16.1. Gametogenesis

Question 1

Gametogenesis

Answer 1

Production of Haploid gametes from diploid somatic (body) cells

Question 2

In mammals, the male gametes are called…

Answer 2

Spermatozoa (sperm)

Question 3

In mammals, the female gametes are called…

Answer 3

ova

Question 4

Spermatogenesis

Answer 4

The formation of male gametes which takes place in tubules in the testes

Question 5

Spermatogenesis Process

Answer 5

• Here, diploid cells divide by mitosis to produce numerous diploid spermatogonia, which grow to form diploid primary spermatocytes.
• The first division of meiosis then takes place,
  forming two haploid secondary spermatocytes.
• The second division of meiosis then produces haploid spermatids, which mature into spermatozoa (haploid).

Question 6

When do the cells become haploid?

Answer 6

• After the first division of Meiosis

- After the second division of Meiosis, cells remain haploid but only have 1 chromatid each

Question 7

Oogenesis

Answer 7

The formation of female gametes which takes place in the ovaries
- Similar to spermatogenesis in that it follows a similar pattern, but many fewer gametes are made, and the process takes much longer, with long ʻwaiting stagesʼ.

Question 8

Oogenesis Process

Answer 8

• Diploid cells divide by mitosis to form oogonia and grow
• These begin to divide by meiosis, but stop when they reach prophase I.
• At this stage, they are called primary oocytes, and they are still diploid. All of this happens before a baby girl is born, and at birth she has around 400 000 primary oocytes in her ovaries.
• When she reaches puberty, some of the primary
  oocytes get a little further with their division by meiosis.
• They proceed from prophase I to the end of the first
  meiotic division, forming two haploid cells.
• However, the division is uneven; one cell gets most of the cytoplasm, and becomes a secondary oocyte, while the other is little more than a nucleus, and is called a polar body.
• Each month, one secondary oocyte is released into
  the oviduct from one of the ovaries.
• If it is fertilised, it continues its division by meiosis, and can now be called an ovum (polar body also forms).

Question 9

Polar Body

Answer 9

Formed in oogenesis after Meiosis 1, when division is uneven; one cell gets most of the cytoplasm. It can be thought of as simply a way of getting rid of half of the chromosomes, and has no further role to play in reproduction.

Question 10

Fertilisation in Humans

Answer 10

• At Fertilisation, the chromosomes of the spermatozoan and the ovum join together to form a single diploid nucleus, and the cell that is made by this process is called Zygote
• The zygote can now divide repeatedly by mitosis to form first an embryo, and then a fetus.

Question 11

In flowers, male gametes are produced in…

Answer 11

the anthers

Question 12

In flowers, female gametes are produced in…

Answer 12

the ovules

Question 13

In flowers, male gametes are called…

Answer 13

the tube nucleus and generative nucleus in a pollen grain

Question 14

In flowers, female gametes are called…

Answer 14

the female gamete (one out of the 8 haploid nuclei in the embryo sac)

Question 15

Flowering plants male gametogenesis

Answer 15

• Inside the anthers, diploid pollen mother cells divide
  by meiosis to form four haploid cells.
• The nuclei of each of these haploid cells then divide by mitosis, but the cell itself does not divide (cytokinesis does not take place), resulting in cells that each contain two haploid nuclei.
• These cells mature into pollen grains, each surrounded by a protective wall made up of a tough exine and thinner intine.
• One of the haploid nuclei is called the tube nucleus, and the other is the generative nucleus.
• These are the male gametes

Question 16

Flowering plants female gametogenesis

Answer 16

• Inside each ovule, a large, diploid, spore mother cell
  develops.
• This cell divides by meiosis to produce four
  haploid cells.
• All but one of these degenerates, and the one surviving haploid cell develops into an embryo sac
• The embryo sac grows larger, and its haploid nucleus
  divides by mitosis three times, forming eight haploid
  nuclei.
• One of these becomes the female gamete.

Question 17

Fertilisation in Plants

Answer 17

Fertilisation occurs when a male gamete from a pollen
grain fuses with a female gamete inside an ovule. This
forms a diploid zygote, which grows into an embryo plant.