Sexual Reproduction in Humans

Describe differences between spermatogenesis and oogenesis in humans. #

spermatogenesis	oogenesis
Continuous after puberty,	In cycles after puberty;
Millions produced,	One/few per cycle;
Occurs 12-65+,	9 –menopause/40;
4 sperm ,	1 ovum per meiosis;
No,	Polar bodies;
All mitotic products used	many mitotic products degenerate/less mitotic replication;
Complete meiosis on release,	Completes meiosis after ovulation/AW;
Primary spermatocyte smaller,	Than primary oocyte/primary oocyte greater growth phase;
Products need to differentiate	no differentiation of products;
Requires testosterone,	Requires oestrogen;

Outline the technique of in-vitro fertilisation (IVF). #

• ref. hormone treatment ;
• results in, superovulation / many oocytes / many follicles, maturing at same time ;
• oocytes harvested ;
• detail of harvesting ;
• mixed with sample of sperm ;
• in special medium ;
• idea of, waiting for three days / wait until 6–8 cell stage ;
• embryos placed in uterus ;
• ref. maintenance of endometrium ;
• sperm / sperm nucleus / sperm DNA, may be injected into oocyte ;

Outline the biological basis of the effect of the oestrogen/progesterone contraceptive pill. #

• (progesterone / oestrogen), reduce the production of, FSH / LH ;
• negative feedback ;
• to, hypothalamus / anterior pituitary ;
• idea of lack of FSH prevents maturation of follicle ;
• lack of LH prevents ovulation ;
• cervical mucus, thick / hostile to sperm ;
• thin uterine lining prevents implantation ;

Describe the changes of the uterus that are caused by the following during the menstrual cycle: #

(ii) oestrogen and [6]
(iii) progesterone [6]

• oestrogen
  • follicular phase;
  • shedding of lining/endometrium(day 1-5)
  • myometrium excitable/muscle sensetice to hormones ;
  • repair;
  • endometrium, supplied with ateries
  • endometrium becomes glandular;
  • up to ovulation/day 14;
• progesterone
  • secretory phase;
  • endometrium thickened;
  • ref coiled arteries;
  • ref coiled glands;
  • venous blood lakes;
  • change in the cervical mucus;
  • secretion from uterine glands;
  • myometrium less excitable;
  • maintains endometrium;
  • from ovulation/day 14-28
  • reduction of steriods;

Describe the follicular stage of the ovarian cycle #

• anterior pituitary gland is secreting follicle-stimulating hormones,FSH
• FSH travels in the bloodstream to ovary
• stimulates development of follicle cells surrounding oocyte
• developing follicle cells secrete oestrogen
• stimulates proliferation of endometrium and its blood supply
• inhibits further secretion of FSH by negative feedback
• stimulates anterior pituitary gland to secrete luteinising hormone,LH
• LH brings about evulation

Describe briefly the process of fertilisation in humans[6]. #

• enzymes from acrosome digest path through follicle cells;
• sperm attach to receptors in zona pellucida;
• (another) enzyme from acrosome digests path through zona;
• sperm and oocyte (cell surface/plasma) membranes fuse;
• enzymes released from, lysosomes/cortical granules, thicken zona/make zona
• sperm-proof/prevent polyspermy;
• male and female (pro)nuclei fuse;

Describe the maturation of a spermatid into a spermatozoon (sperm). #

• ref. differentiation / specialisation ;
• ref. Sertoli cell ;
• forms flagellum ;
• detail (of flagellum) ; e.g. microtubules
• acrosome ;
• detail (of acrosome) ; e.g. contains enzymes / modified lysosome
• many mitochondria ;

Describe the acrosome reaction #

• contact of jelly coat and the sperm
• triggers Ca²⁺ to enter membrane of sperm
• causes acrosome to burst releasing enzymes eg hyluronidase , acrosin..
• sperm filaments attaches to receptor on vitelline membrane
• sperm and egg plasma membranes fuse
• sperm nucleus enters egg, tail and middle piece remain outside.

State the structural features that are characteristic of a wind-pollinated flower and explain how each feature contributes to successful pollination. #

• lots of pollen grains made
  • so more chance of pollination ;
• pollen grains, light/ smooth/ aerodynamic,
  • so easily transported ;
• no/ small, petals/ corolla/ perianth,
  • so stamens/ anthers/ stigma, outside of flower ;
• long filaments
  • so anthers outside of flower ;
• anthers outside of flower/ versatile anthers,
  • so pollen released ;
• long style
  • so stigma outside of flower ;
• stigma outside of flower/ stigma has large surface area,
  • so traps pollen ;

Gametogenesis in Plants #

• In males
  • Takes place in the anther;
  • Pollen mother cells divide by meiosis forming 4 haploid cells;
  • Each of these cells divide by mitosis;
  • cytokinesis does not take place resulting in a cell with two haploid nuclei;
  • These cells mature into pollen grain;
  • one of the nuclei is the tube nucleus and the other is generative nucleus
• In females
  • Takes place in the ovules
  • Similar to the male, Spore mother cell divides by meiosis to produce four haploid cells
  • All but one degenerates, this cell develops into an embryo sac;
  • Embryo sac divides by mitosis 3 times forming 8 haploid nuclei, of which one becomes the female gamete ;

Describe the structural changes that occur after fertilisation, leading to the development of the seed and fruit. #

• zygote divides to become embryo;
• seed:
  • cotyledons, plumule, radicle;
  • mitosis;
  • ref (single) basal cell;
  • suspensor/foot, forms;
  • endosperm nucleus forms endosperm;
  • endosperm develops into food store of cotyledons;
• endosperm persists in some cases;
• nucellus disappears;
• changes in micropyle/described;
• integuments become testa;
• aleurone layer develops;
• becomes, dry/harder/waterproof
• Fruit:
  • ovary wall becomes pericarp;
  • modified for dispersal;
  • dries/becomes fleshy

Describe the passage of sperm from the testes to the oviduct until fertilisation has been completed. #

• from (site of storage) in epididymis;
• via vas deferens;
• receives secretions of, seminal vesicles/prostate gland/
• Cowper’s gland;
• to aid sperm motility/adjust pH/AW;
• pass through urethra;
• semen ejaculated into vagina;
• pass through cervix;
• aided by, muscular contraction/cilia, of uterus/lashing
• of flagella;
• capacitation;
• further detail of capacitation;
• acrosome reaction/enzymes released;
• named enzyme released;
• penetrates zona pellucida/follicle cells/granulosa cells/ corona radiata;
• meiosis completed/triggers meiosis II, in oocyte;
• fusion of nuclei