Component 2

Mitosis, meiosis, human reproduction, plant reproduction

The cell cycle

Series of events cell go through as they grow and divide

Phases include:

1. Interphase

2. Mitosis

Interphase phases

G1 - growth

S synthesis

G2 - growth

G1

first growth phase - protein synthesis, cytoplasm and number of organelles increase rapidly

S synthesis

DNA replicates (amount of DNA doubles) if cell is going to divide

G2

second growth phase - proteins necessary for cell division are synthesised

Cell division

Process by which a cell divides into 2 genetically identical daughter cells

Why do cells need to divide?

Asexual reproduction

Living things grow by producing more cells

Repair of damaged tissue

To replace old or worn out cells e.g. red blood cells and skin cells

DNA

Located in the nucleus and controls all cell activities including cell division

Chromatin

long thread-like DNA in a non-dividing cell

Chromosome

Doubled, coiled short DNA in a dividing cell

Consists of 2 parts - chromatid and centromere

Chromatin to chromosomes

Chromatin duplicates itself and coils up into chromosomes

Centromere

2 identical sister chromatids attached at area in the middle

Human chromosome number

46 chromosomes or 23 pairs

Interphase

Period of cell growth and development

DNA replication, cell growth, replication of all other organelles and normal cell activities take place

The cell spends most of its life cycle in interphase

Mitosis

division of the nucleus into 2 nuclei, each with the same number of chromosomes

occurs in all somatic (body) cells

Why does mitosis occur?

So each new daughter cell has nucleus with a complete set of chromosomes

4 phases of nuclear division

Prophase

Metaphase

Anaphase

Telophase

Prophase

DNA has already replicated

Chromosomes coil up/condense - shorten and thicken

Nuclear envelope disappears

Nucleolus disappears

Centrioles move to opposite sides of the nucleus

Spindle fibres form

Metaphase

The chromosomes arrange themselves on the equator of the spindle

The microtubules are attached to the centromeres

Anaphase

The centromere divides in two

The attached microtubules contract and pull chromatids to opposite poles of the cell, centromeres first

Chromatids now called daughter chromosomes

Telophase

Daughter chromosomes reach the poles and uncoil and lengthen

Nuclear envelope and nucleolus re-form

2 new nuclei are formed

Spindle fibres disintegrate

Cytokinesis

division of the rest of the cell (cytoplasm and organelles) after the nucleus

Significance of mitosis

Mitosis allows the production of cells that are genetically identical to the parent and so gives genetic stability

Living things grow by producing more cells - mitosis leads to the growth of an organism

Repair of damaged tissue and replacement of dead cells

Example of mitosis

In the root tip - embryonic cells also divide by mitosis

Mitosis and cancer

Cancer is an uncontrolled cell division

Cancerous cells divide repeatedly with the formation of tumour

Cancerous cells prevent normal cells/organs from functioning

Cancers are thought to be initiated when changes occur in the genes that control cell division

Tumour

irregular mass of cells

Asexual reproduction

Results in complete offspring that are identical to the parent

Takes place in certain flowering plants where organs such as bulbs, tubers and runners produce large numbers of identical offspring

Comparison of mitosis and meiosis

Mitosis has one division resulting in two daughter cells whereas meiosis has two divisions resulting in 4 daughter cells

Mitosis number of chromosomes is unchanged whereas meiosis number of chromosomes is halved

Mitosis daughter cells are genetically identical whereas meiosis daughter cells are genetically different

Mitosis homologous chromosomes are not associated in pairs whereas meiosis homologous chromosomes pair up

Mitosis crossing over does not occur whereas meiosis crossing over occurs and chiasmata forms

Mitosis no variation between individuals whereas meiosis produces variation

Homologous pair of chromosomes

One chromosome from the mother and one chromosome from the father

Each one of a pair contains the same genes as the other but may have a different allele

Meiosis

Produces cells containing one set of chromosomes - one from each homologous pair

Occurs during sexual reproduction - 1 diploid cell divides to produce 4 haploid cells

2 stages of cell division: meiosis I and II

Somatic cells

Cell which contains diploid number of chromosomes

Gametes

Specialised cell containing half the number of chromosomes (haploid) needed to produce a zygote (diploid)

May be produced by meiosis which causes variety

Diploid

Cell contains homologous chromosomes - 2n

Haploid

Cell contains only one of each chromosome - n

How does meiosis produce variation in offspring?

