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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 MITOSIS

• Meiosis involves 2 cell divisions • Mitosis involves 1 cell division

• Meiosis gives rise to sex cells • Gives rise to somatic cells

• Cells produced are haploid • Cells produced are diploid

• Differs according to gender • Has no gender specification

• 2 phases • 1 phase

• 4 daughter cells formed • 2 daughter cells

• Half the number of chromosomes to the parent cell

• Same number of chromosomes formed as in parent cell

• All cells formed are genetically different to each other and parent cell

• Genetically identical to each other and parent cell

• Cells responsible for genetic variation in reproduction (gametes)

• Cells responsible for growth and repair (somatic cells)

DIFFERENCES BETWEEN MEIOSIS I AND MEIOSIS II

MEIOSIS I MEIOSIS II

• Crossing over occurs in prophase 1 • No crossing over occurs

• Chromosomes arranged at the equator in homologous pairs in metaphase 1

• Chromosomes arranged singly at the equator in metaphase 2

• Homologous chromosome pairs separate

• Sister chromatids separate

• Whole chromosomes are pulled to opposite poles in anaphase 1

• Chromatids are pulled to opposite poles in anaphase 2

• 2 diploid daughter cells produced • 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 Altricial

1. Eyes are open when they hatch 1. Eyes are closed when they hatch

2. Body is covered in soft feathers

called “down”

2. Bodies are not covered by “down”

3. Able to move soon after hatching 3. Unable to move soon after hatching

4. Able to feed themselves 4. Unable to feed themselves

5. Independent of their parents 5. Dependent on parents

6. Eggs provide high energy to

developing embryos

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

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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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- The umbilical vein carries the absorbed nutrients from the mother to the

foetus

- The umbilical artery

- carries all the waste away from the foetus to the mother's blood

GASEOUS EXCHANGE AND NOURISHMENT OF THE FOETUS IN HUMANS

- In the placenta, the mother’s blood comes into close contact with the foetal

blood,

- Oxygen and nutrients diffuse from the mother’s blood into the foetal blood in

the umbilical veins

- This nutrient rich blood is carried to the foetus through the umbilical cord

- CO2 diffuses from the foetal blood in the umbilical artery into the maternal

blood

HUMAN NERVOUS SYSTEM

CENTRAL NERVOUS SYSTEM (BRAIN & SPINAL CHORD) Position & Functions of the following parts:

AUTONOMIC NERVOUS SYSTEM - Every organ/gland is controlled by two sets of nerves/double innervation - that act antagonistically - to control involuntary events - Sympathetic nerves - generally, stimulates a response - Parasympathetic nerves - generally, inhibit a response

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REFLEX ACTION AS A RESULT OF A REFLEX ARC (e.g. finger & flame) - Pain receptors in the skin of the finger (MAKE APPLICABLE TO ANY OTHER EXAMPLE) - convert the stimulus - into an impulse - which is transmitted to the spinal cord - via the sensory neuron - which makes synaptic contact with the - connector neuron - Connector neuron transmits the impulse via - the motor neuronto the - effector/muscles of the arm/hand - causing them to contract - pulling the finger away from the flame

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SENSE ORGANS (Structure of the eye and the ear & functions of parts) THE EYE HOW HUMANS RECEIVE AND INTERPRET LIGHT STIMULI (able to see) - Light enters the eye - through the cornea - which refracts light - It then passes through the aqueous humour - and the pupil - The size of the pupil is adjusted by the iris - to regulate the amount of light that enters the eye - The light then passes through the lens - which further refracts the light - It then passes through the vitreous humour and - reaches the retina - which has the photoreceptors/ rods and cones which convert the light into a nerve impulse - The impulse is sent via the optic nerve - to the cerebrum of the brain - where the image is interpreted PUPIL MECHANISM IN: BRIGHT LIGHT - The circular muscles of the iris contract and - the radial muscles relax - causing the pupil to constrict/ smaller pupil diameter - so that less light enters the eye DIM LIGHT - The radial muscles of the iris contract and - the circular muscles relax - causing the pupil to dilate/ larger pupil diameter - so that more light enters the eye

