Unit 11: Reproduction

Question 1

What are the differences between sexual and asexual reproduction

Answer 1

asexual: 1 parent, mitosis is used, offspring are genetically identical, existing gene combos are maintained (no genetic variation), organisms adapted to an unchanging environment produce offspring that are also adapted

sexual: 2 parents, meiosis is used once per generation, offspring are genetically different to parents, new gene combos are made (genetic variation generated), organisms better adapted to a changing environment

Question 2

What is the role of meiosis and fusion of gametes in the sexual life cycle?

Answer 2

MEIOSIS: During meiosis, the number of chromosomes HALVES. Parental combos of genes are broken up, allowing new combos to form when gametes fuse.

FERTILIZATION (fusion of gametes (haploid cells)): DOUBLES the number of chromosomes each time it occurs.

Question 3

What are the differences between male and female sexes in sexual reproduction?

Answer 3

MALE: male gametes travel to female, smaller gametes, less food reserves, more numbers

FEMALE: female gametes are sessile, larger, have more food reserves (enough for embryo development), and only a few produced/released

Question 4

What is the function of a testis?

Answer 4

produces sperm and testosterone

Question 5

What is the function of a scrotum?

Answer 5

maintains lower temperature than core body temperature

Question 6

What is the function of a epididymis?

Answer 6

coiled section at the back of the testis, stores sperm until ejaculation

Question 7

What is the function of a sperm duct?

Answer 7

transports sperm during ejaculation

Question 8

What is the function of a seminal vesicle and a prostate gland?

Answer 8

secrete fluid containing alkali, proteins and fructose, which is added to sperm to make semen

Question 9

What is the function of a urethra?

Answer 9

transfers semen during ejaculation and urine during urination

Question 10

What is the function of a penis?

Answer 10

penetrates the vagina for ejaculation of semen near the cervix

Question 11

What is the function of an ovary?

Answer 11

produces eggs, oestradiol and progesterone

Question 12

What is the function of a oviduct?

Answer 12

collects eggs at ovulation, provides a site for fertilisation then moves the embryo to uterus

Question 13

What is the function of a uterus?

Answer 13

provides the needs of the embryo and then foetus during pregnancy

Question 14

What is the function of a cervix?

Answer 14

protects foetus during pregnancy and then dilates to provide a birth canal

Question 15

What is the function of a vagina?

Answer 15

provides stimulation to the penis to cause ejaculation, provides a birth canal

Question 16

What is the function of a vulva?

Answer 16

protects internal parts of the female reproductive system

Question 17

Describe changes during the ovarian and uterine cycles

Answer 17

Ovarian + Uterine cycle = Menstrual cycle
(see diagram)

OVARIAN:
1. Follicular phase - when a group of follicles develop in the ovary, then most developed follicle breaks open (ovulation) and releases egg into oviduct
2. Luteal phase - wall of the follicle that released the egg develops into the corpus luteum. if no fertilization, the corpus luteum breaks down to the corpus albicans and the ovary returns to follicular phase

UTERINE: (changes that occur to the endometrium)
1. endometrium becomes thick + more richly supplied w/ blood during the LUTEAL phase
2. if no embryo, thickening starts to break down at the END of the LUTEAL phase
3. this material is shed during menstruation

Question 18

What is FSH?

Answer 18

Follicle Stimulating Hormone (gonadotropin = cause changes to gonads)
- RISES to a peak towards end of follicular phase
- stimulates development of follicles
- stimulates OESTRADIOL secretion by the follicle wall

Question 19

What is oestradiol?

Answer 19

• RISES to a peak towards the end of the follicular phase
• stimulates repair and thickening of the endometrium after menstruation
• stimulates increase in FSH receptors –> make follicles more receptive to FSH, boosting oestradiol production
• if high level, oestradiol inhibits FSH secretion and stimulates LH secretion

Question 20

What is LH?