Through crossing over between homologous chromosomes

Random assortment of homologous chromosomes in meiosis I

Random assortment of chromatid in meiosis II

Random assortment and production of haploid gametes for random fertilisation

Interphase for meiosis

Chromosomes are not yet visible

DNA replicates

Chromosomes are now made of two identical molecules of DNA

More organelles are synthesised

High rate of ATP and protein synthesis

Nuclear envelope and nucleolus still visible

Prophase I for meiosis I

Chromatin condenses, coils and thickens to become visible

Chromosomes are now visible as two chromatids

Centrioles move to opposite poles of the cell

Synapsis

Synapsis

Each homologous pair of chromosomes come together to form a bivalent

Late prophase I for meiosis I

Crossing over takes place between non-sister chromatids in the bivalent

Each chromatid may break and reconnect to another chromatid

Chiasma

Point of crossing over of homologous chromosomes in late prophase I in meiosis I

Metaphase I in meiosis I

Spindle fibres attach to the centromere and move the whole chromosome to the equator of the cell

The bivalents arrange themselves at the equator of the spindle

Homologous chromosomes arrange themselves randomly at the equator of the cell leading to genetic variation

Anaphase I in meiosis I

Spindle fibres attached to the centromere of each homologous chromosome shorten to pull them to opposite poles of the cell

Whole chromosomes are pulled (still made of 2 chromatids)

Telophase I in meiosis I

2 new nuclear envelopes reform

Each nucleus now contains half the number or original chromosomes

The chromosomes are genetically different from those in the original cell

Cytokinesis

Cytokinesis

The organelles, cytoplasm and membrane become evenly distributed in 2 new cells

Prophase II in meiosis II

Chromatin condenses and chromosomes become visible

Centrioles replicate

A new spindle forms at right angles to the first

Nuclear envelope and nucleolus disappear

Metaphase II in meiosis II

The spindle fibres align the chromosomes randomly on the equator of the spindle

Each chromosome is made up of a pair of chromatids

This phase again introduces genetic variation due to the random assortment of chromatids on the equator

Anaphase II in meiosis II

The microtubules contract and the centromeres divide

Chromatids are pulled to opposite poles of the cell by the attached spindle fibres. As soon as they are separated they are called chromosomes.

Telophase II in meiosis II

Chromosomes uncoil - each new chromosome may be genetically different from the original one

Nuclear envelope and nucleolus reappear

Cytokinesis begins

4 haploid cells are produced

Genetic variation of meiosis and fertilisation

During independent assortment of homologous chromosomes, maternal and paternal chromosomes are mixed up

During sexual reproduction, there is random fusion of haploid gametes to form the zygote

Crossing over occurs between homologous chromosomes at the chiasmata. During crossing over, parts of homologous chromosomes may be exchanged producing new allele combinations

Independent assortment of chromatids in meiosis II

Mitosis vs meiosis

One division resulting in 2 daughter cells vs two divisions resulting in 4 daughter cells

Number of chromosomes is unchanged vs number of chromosomes is changed

Daughter cells genetically identical vs daughter cells genetically different

Homologous chromosome not in pairs vs homologous chromosomes pair up

Crossing over does not occur vs crossing over occurs

No variation vs variation

Parts of the female reproductive system

Ovary

Oviduct

Uterus

Cervix

Vagina

Urethra

Ovary

Female sex organs that produce the female gametes and secrete hormones oestrogen and progesterone

Oviduct

Connect the ovary to the uterus

Each tube ends in finger like projections which collect the oocyte at ovulation

Uterus

A compact organ made up of a muscular wall (myometrium) which contracts during child birth and the uterus lining (endometrium) which nourishes and protects the growing foetus.

