Reproductive Systems, Cell Division and Ovarian/Menstrual Cycle Test (11)

List the four main stages of mitosis

prophase, metaphase, anaphase and telophase

Describe the main events occurring in interphase

The period between cell divisions.

G1 phase: during the first growth phase, the cell produces new proteins, grows and carries out normal tasks. The phase ends when the cell’s DNA starts to replicate.

S phase: during the synthesis phase, DNA molecules in the nucleus form exact copies of themselves.

G2 phase:  during the second growth phase, a relatively short phase, the cell prepares for cell division

Describe the main events occurring in prophase

• Two pairs of centrioles become visible, move to opposite poles and microtubules begin to radiate from them.

• The nucleolus disappears, and the nuclear membrane begins to break down.

• Chromatin threads become tightly coiled and can be seen as chromosomes (each with two chromatids) which are easier to distribute to daughter cells.

Describe the main events occurring in metaphase

The chromatid pairs line up on the equator of the cell. The centromere of each pair is attached to a spindle fibre.

Describe the main events occurring in anaphase

Each pair of chromatids separates at the centromere. Once they are independent of each other, they are now considered chromosomes. The new chromosomes are pulled away from one another to opposite poles by the spindle fibres.

Describe the main events occurring in telophase

The two sets of chromosomes form tight groups at each pole of the cell and a nuclear membrane forms around each group. Spindle fibres disappear and the chromosomes gradually uncoil to form chromatin again.

Describe the main events occurring in cytokinesis

Telophase is the last phase of mitosis, however, while telophase occurs, the cytoplasm begins to divide. This is cytokinesis. A furrow develops in cytoplasm between nuclei which deepens until it cuts the cytoplasm into two parts.

Explain the significance of preserving the chromosome number in mitosis

Preserving the chromosome number during mitosis is essential for maintaining genetic stability. Mitosis produces two daughter cells with the same number of chromosomes as the parent cell, ensuring genetic consistency. This process is crucial for growth, development, tissue repair, and asexual reproduction, as it guarantees that new cells carry the correct genetic information for proper functioning.

Define the term gamete

A sperm or egg cell

Define the term gonad

Primary sex organs

Describe the function of the uterus

Contains a lining called the endometrium, protects and nourishes the developing foetus during pregnancy.

Describe the function of the fallopian tubes

 Carries eggs to the uterus, site of fertilization.

Describe the function of the vagina

Receives the penis during sexual intercourse, forms the birth canal.

Describe the function of the fimbriae

Projections at the end of the fallopian tubes direct the eggs into the fallopian tubes.

Describe the function of the clitoris

Contains erectile tissue.

Describe the function of the Labia majora

Fleshy folds of skin at the opening of the vagina.

Describe the function of the cervix

The entrance to the uterus, where the semen is deposited.

Describe the function of the labia minora

Fleshy folds of skin at the opening of the vagina.

Describe the function of the penis

The organ for passing urine and for transferring sperm from the male to the female.

Describe the function of the urethra

Carries semen and urine out of the body.

Describe the function of the foreskin

Covers the head of the penis.

Describe the function of the seminal vesicles

Secretes a fluid rich in sugars and the main component of semen, sugar acts as an energy source for sperm.

Describe the function of the prostate gland

Secretes an alkaline fluid that helps activate sperm, neutralizes vagina.

Describe the function of the bulbourethral gland

Secretes a fluid that becomes part of the semen. Used as a lubricant, pre ejaculation fluid mostly comprised of it.

Describe the function of the epididymis

Stores sperm while they finish maturing.

Describe the function of the scrotum

Holds the testes.

Describe the function of the testes

Produces sperm and hormones.

Describe the function of the erectile tissue

Fills with blood to make the penis erect.

Describe the function of the vas deferens

Carries the sperm to the urethra.

Distinguish between the diploid and haploid number of chromosomes.

Diploid: The number of chromosomes in a cell with both chromosomes from each homologous pair

Haploid: The number of chromosomes in a cell with only one chromosome from each homologous pair; half the diploid number.

Describe the process of meiosis (First meiotic division)

Interphase

• DNA replicates

 Prophase I

• Nucleolus disappears and nuclear membrane breaks down

• Chromatin becomes tightly coiled to form chromosomes
  (with sister chromatids) which begin to migrate towards
  Equator

• Two pairs of centrioles become visible and move to poles.
  Spindles begin to radiate from them.

 Metaphase I

• Homologous pairs are arranged along spindle fibre across the equator of the cell.

 Anaphase I

• Pairs of homologous chromosomes move apart - one chromosome from each pair moves to each pole. Sister chromatids are not separated.

 Telophase I

• Chromosomes decondense and nuclear membrane may reform

• Cytokinesis occurs and cell membrane forms producing two separate cells.

