D3.1 Reproduction

Sexual reproduction

• 2 parents

• Meiosis makes gametes

• Genetically different (variation)

• Gametes needed (sperm/egg)

Asexual reproduction

• 1 parent

• No meiosis

• Genetically identical (clones)

• No gametes

Types of asexual reproduction

• Binary fission - bacteria

• Budding - yeast

• Fragmentation - Starfish

Vertebrae clones

• Occurs in all female species of whiptale

• Offspring are formed as a hybrid of 2 other species

• Eggs are formed by mitosis and develop without fertilising so can reproduce without males

• Therefore eggs do not need to be fertilised

Difference between male and female sexes

Males produce sperm

• large quantities increase likelihood of fertilisation

• Sperm cells are very small

Females produce eggs

• Fewer are produced

• They are larger in size and have a more nutrient dense cytoplasm

The male reproductive system

• Testes - responsible for production of sperm and testosterone

• Epididymis - site where sperm matures and develops ability to be mobile. Matured sperm is stored here until ejaculation

• Vas deferens - long tube connecting tube from testes to the prostate gland

• Seminal vesicle - secretes fluid containing fructose (to nourish sperm), mucus to protect sperm and prostaglandin

• Prostate gland - secretes an alkaline fluid to neutralise vaginal fluid

• Urethra - connects sperm from prostate gland to outside the body via the penis

Female reproductive system

• Ovary - where oocytes mature prior to release, also secretes oestrogen and progesterone

• Fimbria - A fringe of tissue adjacent to the ovary. Sweeps an oocyte into the oviduct (or fallopian tube)

• Fallopian tube - transports oocyte to uterus. typical site of fertilisation

• uterus - where a fertilised egg implants and develops

• Endometrium - mucus membrane lining of uterus. Thickens in preparation for implantation, otherwise lost in menstruation

• Vagina - passage leading to the uterus by which the penis can enter

The 2 cycles in the menstrual cycle

• Ovarian cycle - monthly preparation and release of an egg cell from ovary

• Uterine cycle - Build up and shedding of the uterus lining (endometrium)

FSH

• Released by the anterior pituitary

• Stimulates follicle growth in ovaries

• Stimulates oestradiol secretion

Oestradiol

• Released by ovaries

• Thickens uterus lining

• Inhibits FSH and LH for most the cycle

• Stimulates FSH and LH during pre-ovulation

LH

• Released by anterior pituitary

• Surge in LH causes ovulation

• Thickens corpus luteum

Progesterone

• Released by ovaries

• Thickens uterus lining'

• Inhibits FSH and LH

Positive feedback in the cycle

• Oestradiol secretion - causes an increase in FSH receptors. This makes follicles more receptive to FSH so they produce more oestradiol

• LH causes development of corpus luteum, which secretes progesterone

Negative feedback

• Oestradiol levels (caused by FSH released) become so high that they inhibit FSH secretion

• Another example - Inhibition of FSH and LH by rising progesterone levels

Fertilisation

• Enzymes from acrosome digest the zona pellucida (jelly coat)

• Sperm fuses with the egg membrane - head of sperm enters egg

• Tail and midpiece are destroyed

• Nuclear membrane of egg and sperm dissolve

• 2 sets of chromosomes join up and undergo joint mitosis

• This forms a diploid nucleus

IVF

• Woman is given drugs to suppress natural hormones

• FSH is given in high doses - causes superovulation

• HCG then given to cause eggs to mature

• Eggs collected from ovaries and combined with sperm

• Embryo starts to develop on a petri dish - can be screened

• Progesterone taken to aid embryo implantation

• Up to 3 embryo’s transferred to the uterus

What is gametogenesis

• The process by which diploid pre-cursor cells undergo meiotic division to become haploid gametes

• In males, sperm is made by spermatogenesis

• In females, eggs are made by oogenesis

3 stages of gametogenesis

• Multiplication of precursor cells - occurs by mitosis

• Growth of developing sex cells

• Maturation phase - forms cells which differentiate to haploid gametes

Spermatogenesis

• Spermatogonia - the male germ line cells that remain dormant until adolescence

• Spermatogonia cells undergo mitosis and grow to form primary spermatocytes

• Primary spermatocytes undergo meiosis division to form secondary spermatocytes

• These undergo meiosis again to form spermatids

• Spermatids differentiate to form sperm

Structure of testis

• Contain many seminiferous tubules - make sperm

• The sperm is then stored and matures in the epididymis

• Seminiferous tubules contain sortoli cells, which nourish developing sperm until they are spermatozoa

• Leydig cells make testosterone

Hormonal control of sperm production

• Pituitary gland causes FSH to be released

• FSH starts sperm production

• Pituitary gland also causes LH production

• LH causes leydig cells to produce testosterone

• Testosterone and FSH maintain sperm production and growth of sortoli cells

Oogenesis

• Oogonia undergo mitosis and growth before birth to form primary oocytes

• Oogonia production ceases during pregnancy

• The primary oocytes start meiosis, but stop at prophase 1

• During puberty, meiosis 1 is completed t form a secondary oocyte

• The secondary oocyte is locked in metaphase 2

• If fertilization occurs, the secondary oocyte completes meiosis 2 to form an ovum

• Meiosis is only completed if the egg is fertilised

Ovary structure

Triggering puberty in males

• Gonadotrophin releasing hormone (GnRH) is released from hypothalamus during puberty