Answer 20

Luteinising Hormone (gonadotropin = cause changes to gonads)
- RISES to a sharp peak towards the end of follicular phase
- stimulates completion of meiosis in the oocyte and partial digestion of the follicle wall, allowing it to burst open at ovulation
- promotes post-ovulation development of the corpus luteum follicle wall
- corpus luteum secretes oestradiol (positive feedback) and progesterone

Question 21

What is progesterone?

Answer 21

• RISES at the start of the luteal phase, reaches a peak and then drops back to a low level by the end of this phase
• promotes thickening and maintenence of the endometrium
• inhibits FSH and LH secretion by the pituitary gland (negative feedback)

Question 22

Describe what occurs during human fertilisation

Answer 22

1. Plasma membranes of sperm have receptors that detect chemicals released by the egg
   
   • enables directional swimming of sperm
     towards the egg
2. Egg is surrounded by a cloud of follicle cells and a layer of glycoproteins
3. Sperm pushes between cells and digests its way through the glycoproteins to reach plasma membrane of the egg cell
4. Sperm’s plasma membrane has proteins that bind to the egg cell’s plasma membrane
5. First sperm to penetrate the zona pellucida binds and the membranes of sperm and egg fuse together
6. Sperm nucleus enters the egg cell = moment of FERTILIZATION

Question 23

Describe what happens post human fertilization

Answer 23

1. layer of glycoprotein around egg hardens to prevent any other sperm from entering, ensuring a diploid zygote is produced, rather than an unviable cell with more than 2 sets of chromosomes
2. sperm tail either does not penetrate the egg or is broken down inside the zygote
   sperm mitochondria are usually all destroyed
3. nuclei from both sperm + egg separate until zygote’s first mitosis
   nuclear membranes break down, releasing 23 chromosomes from each nucleus
4. these chromosomes participate jointly in mitosis using the same spindle of microtubules
5. 2 genetically identical nuclei are produced, each with 46 chromosomes.

Question 24

Describe the process of IVF treatment

Answer 24

IVF = in vitro fertilisation (outside of the body)

STAGE 1: DOWN-REGULATION
1. every day, woman has injection or nasal spray containing a drug that suppresses the production of FSH or LH in the pituitary gland, therefore also stopping production of oestradiol and progesterone
2. Intramuscular injections of FSH are then given daily for 7-12 days, stimulating follicles to develop
- aims to generate a much higher FSH concentration than during a normal menstrual cycle, therefore more follicles will develop (8-15)
3. When follicles are 18mm in diameter, they are stimulated to mature by an injection of hCG (human chorionic gonadotropin) = SUPEROVULATION
- normally secreted by a human embryo when it is about a week old, signalling to its mother that it is alive and in need of sustenance from the endometrium

STAGE 2: EGG COLLECTION & FERTILISATION
4. egg collection is performed 34-35 hours after hCG injection (takes about 20 mins)
5. each egg is mixed w/ 50k-100k sperm cells in sterile conditions in a shallow dish, then incubated at 37ºC.
6. if fertilization is successful, one or more embryos are placed in the uterus when they are about 48 hours old.
7. extra progesterone is given as a table placed in the vagina to ensure the endometrium is maintained.
8. if embryo implants = basically a regular pregnancy

Question 25

What is pollination?

Answer 25

transfer of pollen from an anther to a stigma, usually through wind or by animals

Question 26

How does fertilisation occur in plants?