 After the first meiotic division, each daughter cell now only has 23 chromosomes– a major difference form mitosis. Each cell contains a mix of paternal and maternal chromosomes.

Describe the process of meiosis (Second meiotic division)

Prophase II

• New spindle forms and chromosomes gradually move towards equator.

 Metaphase II

• Chromosomes are arranged on the new spindle.

• Centromeres divide so each chromatid is a separate chromosome

 Anaphase II

• New chromosomes migrate to opposite poles of the cell.

 Telophase II

• Nuclear membranes begin to form, and the cytoplasm starts to divide.

 During Meiosis, DNA replicates once but cells divide twice which results in four, haploid daughter cells which are NOT identical to the parent cell.

 Explain the significance of halving the chromosome number in a meiotic division.

Halving the chromosome number during meiotic division is significant because it ensures that when gametes fuse during fertilization, the resulting offspring have the correct diploid number of chromosomes. This reduction is crucial for maintaining genetic stability across generations, as it prevents the doubling of chromosome numbers that would occur if gametes were diploid.

Compare the processes of mitosis and meiosis

Mitosis	Meiosis
One duplication of chromosomes and one nuclear division.	One duplication of chromosomes and two nuclear divisions.
Produces two diploid cells.	Produces four haploid cells.
Homologous chromosomes do not pair.	Homologous chromosomes pair up.
Chromatids separate so that each new cell gets a complete set of daughter chromosomes.	At first meiotic division, members of homologous pairs separate so that new cells get a haploid set of chromosomes. At second division, chromatids separate, giving four haploid cells.
Chromosomes do not change their genetic make-up.	Genetic make-up of chromosomes can be changed through crossing over.
Produces new cells for growth and repair.	Produces haploid gametes for sexual reproduction.

Explain how crossing over leads to variation in offspring

• Occurs during Prophase I

• When homologous pairs line up, chromatids may cross, break and exchange segments – called crossing over.

  • Point at which chromatids cross called chiasma (plural: chiasmata)

• Creates a new combination of alleles along the chromosome and therefore a new combinations of genes – chromosomes passed to offspring not exactly the same as parents.

  • This is called recombination.

Non-disjunction

• Non-disjunction results in one daughter cell receiving an extra chromosome and the other daughter cell lacking a chromosome.

  • Homologous pairs fail to separate in first division

  • One or more of the chromatids fail to separate in the second division.

• In humans, non-disjunction results in gametes with either 22 or 24 chromosomes, meaning after fertilisation, the zygote will have either 45 or 47 chromosomes. This often results in early miscarriage or a number of distinctive birth defects.

• Non-disjunction can result in:

  • Trisomy – individual inherits extra chromosome (three copies)

  • Monosomy – individual is missing a chromosome (one copy)

  • Partial trisomy/monosomy – inherit part of an additional chromosomes, or part of a chromosome is missing.

Describe the process of spermatogenesis.

Spermatogenesis occurs inside the seminiferous tubules of each testis which are lined with immature cells called spermatogonia (sperm mother cells). These are diploid cells which begin dividing by mitosis at puberty providing a constant source of new cells for sperm production.

1. Some daughter cells of spermatogonia are pushed towards the centre of the tubule, where they undergo a period of growth. These enlarged cells are called primary spermatocytes and are diploid cells.

2. Primary spermatocytes undergo the first stage of meiosis to produce secondary spermatocytes – haploid cells.

3. Second meiotic division divides each secondary spermatocyte into two spermatids (totalling four haploid spermatids from one diploid spermatogonium).

4. Finally, spermatids mature into spermatozoa (sperm). During this time most cytoplasm is lost, acrosome appears and  a tail with contractile material forms.

List the hormones involved in spermatogenesis.

Follicle stimulating hormone (FSH) and luteinising hormone (LH)

Describe the process of oogenesis.

• At puberty, the process of follicle growth and maturation begins. As the follicle matures, the primary oocyte contained within it completes the first division of meiosis to produce two haploid cells. These cells are unequal size

  • The large cell, the secondary oocyte, receives half the chromosomes but nearly all cytoplasm.

  • The smaller cell, the first polar body, received the other half of the chromosomes but very little cytoplasm.

• The secondary oocyte immediately starts the second meiotic division but stops at metaphase. At this stage ovulation occurs – the follicle ruptures expelling the secondary oocyte and the polar body.

• If the secondary oocyte is penetrated by a sperm, meiosis is quickly completed producing an additional two haploid cells of unequal size.

  • The larger develops into an ovum  (mature egg)

  • The smaller being the second polar body.

• All polar bodies disintegrate, thus, oogenesis produces a single ovum from each primary oocyte.

