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Topic last updated: 27 Jan 2022
    • Biology Biology
    • Ico Chemistry Chemistry
    • Ico Human Biology Human biology
    • 16+
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Cell division

9  of  10

Sexual reproduction 1 - gametogenesis

In sexual reproduction, both meiosis and mitosis are needed to produce new, genetically unique individuals.

Gametogenesisgametogenesis
 The production of the gametes in the sex organs of an organism. in animals and plants

Meiosis, the cell divisioncell division
 The process by which a parent cell divides into two daughter cells. that halves the number of chromosomechromosome
 A chromosome is like a packet of coiled up DNA. Humans have 23 pairs of chromosomes. They are in the nucleus of every human cell. s in a cell, is a key part of gametogenesis but it is not the whole story. The process of making eggs or sperm in mammals, or ovules and pollen in plants, involves more than meiosis.

In mammals, the male produces large numbers of very small, mobile gametegamete
 The sex cells (ova and sperm) that join together to form a new unique diploid cell in sexual reproduction. s called spermatozoa (sperm). The sperm carry a haploidhaploid
 Have a single set of chromosomes. nucleus. They contain many mitochondriamitochondria
 Organelle(s) within cells that produce ATP, used as a store of chemical energy. Often called the cell's powerhouse to generate the ATP needed for the contraction of the specialised proteins in the tail. This lashes from side to side to help keep the sperm suspended in the semen and moving when released into the female reproductive tract.

Micro Electron
Sperm Cell
Human Ovum

Scanning electron micrograph of mammalian/human ovum (Photo credit: Wellcome Images. Licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license)

Ovum

Human Spermatogenesisspermatogenesis
 The production of sperm.

Spermatogenesis is the production of spermatozoa and it starts in the human male at pubertypuberty
 The time of gradual development of the secondary sexual characteristics and sexual maturity. . Sperm production continues throughout life, although the numbers fall and the quality of the sperm declines with age. Many more mutationmutation
 A change in the arrangement or amount of genetic material in a cell. s appear in the sperm as a man gets older, making the sperm less fertile and increasing the chance of passing on mutated genesmutated genes
 Genes where the sequence of DNA has changed.  to the offspring.

The testes contain primordial germ cells. They divide both by mitosis and meiosis to produce millions of spermatozoa every single day. Each primordial germ cell produces many spermatozoa.

Mitosis: diploiddiploid
 Have two sets of paired chromosomes. primordial germ cells divide several times to form diploid spermatogonia
Growth: spermatogonia grow to form primary spermatocytes
Meiosis I: primary spermatocytes undergo the first meiotic division to produce 2 haploidhaploid
 Have a single set of chromosomes. cells called secondary spermatocytes
Meiosis II: secondary spermatocytes undergo second meiotic division to form four haploid cells called spermatids
Differentiationdifferentiation
 The genetically-controlled process by which an unspecialised stem cell becomes a cell with specialised structures which carries out a particular function. : spermatids differentiate in the tubules of the testes to form spermatozoa
Human Spermo

Oogenesisoogenesis
 The formation of ova. (egg production) in mammals

Oogenesis is the formation of ova. All of the primordial germ cells a girl will ever possess are present in her ovary at birth. Each primordial germ cell will only ever produce one ovum. Because each ovum contains so much more material than a sperm it would be a waste of resources to make too many.

Mitosis: diploiddiploid
 Have two sets of paired chromosomes. primordial germ cells divide several times to form diploid oogonia. Most of these degenerate – only one continues to grow.
Growth: Food stores are built up in oogonia to make a large diploid cell called a primary oocyte.
Meiosis I: primary oocytes undergo the first meiotic division to produce two haploidhaploid
 Have a single set of chromosomes. cells of very unequal size. The large cell is the secondary oocyte. The much smaller cell is the first polar body. No more changes occur until after ovulationovulation
 The release of a ripe egg from its follicle in the ovary.  and fertilisation by spermatozoa.
Meiosis II: Only takes place after fertilisation. The secondary oocyte undergoes second meiotic division to form the haploid ovum and another polar body. The first polar body divides to form two more polar bodies. The three polar bodies simply degenerate and die.
Oogenesis
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Sexual reproduction 2 - fertilisation
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