The internal surface of the endometrium is shed each month (menstruation) if there is no embryo

Cervix

Neck of the uterus, a muscular ring that closes the entrance to the uterus but dilates during birth

Vagina

Muscular tube that leads to the outside of the body

Place of insertion for penis - receives male gametes

Urethra

A tube that leads from the bladder to the vagina and allows the passage of urine

Parts of the male reproductive system

Scrotum

Testes

Urethra

Penis

Vas deferens

Epididymis

Seminiferous tubules

Prostate gland

Seminal vesicle

Scrotum

An external sac that holds the testes outside the body - this gives an optimum temperature for sperm production of 35 degrees

Testes

Male reproductive organs that produce male gametes spermatozoa

Urethra (male)

Tube that connects the bladder to the outside, passes through the penis and transfers urine and semen to the outside - but not at the same time

Penis

Organ that is used to pass semen into the female reproductive system

Vas deferens

Tube that takes the sperm from the testes to the urethra

Epididymis

Sperm collect and mature here

Seminiferous tubules

Tubes found in testes - site of sperm production

Prostate gland

Gland found at base of bladder that produces an alkaline secretion that neutralises any urine left in urethra and aids in sperm mobility

Seminal vesicle

Gland that produces a mucus secretion that helps sperm mobility

Gametogenesis

The production of gametes in the gonads

Spermatogenesis

The formation of sperm in the testes

Oogenesis

The formation of eggs in the ovary

Stages of spermatogenesis

Each germinal epithelial cell in the outer layer of a seminiferous tubule divides by mitosis to produce spermatogonia

The spermatogonia further divide by mitosis and enlarge by cell growth to form primary spermatocytes

Each primary spermatocyte goes through meiosis I to form 2 haploid secondary spermatocytes

Secondary spermatocytes undergo meiosis II to produce spermatids

The haploid spermatids differentiate or mature into spermatozoa - they form their mid piece and tails

Key points of spermatogenesis

During maturation the spermatids obtain nutrients from nearby Sertoli cells - these cells protect the spermatids from the male immune system

The rate of formation of spermatozoa is high and continuous throughout the life of the sexually mature male

Between seminiferous tubules lie cells of Leydig (interstitial cells) that produce testosterone

Mature sperm

The acrosome is a thin vesicle which contains hydrolytic enzymes required to digest through the ovum wall

Haploid nucleus contains paternal chromosome set

Mid section of sperm contains many mitochondria which synthesise ATP to provide energy for the movement of the tail

The axial filament contains protein fibres which add longitudinal rigidity and provide a mechanism of propulsion

Oogenesis stages before birth

Each germinal epithelium cell divides by mitosis to produce many oogonia

The oogonia divide by mitosis and enlarge by cell growth to form primary oocytes

Primary oocytes divide by meiosis I but stops at prophase I

Some germinal epithelial cells also divided by mitosis to form follicle cells which surround the primary oocytes forming primary follicles

Oogenesis stages each month before ovulation

From puberty FSH from the pituitary gland stimulates one primary follicle to develop into a Graafian follicle each month

The primary oocyte inside completes the first meiotic division to form a haploid secondary oocyte and small polar body which degenerates

The secondary oocyte begins meiosis II but stops at metaphase II

Oogenesis stages at ovulation

The Graafian follicle protrudes from the surface of the ovary and ruptures - this projects the secondary oocyte into the oviduct - this is ovulation

Meiosis II will only be completed if fertilisation takes place and it is only then that an ovum is formed

The ruptured Graafian follicle becomes a temporary gland called the Corpus Luteum - this produces progesterone that maintains the endometrium

The ovulated secondary oocyte

The haploid nucleus at metaphase II sits inside the cell with a large volume of cytoplasm

The secondary oocyte and 1st polar body divided unequally at the end of meiosis I, the oocyte gaining the most of the cytoplasm

The secondary oocyte is now surrounded by a membrane called the zona pellucida which is composed of glycoproteins

Cortical granules are present in the cytoplasm just beneath the plasma membrane

Around the outside are follicle cells which form the corona radiata

The menstrual cycle

When hormonal and uterine changes recur

In the absence of an embryo, the endometrium is shed through menstruation

Day 0 of the menstrual cycle

First day of menstruation and the concentration of all 4 hormones are low

The hypothalamus is uninhibited and secretes GnRH (gonadotrophic releasing hormone)

