Reproduction

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Sexual vs Asexual Reproduction

Sexual reproduction involves two parents and the fusion of gamete nuclei to form a zygote, producing offspring that are genetically different from each other.
Gametes are sex cells (sperm and ovum in animals; pollen nucleus and ovum in plants) with a haploid nucleus – half the chromosome number of body cells.
In humans, body cells have 46 chromosomes; gametes have 23. Fusion of gametes restores the diploid number (46) in the zygote.
Asexual reproduction does not involve gametes or fertilisation; only one parent is required, so offspring are genetically identical (clones).
Many plants reproduce asexually; bacteria reproduce by binary fission, producing exact genetic copies.
Key differences between sexual and asexual reproduction include: number of parents, type of cell division, genetic similarity of offspring, sources of genetic variation, number of offspring, and time taken.

Binary fission in bacteria

Meiosis and Gamete Formation

Meiosis is a form of nuclear division that produces haploid cells from diploid cells, forming gametes in plants and animals.
Meiosis occurs in two successive divisions: meiosis I and meiosis II, each with prophase, metaphase, anaphase and telophase.
Crossing over – non-sister chromatids exchange alleles during meiosis I – and independent assortment – random alignment of homologous pairs during meiosis I – create genetic variation.
Random fertilisation – millions of combinations of sperm and egg – further increases genetic variation.
In males, meiosis occurs in the testes producing spermatozoa; in females, in the ovaries producing ova.
In flowering plants, meiosis occurs in the anthers (male) and ovaries (female); male gametes are in pollen grains, female gametes in ovules.

Meiosis

Male and Female Reproductive Systems

You should be able to draw and annotate diagrams of the female and male reproductive systems and recall the function of each structure.
The female reproductive system includes the ovaries (produce eggs), oviducts (site of fertilisation), uterus (where the embryo implants and develops), cervix and vagina.
The male reproductive system includes the testes (produce sperm and testosterone), epididymis (sperm maturation and storage), sperm duct, prostate gland and urethra.
Human females release only one egg cell per menstrual cycle, whereas a male releases many thousands of sperm cells per ejaculation.
This difference reflects the fact that only one sperm cell can fertilise an egg cell, so many sperm are needed to increase the chance of fertilisation.

The male reproductive system

The Menstrual Cycle

The menstrual cycle is a series of changes in the female body leading up to and following the release of an egg; it averages 28 days and is controlled by hormones.
The endometrium (uterus lining) thickens from day 7 to day 28 in preparation for a fertilised egg.
Ovulation occurs about halfway through the cycle on day 14; the egg is released from the ovary and travels down the oviduct.
Eggs develop inside fluid-filled sacs called follicles; after ovulation the follicle becomes the corpus luteum.
If fertilisation does not occur, the corpus luteum deteriorates, progesterone falls, and the endometrium breaks down causing menstruation (days 1–7).
Four hormones control the cycle: FSH and LH from the pituitary gland; oestrogen and progesterone from the ovaries.
FSH stimulates follicle development and oestrogen secretion; LH triggers ovulation and stimulates progesterone production from the corpus luteum.
Oestrogen causes endometrium thickening and egg maturation; progesterone maintains and thickens the endometrium.

The menstrual cycle

Feedback Mechanisms in the Menstrual Cycle

Positive feedback: oestrogen increases FSH receptors on follicles, making them more receptive to FSH, which stimulates more oestrogen production.
Negative feedback: high oestrogen levels inhibit FSH secretion.
Positive feedback: high oestrogen stimulates LH release from the pituitary, causing ovulation around day 14.
After ovulation, LH causes the follicle wall to develop into the corpus luteum, which secretes more oestrogen (positive feedback).
Negative feedback: progesterone thickens and maintains the endometrium but inhibits FSH and LH secretion from the pituitary.
A fall in progesterone as the corpus luteum deteriorates causes the endometrium to break down, resulting in menstruation.

