subject guide notes

D3.1.1—Differences between sexual and asexual reproduction

asexual reproduction produces offspring that are genetically identical to their parents & to each other

offspring produced in short amount of time - time isnt spent on finding a mate

  • is an advantage, as organisms will be well adjusted to the environment and can take over the environment easily

disadvantage - low genetic diversity

  • means low adaptability, so organism can hv difficulty surviving if environment changes

sexual reproduction produces offspring that aren’t genetically identical to parent or to each other

  • so the offspring hv new gene combinations = advantage

genetic variation can occur during formation of gametes & during fertilisation

disadvantage - is time consuming as organisms have to find a mate

D3.1.2—Role of meiosis and fusion of gametes in the sexual life cycle

sexual reproduction involves fusion of two haploid gametes to produce a diploid zygote

meiosis breaks up parental combinations of alleles

crossing over and fusion of gametes produces new combinations

fusion of gametes is also known as fertilization

D3.1.3—Differences between male and female sexes in sexual reproduction

in sexual reproduction, male gamete (sperm) travels to female gamete (egg)

  • for this reason, the sperm is smaller than egg & has fewer food reserves

flagella in sperm allows it to swim up the female reproductive tract & fuse with the egg/ovum

ovum hv food reserves, which provide energy for dividing zygote

D3.1.4—Anatomy of the human male and female reproductive systems

*check diagrams from class & kognity interactives for info on the parts of male & female reproductive systems

male reproductive system

  • produces sperm

  • 2 testes enclosed in sac-like scrotum

  • penis

  • epididymis

  • tubes where sperm pass

  • glands (prostate gland & seminal vesicles)

  • produces male sex hormones

female reproductive system

  • produces ova (through ovaries)

  • fallopian tubes

  • vagina

  • uterus

  • produces female sex hormones

D3.1.5—Changes during the ovarian and uterine cycles and their hormonal regulation

menstrual cycle comprises the ovarian & uterine cycle

ovarian cycle composed of: follicular phase, ovulation & luteal phase

uterine cycle composed of: menstruation, proliferative phase, secretory phase

follicular phase

  • begins with first day of menstrual cycle (bleeding)

  • low levels of ovarian & pituitary hormones

  • anterior pituitary secretes FSH (follicle stimulating hormone) & LH (luteinising hormone)

  • FSH stimulates growth of follicles

  • growing follicle secretes oestradiol

*important - while multiple follicles can begin to develop simultaneously, usually only one of these follicles will mature

ovulation

  • sharp increase in level of oestradiol stimulates anterior pituitary to secrete LH (to a greater extent) & FSH (to a lesser extent)

  • increase in LH causes follicle to rupture

    • releases the mature egg

    • is known as ovulation

  • occurs in middle of menstrual cycle (day 14 of 28-day cycle)

  • egg begins passage to uterus

luteal phase

  • LH & FSH levels drop

  • under influence of LH, ruptured follicle is converted to the corpus luteum

  • corpus luteum secretes oestradiol & progesterone

  • as levels of oestradiol & progesterone increase (to a lesser extent), secretion of FSH & LH is inhibited by negative feedback

proliferative phase

  • coincides with follicular phase & continues till ovulation

  • oestradiol secreted by growing follicle causes thickening of endometrial lining

    • is preparing for possible pregnancy

secretory phase

  • roughly coincides with the luteal phase

  • under influence of progesterone, uterine lining thickens faster

    • it changes into secretory layer, preparing for potential implantation

menstruation

  • towards end of luteal phase, decrease in levels of FSH & LH cause corpus luteum to break down

    • this is cuz no fertilization has occurred

  • the breaking down of corpus luteum means a decrease in oestradiol & progesterone

    • this causes thickened lining of uterus to break down

  • this results in menstrual bleeding

  • in response to low levels of ovarian hormones, anterior pituitary secretes LH & FSH

  • this causes development of another follicle, and next cycle begins

negative feedback can be seen during the follicular phase

  • increase in oestradiol inhibited secretion of FSH, which cause other follicles to regress

also after ovulation, ruptured follicle is converted to corpus luteum

  • secretes oestradiol & progesterone to a lesser extent

  • inhibits secretion of FSH & LH

positive feedback: increase in oestradiol levels stimulated anterior pituitary to release high levels of FSH & LH

D3.1.6—Fertilization in humans

takes place in oviduct

egg released from mature follicle hasn’t undergone meiosis II - referred to as secondary oocyte

cytoplasm of egg has cortical granules, filled with enzymes

head of sperm contains:

