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Biology Chromosomes fertilization

Date post: 07-Aug-2015
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1

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 thepollen grain and the female gamete is an egg cellin the ovule

When the male and female gametes combine, the resulting cell is called a zygote

2Fertilization

pollen nucleus

egg cell

nuclei combine cell division (mitosis)embryo formed

PLANT

sperms

ovumnuclei combine cell 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 andfemale chromosomes combine, the zygote and embryo would have twice as many chromosomes as its parents

If 46 male chromosomes combined with 46 femalechromosomes, the offspring would have 92 chromosomesin 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 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

7

The chromosomes appear,shorten and thicken just asin mitosis

The ‘blue’ chromosomesare from the male parent; the ‘red’ chromosomesare from the female parent

The two long chromosomes and the two short chromosomes are called homologous pairs

8Meiosis 1

Homologous chromosomes come to lie closely alongsideeach other and behave like a single chromosome

9Meiosis 2

The nuclear membrane disappears

A spindle forms

At this stage it is thechromosomes (and notthe chromatids) which areseparated

The cell starts to constrict

10Meiosis 3

One long chromosomeand one short chromosomego to either end of the dividing cell

So the cells now containonly two, rather than fourchromosomes

By this time the chromatidshave become clear

11Meiosis 4

Two new spindles form, at right angles to the originalspindle

At this stage, it is the chromatids which separate and pass to opposite ends of thecells

The cell constricts in the planeat right angles to the first constriction

12Meiosis 5

Cell division is completed,forming four gameteseach with half the numberof chromosomes of the parent cell

gametes

13Meiosis 6

The number of chromosomes in the body cells is called the diploid number

The 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

14

23

46

4623

23

23

23

23

23

46

sperm mothercell

ovummothercell

sperms produced by meiosis

fertilizationzygote

ova produced by meiosis but only one develops tomaturity

15

46 46

46 46

46 46 4646

46 46 46 46

4646

46 46

4646

Cell division continues by mitosis, so all the cells willcontain 46 chromosomes early embryo

16

Genes for any one characteristicoccupy corresponding positions on homologous chromosomes

But they do not necessarily controlthe characteristic in the same way

For example, one of the gene pairresponsible for eye colour mightdetermine brown eyes and its partner determine blue eyes*

gene for brown eyesgene forblue eyes

gene for curly hair

gene forstraight hair

17Genes

Usually only one of a gene pair will be expressed in anindividual

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 tothe gene for blue eyes. The gene for blue eyes is notexpressed in this individual

The gene for blue eyes is said to be recessive to thegene 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 eyesC = gene for curly hairc = gene for straight hair

19Gene combinations

It is just as likely that both ‘blue’ chromosomeswill go to one daughter cell and both ‘red’ chromosomes go to the other

B

b

c

C

One gamete will receive thegenes B and c (brown eyes and straight hair)

The other gamete will receive genes b and C (blue eyes andcurly 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

22Variation

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

bb

sperm ovum 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 aregametes ?

(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


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