LIFE SCIENCES GRADE 12 - PAPER 1 (Answers from past Assessment ...

LIFE SCIENCES GRADE 12 - PAPER 1 (Answers from past Assessment Guidelines)

MEIOSIS

Where in animals and plants? Where in the cell? Events of Interphase + 4 cell division phases Importance Abnormal meiosis

DIFFERENCES BETWEEN MITOSIS AND MEIOSIS

MEIOSIS ? Meiosis involves 2 cell divisions ? Meiosis gives rise to sex cells ? Cells produced are haploid ? Differs according to gender ? 2 phases ? 4 daughter cells formed ? Half the number of chromosomes to the

parent cell ? All cells formed are genetically

different to each other and parent cell ? Cells responsible for genetic

variation in reproduction (gametes)

MITOSIS ? Mitosis involves 1 cell division ? Gives rise to somatic cells ? Cells produced are diploid ? Has no gender specification ? 1 phase ? 2 daughter cells ? Same number of chromosomes formed

as in parent cell ? Genetically identical to each

other and parent cell ? Cells responsible for growth and

repair (somatic cells)

DIFFERENCES BETWEEN MEIOSIS I AND MEIOSIS II

MEIOSIS I ? Crossing over occurs in prophase 1 ? Chromosomes arranged at the

equator in homologous pairs in metaphase 1 ? Homologous chromosome pairs separate ? Whole chromosomes are pulled to opposite poles in anaphase 1 ? 2 diploid daughter cells produced

MEIOSIS II ? No crossing over occurs ? Chromosomes arranged singly at

the equator in metaphase 2

? Sister chromatids separate

? Chromatids are pulled to opposite poles in anaphase 2

? 4 haploid daughter cells produced

THE IMPORTANCE OF MEIOSIS

? Production of haploid gametes ? The halving effect of meiosis overcomes the doubling effect of fertilisation, thus

maintaining a constant chromosome number from one generation to the next ? Mechanism to introduce genetic variation through:

- Crossing over - The random arrangement of chromosomes at the equator

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ABNORMAL MEIOSIS

Non-disjunction and its consequences ? Non-disjunction of chromosome pair 21 during Anaphase in humans to form abnormal gametes with an extra copy of chromosome 21 ? The fusion between an abnormal gamete (24 chromosomes) and a normal gamete (23 chromosomes) may lead to Down syndrome

STRUCTURE AND ARRANGEMENT OF CHROMOSOMES IN A NORMAL HUMAN KARYOTYPE - Each chromosome comprises two chromatids - held together by a centromere - There are 23 pairs/46 chromosomes in - human somatic cells/body cells - which are arranged into homologous pairs - that are similar in length - carry genes for the same characteristics - have alleles of a particular gene at the same lociand - have the same centromere position - Each somatic cell has 22 pairs/44 autosomesand - a pair/2 gonosomes/sex chromosomes/X and Y chromosomes - Autosomes are arranged in pairs from largest to smallest in a karyotype - Males have XY chromosomes - Females have XX chromosomes - The X chromosome is larger than the Y chromosome

BEHAVIOUR OF THE CHROMOSOMES DURING THE DIFFERENT PHASES OF MEIOSIS I - During prophase I - chromosomes pairup/homologous pairs form - Crossing overexchange of genetic material occurs - between chromatids/adjacent chromosome pairs - During metaphase I of meiosis - homologous chromosomes/chromosome pairs are arranged - at the equatorof the cell - in a random way - with the chromosome attached to the spindle fibre - During anaphase I - chromosome pairs separate/chromosomes move to opposite poles - During telophase I - the chromosomes reach the poles of the cell

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EVENTS THAT LEAD TO DOWN SYNDROME - Non-disjunction occurred/A homologous pair of chromosomes - at position 21 - failed to separate - during anaphase - resulting in one gamete with 24 chromosomes / an extra Chromosome /

2 chromosomes at position 21 - The fertilisation of this gamete with a normal gamete/gamete

with 23 chromosomes /1 chromosome at position 21 - results in a zygote with 47 chromosomes - There are 3 chromosomes /an extra chromosome at position 21/ this is Trisomy 21

HOW MEIOSIS CONTRIBUTES TO GENETIC VARIATION Crossing over

- Occurs during prophase I - Chromatids of homologous chromosomes overlap - at points called chiasma/ chiasmata - Genetic material is exchanged - resulting in new combinations of genetic material from both parents

Random arrangement of chromosomes - Occurs during metaphase I / II - Each pair of homologous chromosomes /each chromosome - may line up in different ways on the equator of the spindle - allowing the gametes to have different combinations of maternal and paternal

chromosomes - so that they separate in a random/ independent manner - resulting in new combinations of genetic material

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REPRODUCTION IN VERTEBRATES

EXTERNAL FERTILISATION AND INTERNAL FERTILISATION

External fertilisation: The sperm cell fertilises the egg cell OUTSIDE the body of the female. Water is always required, physical contact not required. E.g. fish, frogs Disadvantages: - Chances of fertilisation is very reduced and large amounts of sperm need to

be produced - Resulting zygote is not well protected from predators

Internal fertilisation: The male deposits the sperm inside the body of the female, where fertilisation occurs inside the female reproductive system. E.g. Birds and mammals Disadvantages: copulatory organ required Advantages: - Chances of fertilisation increased and small amounts of sperm need to be

produced - Resulting zygote is well protected inside the female body.

