D3.1 Reproduction

Sexual reproduction

The fusion of the nuclei of 2 gametes to form a zygote, involves 2 parents, the offspring produced are genetically different from each other, done by fertilisation and meiosis

Gamete

Sex cells involved in reproduction, containing haploid nuclei. In animals, sperm and ovum. In plants, pollen nucleus and ovum

Zygote

A fertilised egg cell that results from the fusion of a female gamete with a male gamete

What are the advantages of sexual reproduction?

Increases genetic variation, species can adapt to new environments

What are the disadvantages of sexual reproduction?

Takes time and energy to find mating partners, difficult for isolated members of a species to reproduce

Asexual reproduction

Does not involve gametes or fertilisation, only 1 parent required, the offspring produced are genetically identical to the parent and to each other, done by mitosis

What are the advantages of asexual reproduction?

Population can increase rapidly, more time and energy efficient

What are the disadvantages of asexual reproduction?

Limited genetic variation, vulnerable to changes in conditions as they may only be suited for one environment

What aspects of meiosis lead to genetic variation?

Crossing over, independent assortment, random fertilisation

Where does meiosis occur in humans?

In the testes of male animals and the ovaries of female animals. In the anthers and ovaries of flowering plants

Features of male gametes

Small size, able to move, produced in high numbers

Features of female gametes

Large size, unable to move, only one egg released per month (per menstrual cycle in humans)

Draw and annotate the female reproductive system

Draw and annotate the male reproductive system

Oviduct (fallopian tube)

Connects the ovary to the uterus, lined with ciliated cells to move the released egg from the ovary to the uterus. Fertilisation occurs here

Ovary

Contains eggs which will mature and develop when FSH is released

Ovulation

A phase in the menstrual cycle where the ovary releases an egg (ovum)

Uterus

Where a fertilised egg will implant and develop to become an embryo (if fertilisation is successful)

Cervix

Connects the uterus to the vagina, keeps the developing foetus in place during pregnancy, widens during birth

Vagina

Passage leading to the uterus, where the male’s penis will enter during sexual intercourse and deposit sperm

Vulva

Protects the internal parts of the female reproductive system

Prostate gland and seminal vesicles

The prostate gland secretes an alkaline fluid which allows sperm to survive in the acidic conditions, the seminal vesicles also produce fluids that mix with sperm to produce semen

Sperm duct

Transports sperm from the testes to the urethra during ejaculation

Urethra

Tube running down the centre of the penis that can carry out urine or semen

Testis

Produces sperm and testosterone

Scrotum

Sac supporting the testes outside the body, ensures that sperm are kept at a temperature slightly lower than core body temperature

Penis

Passes urine out of the body from the bladder and allows semen to pass into the vagina during sexual intercourse

Epididymis

Stores sperm until ejaculation

Erectile tissue

Fills with blood which enlarges and hardens the penis

What happens during the menstrual cycle?

Menstruation takes place on days 1-7, the endometrium thickens from days 7-28, ovulation occurs on day 14. Failure to fertilise the egg leads to menstruation, which is caused by the breakdown of the endometrium

Follicular phase of the menstrual cycle

The first half of the menstrual cycle. A group of follicles develops in the ovaries. In each follicle, the immature egg is stimulated to grow. While this happens, the endometrium starts to repair and thicken. Only the follicle with the most developed egg breaks open, releasing the egg into the oviduct (ovulation). The remaining follicles degenerate

Luteal phase of the menstrual cycle

The cells that make up the follicle which released the egg become the corpus luteum. If fertilisation doesn’t occur, the corpus luteum breaks down, and a new cycle begins

What are the 4 hormones that control the menstrual cycle?

Produced by the pituitary gland: Follicle stimulating hormone (FSH) and luteinising hormone (LH)

Produced in the ovaries: Oestradiol (follicles) and progesterone (corpus luteum)

Role of FSH

Stimulates the development of immature egg cells in follicles in the ovaries, stimulates the secretion of oestradiol by the follicle wall

Role of oestradiol

Stimulates the repair and thickening of the endometrium, at low concentrations it stimulates an increase in FSH receptors which boosts oestradiol secretion (positive feedback), at high concentrations it inhibits the release of FSH (negative feedback) but stimulates the production of LH

Role of LH

Stimulates the completion of meiosis in the oocyte, causes the follicle wall to burst open at the time of ovulation, promotes the development of follicle cells into the corpus luteum, stimulates the production of oestradiol in follicles

Levels of LH

Peaks at the end of the follicular phase

Levels of oestradiol

Peaks at day 14 (ovulation)

Role of progesterone

Maintains and promotes thickening of the endometrium, inhibits FSH and LH secretion

Levels of progesterone

Progesterone stays low from days 1-14 and starts to rise once ovulation has occurred, drops back down at day 28

In vitro fertilisation (IVF)

Procedure that involves fertilising an egg outside the body, then transferring the embryo into the uterus