GnRH stimulates the anterior lobe of the pituitary gland to secrete FSH and LH

This is positive feedback of GnRH on FSH and LH

Follicular phase of the menstrual cycle - day 0-14

GnRH stimulates the anterior pituitary gland to secrete FSH

FSH stimulates the development of a Graafian follicle from a primary follicle in the ovary

FSH stimulates the thecal cells of the Graafian follicle to produce oestrogen which increases

Oestrogen stimulates the rebuilding of the endometrium

Oestrogen inhibits FSH secretion

Oestrogen stimulates LH secretion from the pituitary gland resulting in a surge in LH

LH causes the Graafian follicle to rupture and ovulation on day 14

The menstrual cycle day 14

Main role of LH is to induce ovulation

Its surge in concentration caused by high oestrogen by day 14 causes the Graafian follicle at the surface of the ovary to release the secondary oocyte

Secretory phase of the menstrual cycle - day 15-28

LH stimulates the development of the Corpus Luteum

LH stimulates the Corpus Luteum to secrete progesterone

Progesterone maintains the endometrium and inhibits LH and FSH

The Corpus Luteum starts producing oestrogen which continues the rebuilding of the endometrium and inhibits FSH

With reduced FSH, the Corpus Luteum begins to degenerate which causes the hormones it makes to fall. So progesterone levels fall, no longer maintaining the endometrium which is shed and oestrogen levels fall, so FSH is no longer inhibited and the cycle begins again

The journey of the sperm

The sperm are released from the epididymis and travel along the vas deferens out of the penis through the urethra

Spermatozoa are deposited at the top of the vagina and swim through the cervix and along the lining of the uterus, then up an oviduct to its far end nearest the ovary where they meet the secondary oocyte

Fertilisation occurs here at the beginning of the oviduct nearest the ovary

Capacitation

Spermatozoa can only fertilise an ovum after capacitation

Capacitation of the sperm takes place as they move through the fluid in the uterus (takes about 7 hours) and involves the removal of 3 substances from the sperm

Which 3 substances does capacitation remove?

Removal of the glycoprotein outer layer that was added to the sperm plasma membrane in the epididymis

Removal of plasma proteins that were added to the sperm plasma membrane by seminal fluid from the male seminal vesicles

Removal of cholesterol from the sperm plasma membrane which weakens the membrane and allows calcium ions to enter the sperm

The calcium ions cause the tip of the acrosome membrane to fuse with the sperm head plasma membrane ready for the acrosome reaction. This allows some hydrolytic enzymes to leak from the sperm head membrane.

Calcium ions also caused increased tail activity

Fertilisation and the acrosome reaction

Sperm cells digest their way through the corona radiata due to hydrolytic enzymes leaking from the sperm head

Contact of the sperm head with the zona pellucida results in the acrosome reaction. The membrane ruptures and the hydrolytic protease enzymes are released which digest the zona pellucida

The first sperm to get through the zona pellucida meets the plasma membrane of the secondary oocyte and their plasma membranes fuse

The haploid sperm nucleus enters the secondary oocyte

The fusion of the sperm and the oocyte plasma membranes start the cortical reaction and meiosis II completion of the secondary oocyte to form an ovum

Fertilisation and the cortical reaction

Cortical granules (lysosomes containing enzymes) lie just beneath the plasma membrane of the secondary oocyte

After fusion of the sperm head membrane and secondary oocyte membrane, the oocyte SER release calcium ions which cause the cortical granules to move towards and fuse with the secondary oocyte membrane. The cortical granules release their enzymes into the zona pellucida by exocytosis.

The enzymes expand and harden the zona pellucida to prevent entry of any further sperm (polyspermy). The zona pellucida is now called the fertilisation membrane.

Fertilisation and the formation of a zygote

Fusion of the sperm head plasma membrane with the secondary oocyte plasma membrane stimulates the completion of meiosis II by the secondary oocyte which had been suspended at metaphase II.