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ACCOMODATION of the eye (objects further than 6m) - Ciliary muscles relax - Suspensory ligaments become taut - Tension on the lens increases - Lens becomes less convex - Refractive power of the lens decreases - A clear image falls on the retina ACCOMODATION of the eye (objects closer than 6m) - Ciliary muscles contract - Suspensory ligaments slacken - Tension on the lens decreases - Lens becomes more convex - Refractive power of the lens increases - A clear image falls on the retina

STRUCTURAL SUITABILITY OF THE LENS - Lens is elastic - therefore, can change shape/convexity change to allow for accommodation - Lens is transparent - to allow light rays to pass through - Lens is biconvex - to refract light rays

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VISUAL DEFECTS

THE EAR HEARING - Sound waves are directed by the pinna - through the auditory canal - to the tympanic membrane - causing it to vibrate - The vibrations are transferred to the ossiclesin the middle ear - which causes the oval window to vibrate - This sets up pressure waves in the perilymph - Pressure waves are then transferred to the endolymph in the cochlea - This stimulates the Organ of Corti - in the cochlea - to convert the stimulus into an impulse - which is then transported along the auditory nerve - to the cerebrum - where the sound is interpreted

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BLOCKED EUSTACHIAN TUBE - Air will not be taken in - to equalise pressure - on both sides of the tympanic membrane - Tympanic membrane/ossicles may not vibrate freely - This may lead to the tympanic membrane bursting and - could lead to hearing loss /deafness/ pain AMPLIFICATION OF SOUND

BALANCE - Changes in the direction and speed of movement of the head - causes the endolymph to move in the semi-circular canals - The cristae - found in the ampulla are stimulated and - convert the stimulus into an impulse - which is transmitted via the auditory nerve - to the cerebellum from which - impulses are transmitted via motor neurons - to the skeletal muscles/effector to restore balance to the body - Changes to the position of the head. - causes the endolymph to move in the semi-circular canals - which stimulates the macula - found in the utriculus and sacculus to - convert the stimulus into an impulse - which is transmitted via the auditory nerve - to the cerebellum from which - impulses are transmitted via motor neurons - To the skeletal muscles/ effector to restore balance to the body

HEARING DEFECTS

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HUMAN ENDOCRINE SYSTEM DIFFERENCES BETWEEN AN ENDOCRINE AND EXOCRINE GLAND - Exocrine glands release their secretions into ducts (glands with ducts) - Endocrine glands release their secretions directly into the blood stream (ductless glands) DEFINITION OF A HORMONE - It is a chemical messenger - that is protein in nature - Secreted by glands, but - act on target organs (far from the glands)

THE HUMAN ENDOCRINE SYSTEM

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FUNCTIONS/EFFECT OF ADRENALIN - Increases the heart rate – to supply more O2 and glucose and to remove CO2 - Increases the blood pressure - to supply more O2 and glucose and to remove CO2 to the tissues - Stimulates the conversion of glycogen to glucose by the liver so that more glucose is available for energy ( through cellular respiration) - Increases blood supply to the heart, brain and skeletal muscles to ensure effective functioning of these organs - Decreases blood flow to the digestive system to conserve blood for the vital organs - Decreases blood flow to the skin to conserve blood for the vital organs - Increases rate and depth of breathing to supply more O2 and glucose and to remove CO2 to the tissues - Increases the rate of metabolism/ respiration to provide more energy for the cells - Causes pupils to dilate, to increases visual awareness

HOMEOSTASIS CONTROL OF CO2 (HOMEOSTASIS) - Receptor cells - In the carotid artery/aorta are stimulated - to send impulses to the medulla oblongata in the brain - which then stimulates the heart - to beat faster - and the breathing muscles (diaphragm, intercostal and abdominal) - to contract more actively - thus, increasing the rate and depth of breathing - More CO2 is taken to and exhaled from the lungs - The blood CO2 returns to normal CONTROL OF BLOOD GLUCOSE (HOMEOSTASIS) HIGH GLUCOSE LEVELS - When blood glucose levels rise above normal - the pancreas/ islets of Langerhans are stimulated to - secrete more insulin into the blood - which travels to the liver/ muscle cells - and stimulates them to absorb more glucose from the blood - and convert the excess glucose to glycogen - which decreases the blood glucose levels LOW GLUCOSE LEVELS - When blood glucose levels drop below normal - the pancreas/ islets of Langerhans are stimulated to - secrete more glucagon into the blood - which travels to the liver/ muscle cells - and stimulates them to convert glycogen to glucose - which increases the blood glucose levels