Hormones and the uterus lining

In Vitro Fertilisation (IVF)

IVF is a treatment for infertility where eggs are fertilised by sperm outside the body in controlled laboratory conditions.
First, the woman takes a drug to inhibit FSH and LH secretion from the pituitary, stopping oestrogen and progesterone secretions and temporarily halting the menstrual cycle.
Then injections of FSH and LH stimulate follicle development; the high FSH concentration causes superovulation – many more follicles than normal mature.
Eggs are collected and fertilised by sperm in sterile laboratory conditions; fertilised eggs develop into embryos.
About 48 hours after fertilisation, one or more embryos (tiny balls of cells) are inserted into the mother's uterus.
Extra progesterone is given to ensure the endometrium is maintained.
The success rate of IVF is low (~30%), but medical advancements are improving it.

A sperm cell and an egg cell

Fertilisation and Preventing Polyspermy

Fertilisation is the fusion of a sperm and egg cell nucleus to give rise to a diploid zygote.
Sperm are attracted to the egg by chemical signals; the first sperm to reach the egg fuses its membrane with the egg cell membrane.
The sperm nucleus enters the egg; vesicles from the egg destroy the sperm tail and mitochondria.
Pronuclei form around the two haploid sets of chromosomes; DNA replication occurs before they fuse and the first mitotic division takes place.
Polyspermy is the entry of more than one sperm into a single oocyte; it is prevented by the acrosome reaction and the cortical reaction.
The acrosome reaction: digestive enzymes from the acrosome digest the glycoproteins of the zona pellucida, allowing the sperm to reach the oocyte membrane.
The cortical reaction: cortical granules release enzymes by exocytosis that digest binding proteins and harden the zona pellucida, preventing further sperm entry.

Gametogenesis: Spermatogenesis and Oogenesis

Gametogenesis is gamete formation involving both mitosis and meiosis; spermatogenesis produces sperm, oogenesis produces ova.
Spermatogenesis occurs in the seminiferous tubules of the testes from puberty onwards; interstitial (Leydig) cells produce testosterone.
Spermatogonia divide by mitosis, differentiate into primary spermatocytes, then undergo meiosis I (secondary spermatocytes) and meiosis II (spermatids).
Spermatids mature into sperm cells associated with Sertoli cells, then detach and pass through the epididymis.
Oogenesis begins in the ovaries of the female foetus before birth; oogonia divide by mitosis for the first 7 months of foetal development.
Oogonia grow and enter meiosis I, surrounded by follicle cells to form primary follicles; oogenesis pauses until puberty.
At puberty, FSH stimulates follicle development; meiosis I produces a secondary oocyte and a polar body; meiosis II completes only after sperm entry.
The secondary oocyte becomes an ovum briefly between the end of meiosis II and fusion of nuclei; a second polar body is produced.

Plant Reproduction: Pollination and Fertilisation

In flowering plants, male gametes are produced in the anther and female gametes in the ovule.
Pollen sacs in the anther contain diploid mother cells that undergo meiosis to form four haploid pollen grains; mitosis produces more male gametes.
A diploid cell in the ovule undergoes meiosis to produce four haploid egg cells; only one survives and undergoes mitosis to form female gametes.
Pollination is the transfer of pollen from the anther to the stigma; it can be by insects (or other animals) or by wind.
Insect-pollinated flowers have colourful petals, scent and nectar to attract pollinators; wind-pollinated flowers have lightweight pollen and feathery stigmas.
After pollination, a pollen tube grows down the style to the ovary; two male nuclei travel down it – one fuses with an ovule nucleus to form the zygote, the other forms the food store.
Fertilisation occurs when haploid male and female nuclei fuse to form a diploid zygote; the ovule becomes the seed and the ovary develops into the fruit.

Structure of an insect-pollinated flower

Preventing Self-Pollination and Seed Dispersal & Germination

Self-pollination results in less genetic variation; cross-pollination improves genetic variation and relies on pollinators or wind.
Self-incompatibility mechanisms prevent self-pollination: pollen may fail to germinate, fail to enter the style, degenerate before fertilisation, or the embryo may degenerate.
Seed dispersal distributes seeds away from the parent plant, reducing competition; methods include wind, water, animals and explosions.
Wind-dispersed seeds are lightweight and parachute- or wing-shaped; animal-dispersed seeds may be in fleshy fruit or have sticky/hooked surfaces.
Germination requires water (swells seed, bursts testa, activates enzymes), oxygen (for respiration) and warmth (optimal enzyme activity).
The radicle emerges first, forming the root; the hypocotyl is the first structure above ground, and leaves appear from the cotyledon for photosynthesis.

