20. Gametogenesis

What is the key difference between anaphase I in meiosis and anaphase in mitosis?

A key difference between mitosis and meiosis is that sister chromatids remain joined during anaphase I in meiosis, whereas in anaphase of mitosis they separate.

Spermatozoa

A mature sperm cell

What is inside the zona pellucida?

A fluid filled space before the egg cell membrane.

What happens in telophase II?

A nuclear envelope forms around each set of chromosomes and cytokinesis occurs, producing four daughter cells, each with a haploid set of chromosomes.

Karyotype

A picture of all the chromosomes in a cell arranged in pairs

Spermatogenesis

A process of formation and maturation of sperm in "SMFT" somatic cell division -->"Mitosis" and reproductive cell division --->"Meiosis" a reduction division. Four spermatozoa are produced from one spermatocyte each carries haploid # of 23 chromosomes.

What is a tetrad?

A tetrad is a group of four chromosomes (2 sets of sister chromatids) that come together during sexual reproduction. Crossing-over occurs at the level of the tetrad.

List the events that occur in prophase I.

• The duplicated homologous chromosomes pair. • Crossing-over between the homologous chromosomes occurs. • The nucleolus disappears. • The meiotic spindle forms. • At the end of prophase I, the nuclear envelope disappears and the spindle enters the nucleus.

What is a diploid?

A cell with two sets of chromosomes, contains both parent chromosomes

What is a haploid?

A cell with only one set of chromosomes, used in sexual reproduction.

Fertilisation

1. Fertilisation is the fusion of the male and female nuclei of the gametes 2. The sperm cell (23 chromosomes) and the ovum (23 chromosomes) fuse to form a zygote (diploid - 46 chromosomes) 3. The zygote is the first cell of a new individual who is genetically unique

If a cell has a diploid number of 22, how many total chromosomes should it have?

22

When is meiosis II of the oocyte completed?

Once it is fertilised.

What might be the consequence if spindle fibres failed to attach to a pair of sister chromatids in meiosis II?

If spindle fibres did not attach to a pair of chromatids, then the two gametes resulting from the cell in which this occurred would be abnormal.

What happens in prophase II?

In prophase II, the nuclear envelope breaks down and the spindle apparatus forms.

A female infant is born with several hundred oocytes, each one genetically unique. This is due to ________.

Independent assortment and random crossover

Independent assortment

Independent segregation of genes during the formation of gametes.

What other forms of foetal testing are there?

1. CVS 2. A DNA probe 3. Blood chemistry

Cytokenesis

1. Cell membrane begins to pinch off 2. Two daughter cells each have identical DNA

Metaphase

1. Centromeres of duplicated chromosomes are aligned at plate. 2. Fully formed spindle attach to the sister chromatids from opposite poles 3. Become more centred

Telaphase

1. Chromosomes arrive at the poles of the cell and begin to relax; 2. Two nuclear membranes begin to form 3. Spindle apparatus disassemble

Prophase

1. Chromosomes become visible 2. nuclear envelope dissolves 3. spindles form

Meiosis - gametogenesis

1. Gamete formation 2. 2 consecutive divisions - meiosis I and meiosis II 3. Exchange of genes 4. 4 daughter cells 5. ½ number of chromosomes (46 to 23) 6. Introduction of genetic diversity

What happens in Meiosis I?

1. Homologous chromosomes separate and produce two haploid daughter cells. 2. It is the reduction division making the daughter cells haploid (when the parent cell was diploid). 3. It is during meiosis I that most of the genetic recombination occurs.

Hormonal control

1. Pulsatile release of gonadotropin releasing hormone (GnRH) from hypothalamus 2. Follicle stimulating hormone (FSH) 3. Luteinizing hormone (LH)

What happens in Meiosis II?

1. The daughter cells divide again but the DNA is not replicated. 2. Four haploid daughter cells are produced.

Oogenesis

1. The production of ova in the ovaries 2. Before birth, germ cells in ovaries - oogonium 3. Oogonium divides by mitosis to form primary oocyte 4. They begin meiosis I, but stop at prophase I until puberty 5. From puberty onwards, each month one primary oocyte completes meiosis I 6. One primary oocyte forms a small polar body and a large secondary oocyte (haploid) 7. Meiosis II starts but stops in metaphase and will only be completed once ovulation occurs 8. This will result in a second polar body and a very large ovum (functional gamete)

What are the mechanisms by which genetic variation is produced by meiosis?