Answer 26

1. from each pollen grain on the stigma, a tube grows down the style to the ovary
2. the pollen tube carries male gametes near its growing tip
3. once inside the ovary, the pollen tube grows towards one ovule and into one end of it
4. when the pollen tube reaches the centre of the ovule where the female gamete (egg) is located, the male gametes are released and fertilization occurs.
   - produces a zygote, which develops into an embryo with an embryo root, embryo shoot and either 1 or 2 embryo leaves (cotyledon)

Plants may be hermaphrodites (meaning they can be both male and female parents)

Question 27

Describe the features of an insect pollinated flower

Answer 27

• Flowers have large, brightly coloured petals that advertise the flower, act as a landing stage and guide the insect’s movements to the anther or stigma
• Scent is secreted from the petals to advertise the flower
• Pollen grains are large and spiky so they stick to insects and are attractive as a protein-rich food
• Stigma is large and sticky to collect pollen from visiting insects
• Glands (nectaries) secrete a sugar solution (nectar) that is attractive to insects as an energy source
  -> positioned deep inside the flower so insects can only reach them by brushing past the anthers and stigma

Question 28

Describe the methods of promoting cross-pollination

Answer 28

Cross pollination is the transfer of pollen from an anther in a flower on one plant to the stigma of a flower on another plant.

• leads to fusion of male and female gametes from different plants, so promotes genetic variation and therefore evolution.
• cross pollination also promotes “hybrid vigour” (the improved or increased function of any biological quality in a hybrid offspring)
• The offspring of crosses between genetically unrelated plants tend to be healthy and grow strongly.
  -> may be due to having different alleles of many genes rather than to being homozygous

Question 29

Describe some strategies to prevent self pollination (with examples)

Answer 29

1. adaptations to facilitate transfer of pollen from one plant to another by an outside agent (i.e. vanilla orchid)
2. separation of anthers and stigmas/styles/ovaries in separate male and female flowers on the same plant (i.e. paper birch)
3. separation of anthers and stigmas/styles/ovaries in separate male and female flowers on different plants (i.e. ginkgo tree)
4. anthers and stigmas maturing at different times (protandry = anthers first, protogyny = stigma first) (i.e. foxglove [protandrous], sacred lotus [protogynous])

Question 30

What are the consequences of self-pollination, and describe the self-incompatibility mechanisms and their consequences

Answer 30

Self pollination leads to inbreeding, which decreases genetic diversity and vigour.

Plants with the same self-incompatibility alleles cannot successfully pollinate each other.

However, there are consequences of self-compatibility mechanisms for fruit farmers. Planting a single variety of plants may result in little or no fruit, but if there are two varieties in an orchard that have different alleles of self-incompatibility genes, abundant crops are produced.

Question 31

What is the difference between seed dispersal and pollination?

Answer 31

Pollination occurs when pollen is transferred from the male part of a flower to the female part of the same or a different flower. Seed dispersal is the movement of seeds away from the parent plant.

Question 32

What are some seed dispersal characteristics?

Answer 32

• dry and explosive
• fleshy and attractive for animals to eat
• feathery or winged to catch the wind
• covered in hooks that catch onto the coats of animals

Question 33

What is puberty and what causes it?

Answer 33

Puberty = transition between childhood and sexual maturity

Onset + progress of puberty is controlled by the brain via hormone secretion
- occurs when gonadotropin-releasing hormone (GnRH) is secreted
- several hundred neurons in the hypothalamus synthesise this hormone and secrete it into a blood vessel, carrying it directly into a pituitary gland

Question 34

Describe the process of GnRH secretion

Answer 34

starts 10 weeks after fertilisation, continues throughout pregnancy and stops when the baby is 4-6 months old
GnRH secretion resumes when the brain determines that puberty has arrived
secretion then continues through puberty and adulthood
hormone is released from the hypothalamus in pulses, so the conc. peaks and drops to a minimal level at least once an hour

Question 35

Describe GnRH pulses

Answer 35

lower frequency pulses = stimulate FSH
higher frequency pulses = stimulate LH

Question 36

What is the role of FSH and LH in males

Answer 36

FSH = stimulates testis growth
LH = stimulates testosterone secretion by Leydig cells in the testes, testosterone causes the development of secondary sexual characteristics in boys during puberty

Question 37

What is the role of FSH and LH in males

Answer 37

FSH = stimulates oestradiol secretion + development of follicles in the ovary (oestradiol causes development of secondary sexual characteristics, like uterus enlargement, breast development and body hair)
LH = stimulates development of follicle wall into the corpus luteum after ovulation (progesterone stimulates developmental changes in breasts to prepare for lactation)