Fertilisation occurs when the 2 nuclei fuse

Within 24 hours the first mitotic division sees the sperm and oocyte chromosomes lined up on the equator and the fertilised oocyte is now a zygote

As soon as the first mitotic division is complete, the 2 cells are now known as an embryo

Implantation of the blastocyst

After the first mitosis, the 2 cells of the embryo divide by mitosis many times to form a hollow ball of cells called the blastocyst.

The division of the embryo cells is known as cell cleavage

After 3 days the blastocyst consists of an outer layer of cells called the trophoblast, a large group of cells called the inner mass and a fluid filled cavity called the blastocoele.

The trophoblast cells develop protrusions called trophoblastic villi which penetrate the endometrium

The villi increase surface area for absorption of nutrients from the endometrium

The trophoblast develops into the chorion, an outer membrane surrounding the embryo

The placenta

Made of foetal and maternal tissues

Foetal tissues - trophoblasts develops into the chorion, an outer membrane surrounding the embryo which secretes HCG. The chorion develops into chorionic villi with blood capillaries that connect to the umbilical artery and vein. Chorionic villi cells have microvilli which give a large surface area in contact with the mother’s blood.

Maternal tissues - the endometrium forms projections, between these projections are spaces called lacunae which are supplied with blood from the mother

The amnion

Outer cells of the embryo form a membrane called the amnion which forms a fluid filled sac surrounding the foetus called amniotic fluid.

Functions of amniotic fluid

Acts as a shock absorber and protects foetus during development

Maintains the foetus’ temperature

Contributes to lung development

Allows movements so muscles and bones function before birth

The placenta’s counter current flow

The embryo’s and mother’s blood do not make contact

The umbilical cord develops from the chorion and transfers foetal blood to the placenta in two umbilical arteries (low in e.g. glucose and oxygen). Blood returns to the foetus in an umbilical vein (high in e.g. glucose and oxygen).

Functions of the placenta

As an endocrine organ

Exchange of glucose, oxygen, CO2 and urea etc between mother’s and foetus blood

A physical barrier between foetal and maternal circulation - it protects the fragile capillaries in the foetus from damage by the higher blood pressure of the mother

Provides passive immunity (antibodies) from mother to foetus

Protects foetus from mother’s immune system - the cells of the chorionic villi fuse so that white blood cells cannot get through

Placental problems

The placenta does not always provide complete immunological protection:

Spontaneous abortions are equivalent to rejection of a transplanted organ

An abnormal immune response towards the placenta can result in pre-eclampsia among some women

Rhesus disease in a foetus due to Rhesus incompatibility - destruction of foetus’ blood cells by antibodies made by Rhesus negative mother

Transfer across the placenta of small viruses such as Rubella and drugs such as alcohol and nicotine

Pregnancy hormones

HCG secreted by the embryo moves via the blood stream to the ovaries where it maintains the corpus luteum

The corpus luteum maintains the production of hormones oestrogen and progesterone which maintain pregnancy

Progesterone maintains the uterus lining and prevents uterine contraction until levels drop at end of pregnancy

Oestrogen stimulates growth of uterus to accommodate the foetus and stimulates the growth of the mammary glands

After 2 months the placenta has developed and takes over the production of oestrogen and progesterone from the corpus luteum which degenerates

The presence of HCG in the blood and urine is used as the basis of the pregnancy test

The placenta as an endocrine gland

The placenta steadily increases the concentrations of oestrogen and progesterone in the plasma until the very end of pregnancy.

Oestrogen and progesterone inhibit the secretion of FSH so that no more follicles mature and Prolactin so that milk is not produced.

Progesterone also inhibits LH so no ovulation can occur and Oxytocin, a hormone that causes the myometrium to contract.

Hormones and birth

At 39 weeks, oestrogen and progesterone levels drop which removes the inhibition on oxytocin and prolactin production

Oxytocin

Secreted by the posterior pituitary gland and causes contraction of the myometrium

Mild initial contractions stimulate the production of more oxytocin - positive feedback

Prolactin

Secreted by the anterior pituitary gland and causes the glandular tissue in the mammary glands to synthesise milk

Milk is released when oxytocin causes the muscles around the milk ducts to contract