Similarities between spermatogenesis and oogenesis

• Both create haploid daughter cells (gametes)

• Both involve mitosis, meiosis and maturation stages

• Both take place in the sex organs only

Differences between spermatogenesis and oogenesis

Feature	Spermatogenesis	Oogenesis
Rate of production	Fast	slow
Age at which gametogenesis commences	Puberty	Fetus
Age at which gametogenesis ceases	Death	menopause
Meiosis is completed with or without long pauses	Yes	No
Stage of maturity of cell when released from sex organ	Final (spermatozoa) completed meiosis	Immature secondary oocyte (metaphase 2)
Number of functional cells produced per meiosis	4	1
How cells are nourished when forming	Seminal juices	cytoplasm

Define ovarian cycle

Series of events that takes place in ovary involving Maturation of egg + release into fallopian tube.

Describe the effect of Gonadotropin-releasing hormone (GnRH) in ovarian and menstrual cycle

Source: Hypothalamus. Target Organ: Anterior pituitary gland. Function: Stimulates the secretion of Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH) from the anterior pituitary gland.

Describe the effect of Follicle Stimulating Hormone (FSH) in ovarian and menstrual cycle

Source: Anterior pituitary gland. Target Organ: Ovary Function: Causes immature oocyte and follicle to develop. Increases oestrogen production. Stimulates development of the uterine wall following menstruation.

Describe the effect of Luteinizing Hormone (LH) in the ovarian and menstrual cycle

Source: Anterior pituitary gland. Target Organ: Ovary. Function: Stimulates ovulation. Increases progesterone secretion. Stimulates secretion of oestrogen. Aids in the development of the corpus luteum.

Describe the effect of Progesterone in the ovarian and menstrual cycle

Source: Ovary (corpus luteum). Target Organ: Uterus. Function: Stimulates the thickening of the uterine wall/maintenance of the uterine wall.

Describe the effect of Oestrogen in the ovarian and menstrual cycle

Source: Ovary (ovarian follicle and corpus luteum). Target Organ: Uterus. Function: Stimulates the thickening of the uterine lining (endometrium) Increases LH secretion form the pituitary gland. Maturation of oocytes. Development of female sexual characteristics. Inhibits FSH secretion from the pituitary gland. Increases LH secretion form the pituitary gland.

Describe the effect of Human chorionic gonadotropin (HCG) from placenta in the ovarian and menstrual cycle

Source: Placenta. Target Organ: Corpus luteum. Function: Maintenance of corpus luteum during early stage of pregnancy.

Define term menarche

When menstruation first begins (commencement of puberty)

Define term menstruation

The periodic discharge of blood and tissue fluid due to the breakdown of the lining of the uterus.

Describe how Follicle Stimulating Hormone (FSH) influence the production of sperm in males

Source: Anterior pituitary gland. Target Organ: Seminiferous tubules in testes. Function: Production of sperm (spermatogenesis).

Describe how Luteinising Hormone (LH) influences the production of sperm in males

Source: Anterior pituitary gland. Target Organ: Interstitial cells of testes. Function: Promote secretion of testosterone in testes.

Describe how Testosterone influences the production of sperm in males

Source: Cells in testes. Target Organ: various. Function: Development of immature sperm into spermatozoa, maintenance of male reproductive organs, sex drive.

Maturation of Sperm

Most cytoplasm is lost. Acrosome is formed (contains enzymes which help sperm fertilise the ovum/breaks down corona radiata). Tail is formed (flagellum).

Define menstrual cycle

Series of changes (of endometrium) occurring in uterus, prepares for possibility of pregnancy, closely associated to ovarian cycle.

Describe main event of menstrual cycle

Menstruation Day 1-4/5:

• Degeneration of the corpus luteum into the corpus albicans = massive fall in the amount of progesterone.

• Fall in progesterone trigger for menstruation to start

• Uterine bleeding, accompanied by shredding of endometrium

 

Proliferative Stage Day 4/5-14

• Developing follicle (Graafian) in the ovary is producing large amounts of oestrogen

• Proliferation of endometrial cells i.e. lining starts to develop and increase in thickness

 

Secretory Phase Day 14-28

• Following ovulation the ruptured follicle breaks down into the corpus luteum

• Corpus luteum makes/secretes progesterone

• Progesterone increases the thickness of the endometrium and increases secretion of mucus and blood flow

 

No fertilisation

• No embryo created via fertilisation = no implantation

• Corpus luteum starts to degenerate into the corpus albicans = fall in progesterone = trigger for menstruation day 1

 

Fertilisation

• Embryo created = implantation

• Implantation of the embryo results in an increase in the pregnancy hormone HCG

• HCG maintains the corpus luteum = progesterone production is maintained = no menstruation