Question 38

Describe the process of gametogenesis in humans

Answer 38

gametogenesis - production of gametes
1. mitosis
2. cell growth
3. two divisions of meiosis
4. differentiation

Question 39

Describe the process of spermatogenesis in humans

Answer 39

1. Spermatogenesis occurs in the testes, which are a coiled mass of seminiferous tubules (with small gaps or interstices between the tubules)
   -> These interstitial gaps are filled with testosterone-secreting Leydig cells
   -> Outer layer of cells in the germinal epithelium
   -> Wall of the tubule are large nurse cells (Sertoli cells)
2. Mitosis occurs continuously in the germinal epithelium, generating vast numbers of cells that are gradually displaced inwards towards the fluid-filled centre of the seminiferous tubule
3. As cells migrate inwards, they grow and then divide by meiosis to produce four haploid cells
4. Haploid cells differentiate into sperm by growing a tail and reducing their cytoplasm to a minimum

Question 40

Describe the process of oogenesis in humans

Answer 40

1. Oogenesis happens in the ovaries, with the first stages completed before birth
   -> outer layer of cells in the ovaries of a female foetus is germinal epithelium
2. Mitosis occurs in this layer during foetal development
3. Cells produced migrate inwards to distribute themselves through the cortex of the ovary
4. When foetus is 4-5 months old, these cells grow and start to divide by meiosis
5. By month 7, all are still in first division of meiosis and are surrounded by a single layer of cells
   -> cell that started meiosis + surrounding follicles = called the primary follicle
   —> there are around 400k primary follicles in the ovaries at birth
   -> do not undergo further development until after puberty and no more primary follicles are ever produced
6. a small batch of primary follicles is stimulated to develop by FSH at the start of each menstrual cycle
   -> only 1 goes on to become a mature follicle, containing an egg

Question 41

Describe mechanisms to prevent polyspermy. (x2 reactions)

Answer 41

The acrosome reaction:
Acrosome = large membrane bound sac of enzymes in the head of a sperm
Zona pellucida = coat of glycoproteins that surrounds and protects the egg

Sperm bind to specific glycoproteins in the zona pellucida → triggers release of acrosome contents
Enzymes from the acrosome start to digest the glycoproteins, weakening the zona pellucide.
Sperm can then push through to reach the plasma membrane of the egg

The cortical reaction:
Cortical granules = thousands of enzyme-containing vesicles near the plasma membrane of egg cell

When sperm nucleus penetrates the zona pellucida, it fertilises and activates the egg.

The cortical granules move to the plasma membrane of the cell and release their contents by exocytosis, making the zona pellucida very tough and difficult for other sperm to penetrate

Also changes the glycoproteins in the zona pellucida so the sperm can no longer bind to it.

Question 42

Describe the development of a blastocyst and its implantation in the endometrium

Answer 42

The zygote produced by fertilization has two haploid nuclei (1 from sperm, 1 from egg)
Replication of DNA in each nuclei starts about 6 hours after fertilization and takes 3 hours to complete.
The 2 nuclei undergo major changes in the gene expression and then, after 30 hours the cells divide jointly by mitosis, producing two genetically identical diploid nuclei, which separate when the cell divides.

DAYS 1 TO ~5:
During the following days, the number of cells doubles every 18 hours.
because an egg cell has loads of cytoplasm, the early rounds of the cell cycle can happen without any cell growth, so cell size decreases
embryo is a solid ball of cells

DAY ~6
Embryo becomes hollow, due to unequal cell divisions and cell migration
this is called a blastocyst

DAY ~7
Blastocyst has about 250 cells and is approximately 200 µm in diameter.
It will have migrated to the uterus from the site of fertilization (oviduct)
due to gentle wafting of cilia in the oviduct wall.
Toughened zona pellucida around the egg now breaks down

Blastocyst has used up the reserves of the egg cell and needs an external supply of food
attaches itself to the endometrium (uterus lining) by implantation
outer cell layer of blastocyst develops finger-like projections that grow into the endometrium
exchange materials with the mother’s blood, including absorbing foods and oxygen
inner cell mass of the blastocyst grows and develops into a human body
at 8 weeks, it has started to form bone tissue and is now referred to as a foetus.