OVIPARY, OVOVIVIPARY AND VIVIPARY Ovipary- Eggs are laid and development of the embryo takes place outside the mother's body. (with internal or external fertilisation) Disadvantage: Eggs may be preyed on by predators

Ovovivipary- Internal fertilisation and the eggs are retained inside the mother's body and obtain nourishment from the egg yolk. The young hatch inside the mother's body and are then born. Advantages: Increased chance of offspring being produced, well developed offspring produced. Protection from predators.

Vivipary- Internal fertilisation and the young develop inside the uterus of the mother and obtain nourishment from the mother's placenta. Advantages: Increased protection for developing embryo

AMNIOTIC EGG Has extra-embryonic membranes: chorion, amnion, allantois and yolk sac - Chorion: allows oxygen to enter and CO2 to leave through to the shell and out - Amnion: membrane enclosing the amniotic cavity which contains amniotic

fluid which protects the embryo against mechanical damage. - Allantois: stores waste produced by the embryo; also functions in gaseous

exchange - Yolk sac: provides nourishment to the embryo

Shell: prevents dehydration and gives protection

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PRECOCIAL AND ALTRICIAL DEVELOPMENT

Precocial 1. Eyes are open when they hatch 2. Body is covered in soft feathers

called "down" 3. Able to move soon after hatching 4. Able to feed themselves 5. Independent of their parents 6. Eggs provide high energy to

developing embryos

Altricial 1. Eyes are closed when they hatch 2. Bodies are not covered by "down"

3. Unable to move soon after hatching 4. Unable to feed themselves 5. Dependent on parents 6. Eggs provide less energy to

developing embryos

HUMAN REPRODUCTION

NB: MALE & FEMALE REPRODUCTIVE ORGANS (LABELS & FUNCTIONS)

OOGENESIS

- Under the influence of FSH - diploid (germinal epithelium) cells in the ovary - undergo mitosis to form numerous follicles - One of these follicles enlarges and - undergoes meiosis - Of the four cells produced, only one survives to form the haploid ovum

STRUCTURE AND FUNCTION OF OVUM - Haploid nucleus which - fuses with sperm's nucleus to form a diploid zygote - jelly layer which protects the ovum and - makes the ovum impenetrable once fertilisation has occurred - the cytoplasm which provides nourishment (Be able to draw the ovum with labels)

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HORMONAL CONTROL OF THE MENSTRUAL CYCLE (OVARIAN AND UTERINE cycles) - FSH is secreted - By the pituitary gland - FSH stimulates the development of a primary follicle - into a Graafian follicle - As the Graafian follicle develops, it secretes Oestrogen - Which causes the lining of the uterus/ endometrium - To become thicker/more vascular - In preparation for a possible implantation - The pituitary gland - Secretes LH - Which causes the Graafian follicle to rupture, releasing the ovum - This is called ovulation - The empty follicle changes and becomes the corpus luteum - Which begins to secrete progesterone - Which causes further thickening - Of the endometrium - High levels of progesterone - Inhibit the secretion of FSH - Which prevents the development of a new follicle in the ovary - If there is no fertilisation, the corpus luteum degenerates - Which leads to a drop in the progesterone levels - The endometrium disintegrates and is shed during menstruation - If fertilisation occurs, the corpus luteum remains intact - Which leads to high levels of progesterone - To maintain the pregnancy - The secretion of progesterone is eventually taken over by the placenta

Hormonal control of the female reproductive cycle 6

Hormonal changes taking place at A, B, C and D in the graph

PROCESS OF SPERMATOGENESIS AND FORMATION OF SEMEN - Spermatogenesis takes place under the influence of testosterone - In the seminiferous tubules in the testes - Diploid germinal epithelial cells undergo meiosis - to form haploid sperm cells Formation and transport of semen - Sperm mature and are temporarily stored - in the epididymis - During ejaculation - sperm move into the vas deferens - As it passes the seminal vesicles - the prostate gland and - Cowper's glands - fluids are added that provide nutrition - promotes the movement of the sperm - and neutralize the acids produced in the vagina and urethra - The semen passes through the urethra - of the penis - into the vagina - during copulation - and "swims" up the Fallopian tube where it meets the ovum.

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STRUCTURAL SUITABILITY OF THE SPERM FOR FERTILISATION - The front of the head of the sperm cell has an acrosome - which carries enzymes to dissolve a path into the ovum - The nucleus of the sperm - carries genetic material of the male - The middle piece contains mitochondria - which release energy so that the sperm can move - The presence of a long tail - enables sperm to "swim" towards the ovum - The contents of the sperm cell, such as the cytoplasm, is reduced - making the sperm light for efficient movement

PROCESS OF FERTILISATION - In the fallopian tubes - one sperm cell makes contact with the ovum's membrane - The nucleus of the sperm enters the ovum - and the ovum becomes impenetrable to other sperm - The nucleus of the sperm fuses with the nucleus of the ovum - to form a diploid zygote - This is called fertilisation

EVENTS AFTER FERTILISATION - The zygote divides by mitosis many times - to form an embryo - which first consists of a ball of cells - called the morula - The morula then develops into a hollow ball of cells - called the blastula - which embeds itself into the uterine lining/endometrium - using chorionic villi - This structure is called the placenta

EVENTS AFTER IMPLANTATION - Cells of the embryo continue to divide - and differentiate - to form the different organs and limbs - It is now called a foetus - The foetus is enclosed in a sac/membrane called the amnion - filled with amniotic fluid - which protects the foetus against temperature fluctuations, - dehydration and - mechanical injuries/acts as a shock absorber. - The chorionic villi and the endometrium form the placenta - where the blood of the foetus and the mother - run close to each other - allowing nutrients to diffuse into the blood of the foetus

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