Main steps of IVF

The woman takes a drug to inhibit the secretion of FSH and LH, halting the menstrual cycle and allowing doctors to control the timing of egg production

The woman is given injections of FSH and LH to stimulate the development of follicles, superovulation occurs

The eggs are collected from the woman and fertilised by sperm from the man

The fertilised eggs develop into embryos

One or more embryos are inserted into the mother’s uterus

Fertilisation

The fusion of a sperm and an egg cell nucleus to produce a diploid zygote

Steps of fertilisation

When the sperm cell reaches the egg cell surface, its membrane fuses with the egg cell membrane

The sperm cell nucleus enters the egg cell

Vesicles from the egg destroy the sperm tail and its mitochondria

Temporary membranes form around the two haploid sets of chromosomes

The temporary membranes break down, the two haploid pronuclei come together

Condensed chromosomes undergo mitosis to produce two diploid nuclei

Male gametes in flowering plants

Produced in the anther. The anther contains pollen sacs, each pollen sac contains a diploid mothercell which undergoes meiosis to form four haploid pollen grains (the gametes). Mitosis occurs to produce more gametes

Female gametes in flowering plants

Produced in the ovule. A single diploid cell within the ovule undergoes meiosis to produce four haploid egg cells. Only one of these cells survives and undergoes mitosis to produce the female gamete

Pollination

The process of transferring pollen from the anther of one flower to the stigma of another

Hermaphroditic

Containing both male and female reproductive organs. Self pollination can occur

Cross pollination

The transfer of pollen from one plant to another

Why are all pollination methods sexual?

The gametes are produced by meiosis, there is the fusion of gametes to form a diploid nucleus

Draw and annotate an insect pollinated flower diagram

Function of sepal

Protects the developing flower whilst inside the bud

Function of petal

Attractive color and scent to attract pollinators

Function of anther

Part of the stamen that produces the male gametes

Function of filament

The stalk of the stamen that hold up the anther

Function of stigma

The top of the carpel, captures pollen from pollinators

Function of style

The part of the carpel that supports the stigma

Function of ovary

Contains the ovules until fertilisation then develops into fruit

Function of ovule

Contains the female gametes until fertilisation and then develops into a seed

How does cross-pollination occur?

Transfer of pollen by wind, insects, animals

Other ways to ensure successful cross-pollination

Different maturation times for the pollen and ovules of the same flower, producing flowers that only have either male or female parts

Why is self-pollination bad?

Results in less genetic variation in the offspring in comparison to cross-pollination, as no new alleles are introduced into the offspring, therefore the offspring will not be adapted to suit new conditions

Self-incompatibility mechanism in plants

Each plant has a set of genes that controls the growth of a pollen tube so that when pollen lands on the stigma of a flower of the same plant, it prevents the growth of a pollen tube

Methods of seed dispersal

Wind, water, animals

Seed dispersal

Distributing the seeds far away to reduce competition between the offspring and parent plant

Germination

The start of growth in the seed

Requirements for germination

Water, oxygen, warmth

Process of germination

Embryonic growth is suspended until seed germination, and the developing seedling will rely on the food reserves in its cotyledons until leaves grow for photosynthesis

2 types of sex hormones involved in puberty

Gonadotropin-releasing hormone (GnRH) and steroid sex hormones such as oestradiol and testosterone

What causes puberty?

The increased release of gonadotropin-releasing hormone (GnRH) by the hypothalamus, triggering the increased secretion of LH and FSH

Male secondary sexual characteristics

Larynx increases in size (deepening voice), growth of body hair, continuous production of sperm, increased skeletal development

Female secondary sexual characteristics

Development of breasts, growth of body hair, monthly ovulation and menstruation, widening of pelvis

Gametogenesis

The process by which diploid precursor cells undergo meiotic division to become haploid gametes

Spermatogenesis

The process by which male gametes (sperm) are produced through meiosis in the testes

Oogenesis

The process by which female gametes (eggs) are produced through meiosis in ovaries

Steps of gametogenesis

Cells divide by mitosis to produce many new cells, all of which have the potential to develop into gametes

Cells grow and differentiate

Cells divide by meiosis to produce haploid gametes

Steps of spermatogenesis

Testes contain seminiferous tubules, which are small tubes where sperm is produced

Spermatogenesis begins in the germinal epithelium, the outer layer of seminiferous tubules

Germinal epithelium cells divide by mitosis to produce diploid spermatogonia

One daughter cell will become a sperm cell, the other remains in the germinal epithelium

Spermatogonia migrate towards the lumen of the seminiferous tubules, moving through Sertoli cells. Sertoli cells line the inner surface of the seminiferous tubules

Spermatogonia differentiate into primary spermatocytes

Meiosis I produces secondary spermatocytes. Meiosis II produces spermatids

Spermatids mature into sperm cells (spermatozoa)