Hormones, Puberty and the Placenta

Puberty is triggered by GnRH from the hypothalamus, which stimulates the pituitary to secrete LH and FSH; these enhance the effects of oestrogen and testosterone.
Male secondary sexual characteristics include deepening of the voice, growth of facial and body hair, and increased muscle mass.
Female secondary sexual characteristics include breast development, widening of hips, and growth of body hair.
The placenta is a structure that allows exchange of substances between maternal and foetal blood; it produces hormones including hCG and progesterone.
hCG is secreted in early pregnancy and maintains the corpus luteum, which continues to secrete progesterone to maintain the endometrium.
Substances cross the placenta by diffusion (oxygen, carbon dioxide), facilitated diffusion (glucose), osmosis (water) and endocytosis (antibodies).
Chorionic villi increase in number throughout pregnancy to increase surface area for exchange.

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練習問題

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1.Which of the following best defines a gamete?
Easy
- AA diploid cell containing two copies of each chromosome
- BA haploid sex cell such as a sperm or ovum
- CA fertilised egg cell containing 46 chromosomes
- DA body cell that divides by mitosis
2.A human body cell contains 46 chromosomes. How many chromosomes are present in a human sperm cell?
Easy
- A92
- B46
- C23
- D12
3.Which process produces genetically identical offspring from a single parent without the fusion of gametes?
Medium
- ASexual reproduction
- BMeiosis
- CAsexual reproduction
- DFertilisation
4.Which statement correctly describes crossing over during meiosis?
Medium
- AHomologous pairs of chromosomes line up randomly at the equator during meiosis I
- BNon-sister chromatids exchange alleles during meiosis I
- CSperm and egg cells fuse randomly during fertilisation
- DSister chromatids separate during meiosis II
5.Which of the following are mechanisms that lead to genetic variation in the gametes produced by sexual reproduction? (select all that apply)
Medium
- ACrossing over
- BBinary fission
- CIndependent assortment
- DRandom fertilisation
- EMitosis
6.In humans, each gamete contains 23 chromosomes, while a zygote contains 46 chromosomes.
Easy
True or false?
7.Asexual reproduction produces offspring that are genetically identical to each other and to the parent.
Easy
True or false?
8.Match each flower structure with its function.
Medium
- Sepal
- Petal
- Anther
- Stigma
- Protects the developing flower while inside the bud
- Is colourful to attract pollinators
- Produces the male gametes (pollen)
- Receives pollen during pollination