  • haploid nucleus

  • cap-like acrosome

midpiece of sperm carries mitochondria, which provide energy for the flagella to move

when sperm enters female reproductive tract, it undergoes capacitation in order to activate it

when one sperm reaches the zona pellucida, an acrosome reaction will occur

  • enzymes in the head of the sperm will digest through the zona

complementary receptors present on sperm’s head will reach the cell membrane & bind to proteins on oocyte’s cell membrane

  • so, both the membranes fuse together

binding of sperm activates the oocyte in egg activation

changes that occur in egg & zona pellucida prevent other sperm from fusing with fertilized egg

head of sperm enters cytoplasm of egg

midpiece & tail of sperm are destroyed

sperm nucleus undergoes changes to form sperm pronucleus

nuclear membranes of male & female pronuclei dissolve, chromatin from both nuclei dissolve to form chromosomes & this results in diploid organism

diploid zygote undergoes mitosis

D3.1.7—Use of hormones in in vitro fertilization (IVF) treatment

medication used to suppress normal menstrual cycle

person undergoing treatment is given fertility drugs

  • the drugs contain FSH & result in superovulation (production of multiple eggs)

  • increase in number of eggs = increase in chances of successful fertilisation

person undergoing treatment injected with hCG (human chorionic gonadotropin)

  • causes follicles to mature

  • before they rupture, they’re collected through a process called follicular aspiration

eggs put into petri dish

sperm from donor transferred to dish & sample is reassessed after 16-18 hours to see if fertilization occurred

if fertilization occurred, the fertilized eggs (blastocysts?) grow in a specially formulated culture medium

healthy embryos are selected & transferred to uterus

  • known as implantation

  • up to 3 embryos can be implanted. this increases the chances of fertilization. multiple births may occur

pregnancy test done after 2 weeks will determine if the process has been successful

D3.1.13—Control of the developmental changes of puberty by gonadotropin-releasing hormone and steroid sex hormones

testosterone is primary male hormone & secreted by testes

in males

puberty initiated when hypothalamus releases gonadotropin-releasing hormone (GnRH)

GnRH then acts on the anterior pituitary & stimulates release of FSH & LH

LH & FSH carried by blood to testes

LH stimulates release of testosterone by Leydig cells

FSH plays role in sperm production by stimulating Sertoli cells

surge in testosterone levels plays role in development of secondary sexual characteristics

increase of testosterone levels in bloodstream = inhibits release of GnRH

decreased release of GnRH = inhibited secretion of FSH & LH

Sertoli cells also release inhibin, which also inhibits production of FSH

in females

steroid hormones (oestradiol & progesterone) are secreted by ovaries

GnRH released by hypothalamus, stimulates the release of FSH & LH from anterior pituitary

increase of FSH & LH causes release of female sex hormones (ex: oestradiol & progesterone)

increase in oestradiol levels leads to development of secondary sexual characteristics

D3.1.14—Spermatogenesis and oogenesis in humans

spermatogenesis occurs with onset of puberty

Leydig cells are located between the seminiferous tubules

basement membranes lined by spermatogonia

spermatogenesis

multiplication phase - spermatogonia to primary spermatocytes

  • divide mitotically to produce more cells

  • some will continue to divide

  • others will move away from basement membrane & differentiate to form the primary spermatocyte

growth phase - primary spermatocyte to secondary spermatocyte

  • primary spermatocyte (is diploid) undergo first meiotic division to produce 2 secondary spermatocytes, which are haploid

  • each secondary spermatocyte has 22 autosomes & either X or Y sex chromosome

  • crossing over & independent assortment of chromosomes - introduce genetic variation

maturation phase - secondary spermatocytes to spermatids

  • each secondary spermatocyte undergoes second meiotic division to produce two spermatids

  • spermatids remain haploid

spermiogenesis (spermatid to sperm)

  • spermatids undergo differentiation to form the sperm or spermatozoa

  • process supported by secretion of Sertoli cells

  • i think testosterone also stimulates the differentiation of the spermatid

so, each primary spermatocyte produces 4 haploid sperm

oogenesis

  • begins during foetal development

  • begins with diploid oogonia

oogonia undergo mitosis

some oogonia undergo mitosis to form primary oocytes

diploid primary oocyte begins meiosis I, but this is arrested at prophase 1

after puberty begins

  • only one primary oocyte begins maturation

    • *remember - FSH stimulates development of multiple follicles, but once one follicle has matured, the other follicles regress. so the primary oocyte mentioned here is one that has matured