• This causes anterior pituitary to release FSH and LH

• LH causes testosterone release

• FSH starts spermatogenesis

• Testosterone surge alters gene expression to trigger secondary sexual characteristics

Triggering puberty in females

• GnRH released from hypothalamus

• Causes anterior pituitary to release FSH and LH

• This causes production of oestradiol and progesterone

• Oestradiol → secondary female characteristics

Human fertilisation - capacitation

• When released, sperm is protected by a cholesterol coat

• Uterine chemicals direct the sperm and dissolve this coat

• This increase sperms motility

Human fertilisation - acrosome reaction

• Sperm binds to a docking protein on the zona pellucida

• Enzymes are released from the acrosome

• This digests the eggs jelly coat

• The sperm nucleus enters the egg cell

• Helps to reduce polyspermy

Human fertilisation - cortical reaction

• Once the sperm nuclei enters the gg, the egg detects this and is depolarised

• This causes cortical granules to rupture and release a chemical

• This chemical removes attachment sites and hardens the jelly coat

• Prevents polyspermy

Embryogenesis

• Day 0 - Zygote - one diploid cell after fertilisation

• Day 4 - Morula - Zygote divides to form a group of cells called morula

• Day 7 - Blastocyst - contains 3 sections: inner mass that will become foetus, trophoblast layer that surrounds the blastocyst and will become the placenta and a fluid filled cavity called the blastocoele

• Trophoblast cells bind and embed the blastocyst into the endometrium

HCG

• A hormone that is in high levels in pregnant women

• Maintains the corpus luteum

• Maintains production of progesterone and thus the uterus lining

• This occurs until the placenta is formed, which takes over progesterone production

Pregnancy tests - reactions site

• Contains free antibodies attached to dye enzymes

• The antibodies will bind to HCG if present

Pregnancy tests - test site

• Contains a fixed antibody and dye substrate

• The fixed antibody binds to the HCG attached to the free antibody + dye enzyme if pregnant

• This causes dye to be released

• If not pregnant, fixed antibodies cannot bind to free antibodies so no dye is released

Pregnancy tests - control site

• Contain a fixed antibody that traps the free antibody even if no HCG is present

• This proves the test has worked

The Placenta

• Delivers nutrients to a developing baby, facilitating exchange between mother and foetus

• Also secretes hormones to maintain pregnancy

Chorionic villi

• A large surface area of foetal blood vessels and chorionic villi extend into intervillous spaces

• This enables exchange to take place between foetus and mother

• The placenta enables the foetus to be retained int he uterus until later stages of development

Positive feedback when giving birth

• Baby pushes against the cervix, causing it to stretch

• Nerve impulses are sent to the brain

• Stimulate the posterior pituitary to produce oxytocin

• Oxytocin causes smooth muscle lining in the uterus to contract

Hormonal control of birth

• Oxytocin - stimulates uterus contractions

• A drop in progesterone and oestradiol causes childbirth

• A high level of oxytocin causes labour

The menopause

• Menopause = caused by a drop in oestradiol and progesterone

• Symptoms:

• Changes in body temp

• Increase heart rate

• Joint and muscle stiffness

• Excess sweat

Hormone replacement therapy (HRT)

• Can help some menopause symptoms

• Low levels of oestradiol and progesterone

• Evidence from epidemiology studies of reduced risk of heart disease if taking HRT

• Clinical studies did not support this - there was no cause and effect relationship

• New hypothesis - those on HRT tend to have higher socio economic status

Structure of the flower

Pollination

• The transfer of pollen from the anther to stigma

• A mutualistic relationship - insects drink nectar from plants, insects transfer pollen to other plants stigma

Stages of plant reproduction

• Pollination

• Fertilization

• Seed dispersal

Self pollination

• When the pollen is produced and transferred to the stigma of the same plant

• Advantage - preserves good genomes suited to the environment

• However, reduces variation, decreases genetic diversity, greater chance of 2 recessive alleles combining that may not be desirable

Cross pollination pros and cons

• Pros - increase variation, good if there is environmental changes (natural selection)

• Cons - less change of pollen reaching stigma, dependent on insects/wind

Mechanisms to encourage cross pollination

• different male and female flowers

• Stigma and anther far apart or at different heights

• Stigma and stamen mature at different times

Self incompatibility mechanisms

• Stigma rejects any pollen with protein markers that show it is from the same flower

• prevents in breeding

Seed stucture

• Largely contains a cotyledon - contains food reserves

• Hard case around the outside called a tesla - protects the seed

• Contains a radicle, has an embryo stem and root

Formation of seeds after ferilisation

• Zygote grows by repeated mitotic division

• Produces cells that form an embryonic plants, embryo root, stem and single/2 cotyledons

• Formation of stored food reserves - absorbed into cotyledons

• As seed matures, outer layers form protective coat (tesla)

ways of seed dispersal

• Wind

• Water

• Animal

Seed germination conditions

• Water uptake so seed is hydrated

• Oxygen present for aerobic respiration

• suitable temperature for enzymes involved in mobilisation of food reserves

Seed germination process

• Water is absorbed by seed

• Water activates gibberellic acid

• This activates gene expression of hydrolytic enzymes

• Starch is broken down into glucose, food reserves are mobilised, used for respiration

• Proteins are broken down to amino acids and used for growth

• When food reserves are gone, plant uses photosynthesis to survive