もっと問題

9.Arrange the stages of the cortical reaction in the correct order.
Medium
- Sperm nucleus enters the oocyte and meiosis II completes
- Cortical granules move from the cytoplasm to the cell surface membrane
- Enzymes are released by exocytosis
- Binding proteins on the ovum surface are digested and the zona pellucida hardens
- No further sperm can enter the ovum
10.Match each substance transported across the placenta with its transport mechanism.
Medium
- Carbon dioxide and oxygen
- Glucose
- Antibodies
- Water
- diffusion
- facilitated diffusion
- endocytosis
- osmosis
11.Arrange the events of the acrosome reaction and fertilisation in the correct sequence.
Medium
- Sperm releases digestive enzymes from the acrosome
- Sperm digests glycoproteins of the zona pellucida
- Sperm membrane fuses with the oocyte cell surface membrane
- Sperm nucleus enters the oocyte
- Meiosis II completes and the second polar body is released
12.Which of the following are roles of progesterone in the menstrual cycle? (select all that apply)
Medium
- AThickens and maintains the endometrium
- BInhibits secretion of FSH and LH from the pituitary gland
- CStimulates ovulation on day 14
- DCauses the endometrium to break down when its level falls
- EStimulates the development of immature egg cells in follicles
13.Which of the following statements about the placenta are correct? (select all that apply)
Medium
- AIt is primarily made of finger-like projections called chorionic villi
- BIt produces hormones such as hCG and progesterone
- CIt allows maternal and fetal blood to mix directly
- DThe number of villi increases throughout pregnancy
- EIt transports oxygen from the fetus to the mother
14.Which of the following are required for successful seed germination? (select all that apply)
Medium
- AWater
- BOxygen
- CWarmth
- DLight
- ESoil
15.Which of the following are advantages of asexual reproduction? (select all that apply)
Medium
- AOffspring are genetically identical to the parent
- BOnly one parent is required
- CIt produces genetic variation in the offspring
- DIt is rapid and can produce many offspring quickly
- EIt requires the fusion of gametes
16.Which hormone is produced by the corpus luteum after ovulation?
Medium
- AProgesterone
- BFSH
- CLH
- DOestrogen
17.What is the role of LH in the menstrual cycle?
Medium
- AIt causes ovulation and stimulates the corpus luteum to produce progesterone
- BIt stimulates the development of immature egg cells in follicles
- CIt causes the endometrium to break down during menstruation
- DIt inhibits the secretion of FSH from the pituitary gland
18.In IVF, why is medication containing FSH given to the woman?
Medium
- ATo stimulate the development of multiple follicles (superovulation)
- BTo prevent ovulation from occurring
- CTo cause the endometrium to break down
- DTo inhibit the secretion of LH
19.A fertility clinic advises women with a BMI greater than 30 kg/m² to lose weight before IVF. Which statement best explains this advice based on the data?
Medium
- AWomen with BMI >30 have a lower percentage of successful implantation and healthy baby take-home
- BWomen with BMI >30 have a higher percentage of successful implantation
- CBMI has no effect on IVF success
- DWomen with BMI <18 have the lowest success rates
20.The cortical reaction involves the release of enzymes from cortical granules that harden the zona pellucida and prevent polyspermy.
Easy
True or false?
21.In flowering plants, the pollen tube grows from the stigma down the style to the ovary, and two male nuclei travel down it.
Medium
True or false?
22.Arrange the stages of the menstrual cycle in the correct order, starting from day 1.
Medium
- Menstruation occurs
- Endometrium thickens
- Ovulation occurs
- Corpus luteum forms and secretes progesterone
- Progesterone levels fall and endometrium breaks down
23.A human cell in the germinal epithelium of a seminiferous tubule is observed to contain 46 chromosomes and is about to divide by meiosis. After meiosis I, how many chromosomes would be present in each secondary spermatocyte?
Medium
- A46
- B23
- C92
- D12
24.A student counts the cells in a cross-section of a seminiferous tubule and records the following: 120 spermatogonia, 60 primary spermatocytes, 30 secondary spermatocytes and 60 spermatids. If each primary spermatocyte divides by meiosis to form four spermatids, what is the expected number of spermatids if all 60 primary spermatocytes completed meiosis?
Hard
- A60
- B120
- C240
- D480
25.The graph of testosterone levels in males and females from before birth to old age shows a sharp rise in males beginning at around age 12–14. Which of the following best explains the difference between males and females at this point?