1. Through the way the paternal and maternal chromosomes segregate 2. by the process of crossing-over.

How many chromosomes are sex chromosomes?

2 (one pair)

The maternal and paternal chromosomes of each homologous pair separate and reach the poles of the cell, nuclear envelopes form around them, and cytokinesis follows to produce two cells.

Telophase I

What is genetic variation?

Differences among individuals in the composition of their genes or other DNA segments. Crossing over and exchange of chromosomal parts that occur during meiosis II.

What type of cells are body cells?

Diploid (2n) - contain 2 sets of chromosomes

How many chromosomes are homologous (autosomes)?

44 (22 pairs)

How many chromosomes are in a diploid human cell?

46

How long will the ovum survive once ovulation occurs?

48 hours

What is a eukaryotic cell?

A cell characterized by the presence of a nucleus and other membrane-bound organelles. Eukaryotes can be unicellular (protists) or multicellular (fungi, plants and animals).

Testosterone

A hormone produced by the testes that affects the production of sperm, the development of male secondary sex characteristics, & the sex drive.

Spermatogonia

Diploid cells that undergo meiosis to produce four haploid sperm of equal size

What is the most common type of foetal testing?

Amniocentesis

Spermatogonium

Cell which undergoes chromosome replication to produce a spermatocyte

Oogonium

Cell which undergoes chromosome replication to produce an oocyte

What holds duplicated chromosomes together?

Centromere

Anaphase

Centromere split in half, spindle fibres pull chromosomes to opposite pole.

What is polygene inheritance?

Polygenic inheritance is where a trait is produced from the cumulative effects of many genes. In humans, height, weight, and skin colour are examples of polygenic inheritance.

Crossing over

Process in which homologous chromosomes exchange portions of their chromatids during meiosis.

Genetic variation of individual chromosomes occurs during __________.

Prophase I

What meiosis phase do primary oocytes sit in until puberty?

Prophase I

What happens in Anaphase II?

The centromeres separate and the sister chromatids — now individual chromosomes — move toward the opposite poles of the cell.

What happens in metaphase II

The chromosomes become arranged on the metaphase plate, much as the chromosomes do in mitosis, and are attached to the now fully formed spindle.

Spermatogenesis (Powerpoint)

1. Formation of sperm in the seminiferous tubules of testes 2. Stem cells from which all sperm are formed - spermatogonia 3. These divide by mitosis into 2 daughter cells - one of the new cells - stays a stem cell and will divide again later 4. The other new cell - primary spermatocyte 5. Primary spermatocyte begins meiosis I 6. 2 cells at the end of meiosis I - secondary spermatocyte (haploid) 7. Secondary spermatocytes undergo meiosis II to produce spermatids 8. Undergo further maturation to become spermatozoa (sperm)

How long do sperm survive once ejaculated?

Up to 72 hours.

What hormones stimulate the primary oocyte to divide?

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

What is the product of spermatogenesis?

Formation of haploid spermatozoa

What is the zona pellucida?

Glycoprotein rich layer on the surface of the oocyte beneath the follicular cells that contains proteins that act as spermatocyte receptors which are critical fro normal fertilization.

Which type of cell is released during ovulation?

Secondary oocyte

Tetrad

Structure containing 4 chromatids that forms during meiosis 1. Crossing over of genes.

What are chiasmas?

The point where two homologous non-sister chromatids exchange genetic material during chromosomal crossover during meiosis. When each tetrad, which is composed of two pairs of sister chromatids, begins to split, the only points of contact are at the chiasmata.

What happens when the sperm reaches the zona pellucida?

The sperm attach to specialised sperm receptors on the zona pellucida surface. This triggers their acrosomes to release digestive enzymes that enable the sperm to burrow into the layer.

When luteinising hormone (LH) peaks what happens?

Triggers the release of the ovum.