    • completes first meiotic division

      • results in 2 haploid cells

      • these cells differ in size

        • are referred to as: secondary oocyte & first polar body

  • first polar body disintegrates

  • secondary oocyte begins meiosis II, but is arrested at metaphase II

  • at time of ovulation, secondary oocyte released from ovary

  • i think this all happens in the follicle, and secondary oocyte is released from the mature follicle when it ruptures

if fertilization occurs

  • secondary oocyte completes meiosis II

  • forms ovum & second polar body

  • if fertilisation doesnt occur, secondary oocyte discharged from body during menstrual flow

NOTE - each primary oocyte gives rise to one ovum

D3.1.15—Mechanisms to prevent polyspermy

sperm bind to specific binding sites on zona pellucida & begin acrosome reaction

in order to prevent polyspermy, the following changes are seen after fertilization

  • plasma membrane undergoes changes to its membrane potential, making it impermeable to other sperm

  • cortical reaction occurs

    • enzymes of cortical granules destroy sperm receptors on zona pellucida, preventing binding of other sperm

  • zona pellucida hardens, preventing the sperm from progressing towards the egg

so acrosome reaction enables sperm to progress towards egg

cortical reaction opposes movement of sperm

D3.1.16—Development of a blastocyst and implantation in the endometrium

soon after fertilization, the diploid zygote divides mitotically

  • is known as cleavage

it quickly transitions from a two-cell stage to a four-cell stage to an eight-cell stage, and so on

16-cell stage is known as morula & is same size as zygote

but rapid cell division prevents growth, so the 16-cell stage & zygote are same size

around day 4, the dividing cells arrange themselves to form an outer trophoblast and inner cell mass

blastocoel develops in centre

at this point, the embryo is known as the blastocyst

blastocyst sheds the zona pellucida in a process known as hatching

trophoblast differentiates into structures that help the implantation of embryo to the uterine endometrium

most of inner cell mass differentiates into structures of embryo

by the 7th or 8th day after fertilization, blastocyst adheres itself to the uterine endometrium, resulting in implantation

if blastocyst doesn’t attach to uterine wall, it is shed during menstrual flow

*check interactive on kognity for this section

D3.1.17—Pregnancy testing by detection of human chorionic gonadotropin secretion

soon after implantation, cells of trophoblast secrete hCG (human chorionic gonadotropin)

  • this maintains the corpus luteum, so that it continues to secrete progesterone

by 3rd month, placenta (formed from the trophoblast) begins to secrete progesterone & oestradiol, which maintain the pregnancy

pregnancy test kits test for presence of hCG in women’s urine

the stick on the test is coated with monoclonal antibodies which’re specific to hCG

if hCG present in women’s urine, it’ll bind to the monoclonal antibodies

D3.1.18—Role of the placenta in foetal development inside the uterus

once blastocyst implanted, extraembryonic structures begin to develop

  • the umbilical cord

  • chorion

chronic villi begin to form by 14th day and this represents the beginning of the placenta

the villi extend into the intervillous space & increase the surface area'

  • also has thin membrane to allow for short diffusion distance

maternal blood from open endometrial arteries pools into the intervillous space, so the chorionic villi are bathed in maternal blood

*but note - maternal & foetal circulatory systems aren’t connected

maternal blood that bathes the chorionic villi is high in oxygen & nutrients

  • it travels down/diffuses? the umbilical cord to the foetal circulation

waste produces from foetus travel down the umbilical arteries to the placenta

  • they diffuse into the maternal blood that’s present in the intervillous space & are removed by the mother

D3.1.19—Hormonal control of pregnancy and childbirth

pregnancy is maintained by the progesterone secretion from the corpus luteum, then from the placenta

in anticipation of delivery, progesterone levels will plateau, then start declining

growth of baby causes uterus walls to expand, which is detected by receptors on uterine wall

this causes secretion of stress hormones, like cortisol

increase in stress hormone levels causes release of oestradiol

oxytocin released by posterior pituitary & causes uterine contractions

foetus secretes prostaglandins, which intensify the contractions

this causes posterior pituitary to release more oxytocin

so this positive feedback loop leads to the birth of the baby & expulsion of placenta

D3.1.20—Hormone replacement therapy and the risk of coronary heart disease

Hormone replacement therapy is a treatment that relieves symptoms of menopause

the person undergoing HRT is given medication that contains low doses of oestradiol, or a combination of oestradiol - progestin (progestin is synthetic derivative of progesterone)