topic 2.4 & 2.5 - sexual reproduction in mammals & plants

Question 1

what is gametogenesis

Answer 1

the formation of gametes by meiosis in the sex organs

Question 2

what is spermatogenesis

Answer 2

formation of the spermatoza in the testes

Question 3

process of spermatogenesis

Answer 3

• the diploid primordial germ cell divides several times by mitosis to form diploid SPERMATOGONIA.
• the SPERMATOGONIA then grows (without further division) until they are big enough to be called PRIMARY SPERMATOCYTES
  -the spermatocytes undergo meiosis. first meiotic division results in 2 haploid cells called SECONDARY SPERMATOCTYES
• the second meiotic division results in 4 haploid cells called SPERMATIDS
• the spermatids then differentiate in the tubules of the testes to form SPERMATOZOA - sex gametes

Question 4

function & adaptation of spermatozoa

Answer 4

• mini, many, motile
• must penetrate protective barrier around ovum (zona pellucida) and deliver the male haploid genome inside.

1. acrosome (tip) contains digestive enzymes which breaks down layers surrounding ovum and allows sperm to penetrate egg
2. flagellum (tail) allows sperm to move/swim towards egg
   lots of mitochondria tightly packed in middle section to provide ATP for rotation of flagellum to allow it to move.
3. haploid nucleus to allow restoration of full set of chromosomes at fertilisation.

Question 5

what is oogenesis

Answer 5

formation of ova in the ovaries

Question 6

process of oogenesis

Answer 6

• diploid primordial germ cell divides several times by mitosis to form diploid OOGONIA - most oogonia don’t develop further - they degenerate
• only 1 continues to grow & lots of storage material goes into it - making it large = PRIMARY OOCYTE
• oocyte undergoes meiosis. 1st meiotic division = 2 very unequal sized cells.
• larger cell = SECONDARY OOCYTE
  -smaller cell = 1st POLAR BODY
  -second meiotic division completed after fertilisation occurs
• secondary oocyte divides to form haploid OVUM and 2nd POLAR BODY
• the polar body divides to from 2 more polar bodies
• total 3 polar bodiesand 1 haploid ovum
• polar bodies degenerate as they have no function other than to receive chromosomes

Question 7

function & adaptation of ova

Answer 7

• few, fat, fixed
• connect with the male sperm cell in order to become fertilised and develop the embryo

1. contains Zona Pellucida (protective coating) that sperm has to penetrate through.
   - cortical granules release substances which cause the zone pellucida to harden - so only one sperm can penetrate through.
2. haploid nucleus that allows a full set of chromosomes to be restored at fertilisation.
3. cytoplasm contains lots of food for developing zygote

Question 8

events of fertilisation

Answer 8

1. acrosome reaction
   - front of sperm touches zona pellucida, acrosome bursts and releases enzymes whivh digest a channel in the zona pellucida
2. membrane fusion
   - surface membranes of sperm and egg fuse, allowing the haploid nucleus from sperm to enter cytoplasm of egg
3. cortical reaction
   - vesicles - cortical granules - inside egg fuse with cell membrane and release their contents
   - changes to surface layer of egg, preventing more sperm from entering
4. meiosis restarted
   - egg is secondary oocyte and presence of sperm causes 2nd meiotic cell division
5. fertilisation
   - chromosomes from haploid sperm and from haploid egg combine to restore the diploid number - fertilisation

Question 9

what happens following fertilisation?

Answer 9

fertilised egg/zygote is totipotent - has potential to develop into all different cell types.

1. Cleavage - special kind of mitosis where cells divide repeatedly without interphase between.
2. Result of cleavage is a mass of small, identical undifferentiated cells forming hollow sphere (Blastocyst)
3. one large zygote cell forms a large number of small cells in early embryo - embryonic stem cells. these are undifferentiated cells and have potential to develop into many types of cells
4. earliest cells in embryo are totipotent like zygote, but in Blastocyst the outer layer form placenta and inner layers already lost some of their ability to differentiate
5. can form MOST, NOT ALL cells & are pluripotent (pluripotent embryonic stem cells)
6. Blastocyst ‘hatches’ /breaks free from outer layer and begins implanting in lining of uterus after about 7 days

Question 10

what is cell determination?

Answer 10

• cells are already predestined / determined to become a particular type of tissue from early in development of the embryo
• even if removed and placed somewhere else will still develop in what it was destined to develop into
• closely linked to the position of the cells in the embryo

Question 11

internal fertilisation

Answer 11

internal
- fertilisation of the female and male gamete where the male gametes are transferred directly into the female and takes place inside the body of the mother

Question 12

external fertilisation

Answer 12

external
- Fertilisation where female and male gametes are released outside the paternal bodies to meet and fuse in the environment - outside the body

Question 13

where is pollen formed

Answer 13

• in the anther
• each anther contains 4 pollen sacs where the pollen grain develop

Question 14

gametogenesis in plants

Answer 14

1. diploid microspore / pollen mother cells in anther undergo MEIOSIS and form 4 haploid microspores
2. the haploid microspores undergo MITOSIS to mature into pollen grains
3. pollen grains consist of 2 nuclei - the generative and the pollen tube nucleus
4. the 2 haploid microgametes are formed by the mitosis of the generative nucleus

Question 15

function of pollen tube nucleus

Answer 15

penetrates through stigma & ovary and into the ovule

Question 16

function of generative nucleus

Answer 16

fuses with the nucleus of the ovule to form a new individual

Question 17

formation of egg cell in plants - megagamete

Answer 17

egg formed in ovule in ovary of plant.

• diploid megaspore mother cells in the ovule undergo MEIOSIS, forming 4 haploid megaspores - 3 are DEGENERATE and 1 that will continue to develop
• the megaspore undergoes 3 MITOTIC division that results in an embryo sac containing 1 egg cell (female megagamete), 2 polar nuclei, and other small cells

[as it goes through 3 mitotic division in end result there’s 8 nuclei’s but only 1 is the gamete]

Question 18

what is pollination

Answer 18

• the transfer of pollen from the anther to the stigma of another plant

transferred by wind, insects, birds

Question 19

fertilisation in plants

Answer 19

1. the pollen grain (composed of pollen tube nucleus + the generative nucleus) adheres to the stigma where it eventually germinates
2. after pollination, the pollen grain begins to ‘grow’ or ‘germinate’ + a pollen tube grows from the pollen grain down to the ovary of the plant into the embryo sac. this is due to the digestive enzymes secreted which digest the surrounding tissue (using it as source of nutrients)
3. as the pollen travels down the pollen tube, the generative nucleus divides by mitosis to form a further two haploid male nuclei (male gametes) which then travel down the pollen tube towards the female ovule.
4. as the pollen tube reaches the ovule, the pollen tube nucleus breaks down and the two haploid male nuclei pass into the ovule so that fertilisation can occur.
5. Double fertilisation occurs:
   - one of the male gametes fuses with the female nucleus to form a diploid zygote
   - the other male gamete fuses with two polar nuclei to form a triploid endosperm nucleus which serves as a source of nutrients for the embryo

fertilisation is complete and the development of the seed & the embryo within can begin