Medium
- AFemales produce no testosterone at any age
- BMales produce a surge of testosterone at puberty, while females maintain lower, relatively stable levels
- CTestosterone levels in females peak before birth and then decline to zero
- DMales and females have identical testosterone profiles throughout life
26.The graph of reproductive hormones A–D during a menstrual cycle shows line A rising steeply and peaking just before day 14, then falling sharply. Which hormone does line A represent, and why?
Medium
- AProgesterone, because it peaks just before ovulation
- BOestrogen, because it peaks just before ovulation and triggers the LH surge
- CFSH, because it peaks at ovulation and causes the corpus luteum to form
- DLH, because it peaks just before ovulation and causes the endometrium to thicken
27.A fertility clinic's data show that women with a BMI greater than 30 kg/m² have a lower percentage of successful implantation, ongoing pregnancy and live birth following IVF. Which statement best explains this advice?
Hard
- AHigh BMI reduces the number of eggs collected during IVF
- BHigh BMI is associated with lower success rates at each stage of IVF, so losing weight may improve the chance of a healthy baby
- CHigh BMI increases the risk of polyspermy during fertilisation
- DHigh BMI prevents the cortical reaction from occurring
28.During IVF, a woman is given a drug that inhibits the secretion of FSH and LH from the pituitary gland. What is the main purpose of this step?
Medium
- ATo increase the number of eggs released naturally
- BTo temporarily halt the menstrual cycle so doctors can control the timing and quantity of egg production
- CTo stimulate the endometrium to thicken before implantation
- DTo prevent the corpus luteum from forming
29.Which of the following statements about the cortical reaction are correct? (select all that apply)
Hard
- AIt involves the release of enzymes from cortical granules by exocytosis
- BIt causes the zona pellucida to harden
- CIt breaks down binding proteins on the ovum cell surface membrane
- DIt allows multiple sperm to enter the ovum
- EIt occurs before the acrosome reaction
30.A student is comparing spermatogenesis and oogenesis. Which of the following are similarities between the two processes? (select all that apply)
Medium
- ABoth begin with cells in the germinal epithelium
- BBoth involve mitosis and meiosis
- CBoth produce four mature gametes from each starting cell
- DBoth produce haploid gametes
- EBoth are completed before birth in humans
31.In humans, the secondary oocyte completes meiosis II before ovulation.
Medium
True or false?
32.Arrange the stages of oogenesis in the correct order, starting from the earliest event.
Medium
- Oogonia divide by mitosis in the germinal epithelium
- Oogonia grow and enter meiosis I, forming primary follicles
- FSH stimulates primary follicles to continue development
- Meiosis I produces a secondary oocyte and a polar body
- The secondary oocyte is released at ovulation
33.Which factor is the independent variable in this investigation?
Medium
- ANumber of germinated seeds
- BTemperature
- CVolume of water
- DTime
34.The results show that germination is highest at 25°C and lower at 5°C and 35°C. Which statement best explains this pattern?
Medium
- AEnzymes controlling germination work best at an optimum temperature
- BSeeds require light to germinate
- CWater is not needed at high temperatures
- DOxygen is only available at 25°C
35.A student sets up four test tubes, each containing 20 cress seeds on damp cotton wool. The tubes are placed in different conditions for 5 days. Which tube is the correct control to show that water is required for germination?
Medium
- ASeeds on dry cotton wool, kept at 25 °C with oxygen available
- BSeeds on damp cotton wool, kept at 5 °C with oxygen available
- CSeeds on damp cotton wool, kept at 25 °C with no oxygen
- DSeeds on damp cotton wool, kept at 25 °C with oxygen available
36.A student investigates germination of dandelion seeds at different temperatures. They place 50 seeds in each of five petri dishes containing damp filter paper and incubate them at 5 °C, 15 °C, 25 °C, 35 °C and 45 °C. After 7 days they count the number of germinated seeds. Which variable is the dependent variable?
Medium
- AThe number of seeds that germinate
- BThe temperature of the incubator
- CThe number of seeds placed in each dish
- DThe volume of water added to the filter paper
37.A student investigates the effect of temperature on germination of bean seeds. They set up five petri dishes, each with 30 seeds on damp cotton wool, and place them at 5 °C, 15 °C, 25 °C, 35 °C and 45 °C. After 6 days, the numbers of germinated seeds are 8, 18, 27, 21 and 9 respectively. Which conclusion is best supported by these data?
