The drawing represents a sperm fertilizing an ovum in a mammalian cell. Only one of the swarm of sperms will penetrate the ovum and bring about fertilization
The left hand drawing represents a vertical section through the ovary of a plant. When the pollen grains land on the stigma they produce pollen tubes which grow through the ovary to reach the ovule. In the drawing on the right, many thousands of sperm cells swim towards the ovum but only one of them will penetrate and fertilise it.
The chromosomes have already replicated but the chromatids are not obvious at this stage
In some organisms the cell divides completely at this stage.
Human body cells contain 46 chromosomes, but the gametes contain only 23. At fertilisation, the number is restored to 46. In humans and most other mammals, the ovum mother cell produces four cells by meiosis but only one of these goes on to become a gamete.
*This example is solely for illustration of the principle. Eye colour and hair curliness are not controlled by single genes.
The genes b and B are said to control contrasting characters , i.e. either blue or brown eyes. Similarly, curly hair and straight hair are contrasting characteristics
Fertilization 2 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
0.1 mm 4 pollen grainovaryovule egg cell This sperm will fertilize the ovum the pollen cell reaches ANIMAL the egg cell through a PLANT pollen tube
5As a result of fertilization, the chromosomes from themale and female cells are combined in the same nuclearmembrane Do you see a problem with this?
6The problem is that if the full complement of male andfemale chromosomes combine, the zygote andembryo would have twice as many chromosomesas its parentsIf 46 male chromosomes combined with 46 femalechromosomes, the offspring would have 92 chromosomesin their cellsAnd the next generation would have 184 chromosomes,and so onIn fact, when the gametes are formed, the number ofchromosomes is halved so that the zygote ends up withthe same number of chromosomes as its parents.
7The type of cell division which gives rise to gametesis 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
Meiosis 1 8The chromosomes appear,shorten and thicken just asin mitosisThe ‘blue’ chromosomesare from the male parent;the ‘red’ chromosomesare from the female parentThe two long chromosomesand the two short chromosomesare called homologous pairs
Meiosis 2 9Homologous chromosomescome to lie closely alongsideeach other and behave likea single chromosome
Meiosis 3 10 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
Meiosis 4 11One long chromosomeand one short chromosomego to either end of thedividing cellSo the cells now containonly two, rather than fourchromosomesBy this time the chromatidshave become clear
Meiosis 5 12 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
Meiosis 6 13 Cell division is completed, forming four gametes each with half the number of chromosomes of the parent cell gametes
14The number of chromosomes in the body cells is calledthe diploid numberThe number of chromosomes in the gametes is calledthe haploid number (half the diploid number)The diploid number in human cells is 46. The haploidnumber in the gametes is 23
15 23 sperms produced 46 by meiosis 23spermmother 23cell 23 fertilization zygoteovum 23mothercell 23 23 46 46 ova produced by meiosis but only one develops to maturity
16Cell division continues bymitosis, so all the cells willcontain 46 chromosomes early embryo 46 46 46 46 46 46 46 46 46 46 46 46 46 46 46 46 46 46
Genes 17 gene for gene for brown eyes blue eyes Genes for any one characteristic occupy corresponding positions gene for on homologous chromosomes curly hair But they do not necessarily control the characteristic in the same way For example, one of the gene pair gene for responsible for eye colour might straight determine brown eyes and its hair partner determine blue eyes*
18Usually only one of a gene pair will be expressed in anindividualA person inheriting the gene for brown eyes and the genefor blue eyes will have brown eyesThe gene for brown eyes is said to be dominant tothe gene for blue eyes. The gene for blue eyes is notexpressed in this individualThe gene for blue eyes is said to be recessive to thegene for brown eyes
Gene combinations 19 In the first stage of meiosis, the illustration (slide 10) showed one ‘red’ and one ’blue’ chromosome going to each daughter cellOne gamete will receive thegene combination for brown Ceyes and curly hair. The otherwill receive the genes for blue Beyes and straight hair B b c = gene for straight hair C = gene for curly hair c b = gene for blue eyes B = gene for brown eyes
20 It is just as likely that both ‘blue’ chromosomes will go to one daughter cell and both ‘red’ chromosomes go to the otherOne gamete will receive thegenes B and c (brown eyesand straight hair) c B bThe other gamete will receivegenes b and C (blue eyes and Ccurly hair)
21So, there could be 4 types of gamete with differentcombinations of the genes BC brown eyes, curly hair bc blue eyes, straight hairBc brown eyes, straight hair bC blue eyes, curly hair
Variation 22 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
23Rearrangement of genes can also takeplace at fertilizationA 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 combinationscan occur
24 sperm ovum 4 Possible combinations B B BB Bb fertilization bB b b bbAlthough there are 4 possible combinations of genesBB, Bb and bB have the same effect of producing brown eyesOnly bb gives rise to blue eyes
Question 1Which of the following are gametes ?(a) sperms(b) dividing cells(c) ova(d) nuclei
Question 2In flowering plants, which of the following aregametes ?(a) egg cell(b) ovule(c) pollen grain(d) pollen cell
Question 3Which of the following occur in both mitosisand meiosis ?(a) chromatids separate(b) homologous chromosomes separate(c) nuclear membrane disappears(d) four cells are formed
Question 4What is the correct sequence of events in meiosis ? (a) (b) (c) (d) (e) (f)(a) a, b, d, c, e, f (c) b, d, a, c, e, f(b) b, a, d, c, e, f (d) a, b, d, c, e, f
Question 5Which of the following represent variationwithin a species ?(a) black cats and tabby cats(b) collie dogs and dachshunds(c) goldfinch and greenfinch(d) shire horses and race horses