Which hormone is absolutely necessary for ovulation to occur?

LH

What is the corona radiata?

Less dense so easier for the sperm to get through.The innermost layer of follicular cells, closest to the zona pellucida; it will stay with the oocyte after ovulation until meiosis 2 begins

Spermatogenesis (Order)

Meiosis I 1. Stem cell 2. Primary spermatocyte 3. Secondary Spermatocyte (haploid) Meiosis II 4. Spermatids 5. Spermatozoa

The process of Meiosis

Meiosis consists of two successive nuclear divisions, meiosis I and meiosis II. Each division consists of these stages: prophase, metaphase, anaphase, and telophase.

What is Meiosis?

Meiosis involves two successive divisions of a diploid (2N) eukaryotic cell of a sexually reproducing organism that result in four haploid (N) progeny cells, each with half of the genetic material of the original cell. Through the mechanisms by which paternal and maternal chromosomes segregate, and the process of crossing-over, genetic variation is produced in the haploid cells.

What is the stage of meiosis during which the pairs of chromosomes (the bivalents) become arranged on a plane equidistant from the poles?

Metaphase I

What is monogenic inheritance?

Monogenic inheritance refers to the kind of inheritance where a trait is determined by the expression of a single gene or allele. AKA Mendelian inheritance.

Learning outcomes - Gametogenesis

On successful completion of this topic you will be able to : 1. describe the steps in meiosis resulting in the formation of haploid gametes 2. name sites and cells involved in oogenesis and spermatogenesis 3. explain the influence of gonadotrophins 4. explain the process of fertilisation in humans

How many functional gametes are produced by oogenesis?

One

What is a polar body?

One of the two cells formed when a primary oocyte divides; it gets almost no cytoplasm and eventually disintegrates.

What is a chromatid?

One of two identical strands into which a chromosome splits during Anaphase

Karyotype (Image)

Tall to small in maternal/paternal pairs

What is a zygote?

The diploid (2n) cell resulting from the fusion of male and female gametes (fertilization)

During the secretory phase of the uterine cycle __________.

the endometrium prepares for implantation

Heterozygous

An organism that has two different alleles for a trait

Homozygous

An organism that has two identical alleles for a trait

The two chromosomes in each bivalent separate and migrate toward opposite poles.

Anaphase I

In what stage of meiosis do the centromeres separate, and the two chromatids of each chromosome move to opposite poles on the spindle?

Anaphase II

Autosomes

Any chromosome that is not a sex chromosome

What is sex-linked inheritance?

Sex-linked inheritance refers to the inheritance depending on genes located on the sex chromosomes. Where the Y chromosome does not cover the X completely leaving the recessive visible.

Order of oogenesis

Stem cell Primary oocyte (present at birth) ----->meiosis I Secondary oocyte + polar body (monthly from puberty) ----->meiosis II Ovum + polar body (only on fertilisation)

A nuclear envelope forms around each set of chromosomes and cytokinesis occurs, producing four daughter cells, each with a haploid set of chromosomes.

Telaphase II

What does the release of GnRH from the hypothalamus to the anterior pituitary gland do?

Tells the anterior pituitary gland to release FSH which stimulates the oocyte to mature ready for ovulation.

Why do sperm release enzymes when reached the ovum?

The enzymes help the sperm get through the corona radiata

What is fertilisation?

The fusion of the male and female nuclei of the gametes.

If a diploid cell entering meiosis has 6 chromosome pairs, what is the number of possible chromosome combinations in the haploid nuclei?

The general formula for the number of possible chromosome combinations is 2n, where n is the number of chromosome pairs. For 6 chromosome pairs, the number of possible chromosome combinations is 2(to the power of)6 = 64.

What happens when the egg and sperm membranes touch?

They fuse and the egg pulls the sperm inside the membrane. This causes changes (calcium productions) in the outer membrane which stops any other sperm from entering.

Deletion

When part of a chromosome is lost. (1) A deficiency in a chromosome resulting from the loss of a fragment through breakage. (2) A mutational loss of one or more nucleotide pairs from a gene.

Formation of a secondary oocyte occurs during __________.

the follicular phase