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NEGATIVE FEEDBACK BETWEEN TSH and THYROXIN - When thyroxin levels are low - the pituitary gland is stimulated - to secrete more TSH - which stimulates the Thyroid gland to secrete - more thyroxin - which causes an increase in metabolic rate - to increase energy production - When thyroxin levels are high - the pituitary gland - secretes less TSH - which causes the thyroid gland to secrete - less thyroxin (Also learn Negative feedback between Insulin & Glucagon as well as between Progesterone & FSH)

OSMOREGULATION (low water levels in blood) - Due to dehydration/exercise/ sweating - osmoreceptors in the hypothalamus are stimulated - An impulse is sent to the pituitary gland - and more ADH is secreted - The ADH causes walls of the renal tubules - to become more permeable to water - More water is reabsorbed - and the blood volume increases - Less urine is produced - and the urine is more concentrated OSMOREGULATIO FOR high water levels in blood - just the opposite reactions

SALT BALANCE and the role of ALDOSTERONE - When sodium (salt) levels are low in the blood - the adrenal glands are stimulated - to secrete more aldosterone - which causes the walls of the renal tubules - to become more permeable - allowing for a greater reabsorption of sodium ions - from the filtrate into the blood capillaries - and a decreased excretion of sodium ions - from the blood into the filtrate FOR HIGH SALT CONCENTRATION - just the opposite reactions

THERMOREGULATION (during exercise or high environmental temperatures) - Increased respiration (as a result of exercise) - causes increased body temperature - Hypothalamus is stimulated which - sends impulses to the muscle layer in the walls of the skin's blood vessels - This leads to the dilation of blood vessels/vasodilation - and more blood flows to the skin - More heat is lost by radiation - More blood flows to the sweat glands - which become more active/produce more sweat - to lower the body temperature THERMOREGULATION (in a cold environment - opposite reactions)

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PLANT RESPONSES TO STIMULI FROM THE ENVIRONMENT PLANT HORMONES FUNCTIONS OF AUXINS - Responsible for photo- and geotropism/gravitropism - Promote cell division - Promote cell elongation - Responsible for apical dominance - by inhibiting lateral branch growth - and promoting growth in the apical buds - Promotes root development FUNCTIONS OF GIBBERELLINS - Bring about elongation of internodes - Stimulates root growth - Promote development of flowers - Stimulates the germination of seeds - Increases fruit size FUNCTIONS OF ABSCISIC ACID - Causes seed dormancy - Promotes the aging and falling of leaves and fruit - Causes closing of stomata during drought periods

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PHOTOTROPISM - Auxins are sensitive to light - Light stimulus from one side (unilateral light) causes auxins to move to the - shaded side/destroyed on the illuminated side - Auxin concentration is higher on the shaded side - This promotes cell elongation on the shaded side of the plant - Resulting in more growth on this side - The stem grows towards the light stimulus - This is called phototropism NEGATIVE GEOTROPISM/GRAVITROPISM (STEMS) (for a stem placed horizontally) - Auxins will accumulate on the lower side of the stem - as they are attracted by gravity - and stimulate cell elongation on the lower side - while the upper side grows slowly - causing the stem to grow upward POSITIVE GEOTROPISM/GRAVITROPISM (ROOTS) (for a root placed horizontally) - Auxins will accumulate on the lower side of the root - as they are attracted by gravity - and inhibit cell elongation on the lower side - while the upper side grows faster - causing the root to grow downward

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HUMAN IMPACT ON THE ENVIRONMENT

USE THE MINDMAP ON THIS TOPIC TO UNDERSTAND THE "BIG PICTURE"

KNOW THE TERMS WITH DEFINITIONS IN THIS TOPIC

BE ABLE TO INTERPRET EXTRACTS, GRAPHS, TABLES TO EXPLAIN CONCEPTS

EFFECT OF SEWAGE/FERTILISERS ON WATER QUALITY - The sewage/fertiliser contains nutrients - The amount of nutrients in the water increases/eutrophication occurs - causing an algal bloom - The algae covers the surface of the water and - blocks out the sunlight - causing the water plants to die - thereby reducing photosynthesis - Aquatic animals also die due to a lack of food - This increases decomposition (of plants and animals) - which reduces the oxygen content in the water FOOD SECURITY

DEFORESTATION LEADS TO GLOBAL WARMING