Unit 11: Reproduction Flashcards

Question

What is pollination?

Answer 1

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

Answer 2

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)

Answer 3

- 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

Answer 4

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

Answer 5

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])

Answer 6

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.

Answer 7

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.

Answer 8

- 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

Answer 9

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

Answer 10

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

Answer 11

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

Answer 12

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

Answer 13

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)

Answer 14

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

Answer 15

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

Answer 16

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

Answer 17

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.

Answer 18

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.

Answer 19

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.

Answer 20

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

Answer 21

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

Answer 22

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.