sperm may carry a gene for brown eyes ( B ) or a gene for blue eyes ( b ) An ovum may carry a gene for brown eyes ( B ) or a gene for blue eyes ( b ) At fertilization, four possible combinations can occur 23

Bb BB bB bb b b spermovum 4 Possible combinations BB, Bb and bB have the same effect of producing brown eyes Only bb gives rise to blue eyes Although there are 4 possible combinations of genes B B 24 fertilization

Question 1 Which of the following are gametes ? (a) sperms (b) dividing cells (c) ova (d) nuclei

Question 2 In flowering plants, which of the following are gametes ? (a) egg cell (b) ovule (c) pollen grain (d) pollen cell

Question 3 Which of the following occur in both mitosis and meiosis ? (a) chromatids separate (b) homologous chromosomes separate (c) nuclear membrane disappears (d) four cells are formed

Question 4 What is the correct sequence of events in meiosis ? (a)(b) (c)(d)(e) (f) (a) a, b, d, c, e, f (b) b, a, d, c, e, f (c) b, d, a, c, e, f (d) a, b, d, c, e, f

Question 5 Which of the following represent variation within a species ? (a) black cats and tabby cats (b) collie dogs and dachshunds (c) goldfinch and greenfinch (d) shire horses and race horses

Answer Correct

Answer Incorrect