Medium
- AGermination is highest at 25 °C and decreases at both lower and higher temperatures.
- BGermination increases steadily as temperature rises from 5 °C to 45 °C.
- CGermination is unaffected by temperature between 5 °C and 45 °C.
- DGermination is highest at 5 °C and decreases as temperature rises.
38.A student investigates the effect of light on germination of lettuce seeds. They set up two groups: one in continuous light and one in continuous darkness. Both groups have 50 seeds on damp filter paper at 20 °C. After 5 days, 44 seeds germinate in the light and 45 seeds germinate in the dark. Which statement is the most valid conclusion?
Hard
- ALight does not significantly affect germination of lettuce seeds under these conditions.
- BLight is required for germination of lettuce seeds.
- CLight inhibits germination of lettuce seeds.
- DDarkness is required for germination of lettuce seeds.
39.In an investigation into seed dispersal, students measured the mass and wing length of sycamore seeds and the distance each seed travelled from the parent tree. They found that seeds with longer wings travelled further. Which is the most likely explanation for this result?
Hard
- ALonger wings increase air resistance and allow seeds to stay airborne longer, so they are carried further by wind.
- BLonger wings make seeds heavier, so they fall faster and land closer to the parent tree.
- CLonger wings attract animals that carry the seeds further away.
- DLonger wings store more food, giving seedlings more energy to grow.
40.A student is given a sample of seeds and asked to determine whether they are dispersed by wind or by animals. Which observation would provide the strongest evidence that the seeds are dispersed by wind?
Hard
- AThe seeds are lightweight and have wing-like structures.
- BThe seeds are brightly coloured and have a fleshy outer layer.
- CThe seeds have hooks that catch onto fur.
- DThe seeds are large and heavy with a hard coat.
41.Complete the sentence about gametes and fertilisation.
Easy
A gamete has a ____ nucleus, and when two gametes fuse they form a zygote with a ____ nucleus.
42.Name the type of nuclear division that produces haploid cells from diploid cells during the formation of gametes.
Easy
43.Complete the sentence about fertilisation in flowering plants.
Medium
After pollination, a pollen tube grows down the ____ to the ovary, and the male nuclei travel to the ____ where fertilisation occurs.
44.How many chromosomes are present in a human zygote?
Easy
45.Name the structure that produces testosterone in the male reproductive system.
Medium
46.Give the name of the process by which a seed starts to grow.
Medium
47.A human zygote is formed by the fusion of a sperm and an egg. How many chromosomes are present in the zygote?
Medium
48.Complete the sentence about the acrosome reaction.
Medium
During the acrosome reaction, the sperm releases ____ enzymes from the acrosome to digest the ____ of the zona pellucida.
49.Name the structure from which the sperm releases digestive enzymes during the acrosome reaction.
Medium
50.At 25°C, 45 out of 50 seeds germinated. Calculate the percentage germination.
Medium
51.The average menstrual cycle is 28 days long. On which day of the cycle does ovulation normally occur?
Easy
52.A human body cell contains 46 chromosomes. How many chromosomes are present in a human ovum?
Easy
53.Describe the term 'placental mammal'.
Medium
54.Explain how the structure of an insect-pollinated flower is adapted to attract insects and ensure pollination.
Hard
55.Describe the steps of the cortical reaction and explain how it prevents polyspermy.
Hard
56.Outline the events that occur during days 14–28 of the menstrual cycle in the absence of fertilisation, including the hormones involved.
Hard
57.Explain why the number of chorionic villi in the placenta increases throughout pregnancy.
Medium
58.How many chromosomes are present in a human zygote?
Easy
- A51 chromosomes
- B46 chromosomes
- C41 chromosomes
- D56 chromosomes
59.A human zygote is formed by the fusion of a sperm and an egg. How many chromosomes are present in the zygote?
Medium
- A51 chromosomes
- B56 chromosomes
- C41 chromosomes
- D46 chromosomes
60.At 25°C, 45 out of 50 seeds germinated. Calculate the percentage germination.
Medium
- A81 %
- B90 %
- C99 %
- D108 %
61.The average menstrual cycle is 28 days long. On which day of the cycle does ovulation normally occur?
Easy
- A16 day
- B14 day
- C13 day
- D15 day
62.A human body cell contains 46 chromosomes. How many chromosomes are present in a human ovum?
Easy
- A25 chromosomes
- B27 chromosomes
- C23 chromosomes
- D21 chromosomes

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