Question 42

Describe how pregnancy tests work

Answer 42

human chorionic gonadotropin (hCG) is a medium sized protein.
produced by the embryo’s trophoblast cells from the blastocyst stage onwards
continuity of hCG production is essential to maintain pregnancy and avoid miscarriages

in the first ~10 weeks, hCG stimulates the corpus luteum to develop and to secrete progesterone

the hormone helps to support the pregnancy by preventing degeneration of the uterus lining

After 8-12 weeks, the placenta starts to secrete progesterone in response to the hormone hCG. The corpus luteum stops hormone production and breaks down.

Trophoblast cells in the placenta continue to secrete hCG throughout the remainder of the pregnancy, stimulating progesterone secretion by placenta cells and thereby maintenance of the endometrium.

Question 43

What is the role of the placenta in foetal development inside the uterus?

Answer 43

Humans need a placenta because the body-surface-area-to-volume ration becomes smaller as the foetus grows larger.

PLACENTA
made of foetal tissues
in intimate contact with maternal tissues in the uterus wall
foetus also develops membranes that form the amniotic sac
contains amniotic fluid (supports and protects the developing foetus)
basic functional unit of the placenta = placental villus (finger like piece of foetal tissue)
villi increase in number during pregnancy to cope with the increasing demands of the foetus for the exchange of materials with the mother

Maternal blood flows in spaces around the villi (intervillous spaces).
Foetal blood circulates in blood capillaries that are close to the surface of each villus.
The cells that separate maternal and foetal blood form the placental barrier.
must be selectively permeable, allowing some substances to pass, but not others

PLACENTAL VILLI HAS A LARGE SURFACE AREA

Question 44

What is the hormonal control like during pregnancy and childbirth?

Answer 44

9th week of pregnancy, the placenta has started to secrete oestradiol and progesterone in large enough quantities to sustain the pregnancy, so corpus luteum is no longer needed.
danger of miscarriage of this switch fails

During pregnancy, progesterone inhibits the secretion of oxytocin (stimulates contractions of the muscle fibres in the myometrium)
At the end of the pregnancy, hormones produced by the foetus signal to the placenta to stop secreting progesterone so oxytocin starts to be secreted
Oxytocin contractions are detected by the stretch receptors → signalling to pituitary gland to increase oxytocin secretion, making contractions more frequent and more vigorous, causing even more oxytocin secretion (positive feedback)

Relaxation of muscle fibres in the cervix causes it to dilate
Uterine contraction then bursts the amniotic sac and the amniotic fluid passes out
Further uterine contractions finally push the baby out through the cervix and vagina
Umbilical cord is broken, baby achieves physiological independence from its mother

Question 45

What is hormone replacement surgery and what are the risks of using it? What are the links between HRT and coronary heart disease?

Answer 45

Hormone replacement therapy is a treatment used to reduce menopausal symptoms
supplements oestrogen and progesterone
helps relieves hot flushes, night sweats, mood swings, vaginal dryness and reduced sex drive
can help prevent weakening of bones (osteoporosis)

In early epidemiological studies, it was argued that women undergoing hormone replacement therapy (HRT) had reduced incidence of coronary heart disease (CHD) and this was deemed to be a cause- and-effect relationship. Later randomized controlled trials showed that use of HRT led to a small increase in the risk of CHD. The correlation between HRT and decreased incidence of CHD is not actually a cause-and-effect relationship. HRT patients have a higher socioeconomic status, and this status has a causal relationship with lower risk of CHD.