11.4 - sexual reproduction Flashcards
1
Q
what is oogenesis and Spermatogenesis?
A
- the production of egg cells in the ovaries. This process starts in the ovaries of a female foetus
- the production of sperm cells in the testes.
- Together, these processes are called gametogenesis
2
Q
what are the similarities between oogenesis and spermatogenesis?
A
- Mitosis occurs where the germinal epithelium cells divide to produce large numbers of diploid cells (2n).
- Cell growth occurs so that the cells have enough resources to undergo the two divisions of meiosis.
- Meiosis occurs to produce haploid cells (n).
- Differentiation occurs so that the haploid cells develop into gametes with the structures needed for fertilisation
3
Q
what is the process of Oogenesis?
A
- Mitosis produces the germ cell layer and the oogonia (small diploid cell that increases in size, and matures to form the primary follicle). These cells will be distributed in the cortex or the ovary.
- After mitosis, the germ cells undergo meiosis, during this stage the germ cells are known as oocytes.
- Primary oocytes undergo meiosis I and secondary oocytes undergo meiosis II.
- The result of meiosis I is one polar cell and one secondary oocyte.
- The result of meiosis II is one ootid (a haploid cell) and a second polar cell.
- This unequal division removes excess genetic material and allows the maximum amount of cell material to stay with the future ovum.
- The polar cells contain unwanted genetic material and very little cytoplasm, and thus degenerate with time.
- The ootid goes on to develop into a
mature ovum (egg).
4
Q
what is the process of Spermatogenesis?
A
- Mitosis produces the germ cell layer and the spermatogonia (undifferentiated cell that undergoes maturation).
- After mitosis, the germ cells undergo meiosis, and during this stage the germ cells are known as spermatocytes.
- Primary spermatocytes undergo meiosis I and secondary spermatocytes undergo meiosis II.
- The resulting cells are called spermatids, which are haploid cells.
- The spermatids then differentiate into mature and functional spermatozoa (sperm).
- Four spermatids and spermatozoa are produced
5
Q
what are the differences in gametogenesis outcomes?
A
- duration of production of sperm in men begin from puberty onwards whereas women produce eggs between puberty and menopause besides during pregnancy
- number of gametes produced are different as men produce millions of sperm each day and can be released at any time whereas women produce and release just one egg every 28 days
- amount of cytoplasm differs as nearly all the cytoplasm is removed during latter stages of spermatogenesis whereas egg cells have more cytoplasm (both occurs during differentiation)
- number of gametes produced from one germ cell differs as there are four spermatozoa produced from spermatogenesis whereas only one gamete is formed from oogenesis.
- Spermatogenesis does not form polar cells whereas oogenesis forms two polar cells
6
Q
what is a primordial follicle?
A
- Primordial follicles contain egg cells that have been arrested in prophase I (primary oocytes)
- a central oocyte surrounded by a single layer of follicle cells.
- Every menstrual cycle, a few primary follicles start to develop and the oocyte completes the first division of
meiosis.
7
Q
what is a secondary follicle?
A
- the follicle cells proliferate, a fluid-filled cavity develops and the oocyte starts the second division of meiosis. - The ruptured follicle develops into the corpus luteum that secretes key
ovarian hormones but degenerates
8
Q
how are gametes produced in the seminiferous tubule?
- primary
A
- An outer layer called germinal epithelium cells divide endlessly by mitosis to produce more diploid cells.
- Each primary spermatocyte undergoes meiosis I to form two secondary spermatids (haploid cells)
9
Q
how are gametes produced in the seminiferous tubule?
- secondary
A
- Spermatids become associated with a nurse cell called a Sertoli cell that nourishes the cell. It also facilitates spermatid differentiation into spermatozoa (haploid cells).
- Spermatozoa detach from Sertoli cells and eventually carried out of the testis by the fluid in the centre of the seminiferous tubule.
10
Q
Annotating a diagram of an egg.
A
- large and non-motile.
- has the cytoplasm and organelles needed for a zygote.
- cortical granules inside the egg release contents upon fertilisation to prevent polyspermy
- Diameter of cell is ~110mm
- The cytoplasm contains droplets of fat.
- two layers: zone pellucida is the glycoprotein gel barrier against sperm entry, and corona radiata that is a layer of follicle cells that nourish and support the egg cell
- Mature human ovum are arrested in metaphase II until fertilisation so do not contain a condensed nuclei.
-but, the nuclei is typically included in a biological diagram of egg cells to represent the presence of haploid DNA.
11
Q
Diagram of a sperm.
A
- small and highly motile and only contributes the haploid nucleus to the zygote.
- typical spermatozoa can be divided into three sections: head, mid-piece and tail.
- The head region contains the haploid nucleus with the paternal DNA, an acrosome cap with hydrolytic enzymes used to penetrate the jelly coat of the egg, and a pair of
centrioles needed by the zygote for division (centrioles are removed in polar bodies of egg cells). - The head is about 3mm wide and 4mm long.
- The mid-piece contains large numbers of mitochondria which provides the energy (ATP) needed for the tails to move.
- The mid-piece is ~7mm
- The tail (or flagellum) is composed of microtubule structure called axoneme, which bends to facilitate movement.
- The axoneme is arranged in a 9 + 2 arrangement, and protein fibres are used to strengthen the tail.
- The tail is long ~40mm.
12
Q
what is external fertilisation?
A
- Aquatic animals often release their gametes directly into water in a process that leads to fertilisation outside the female’s body
-These animals often have behaviours that bring sperm near eggs - are risks involved with this type of fertilisation including predation and effects of the environment such as temperature, pH and pollution
13
Q
what is internal fertilisation?
A
- terrestrial animals are dependent on internal fertilisation as the gametes will dry out otherwise
- The sperm and ova are placed in close proximity and are fertilised in suitable environmental conditions within the female’s body
14
Q
what is fertilistation?
A
- the fusion of a sperm and egg to form a zygote
15
Q
what is Polyspermy?
A
- occurs when an egg is fertilised with more than one sperm, which in most
cases results in the death of the embryo due to three sets of chromosomes. - Multiple sperm arrive at the egg as sperms have receptors that detect chemicals released by the eff, allowing sperm to swim directly to the egg, but many events take place so that only one sperm successfully fuses with the egg
16
Q
what is the The acrosome reaction?
A
- The sperm try to push through the layers of the follicle cells (corona radiata) around the egg.
- The first sperm to break through the layers of the follicle cells will bind to the zone of pellucida, which is a coat of glycoproteins that surrounds the egg; and this triggers the acrosome reaction.
- The acrosome is a membrane-bound sac of proteases that are released and digest a route for the sperm through the zone pellucida
17
Q
what is the Penetration of the egg membrane?
A
- The acrosome reaction allows the first sperm to reach the plasma membrane of the egg.
- The plasma membranes of the egg and sperm fuse, and the sperm nucleus enters the egg to join the egg nucleus.
- This is the moment of fertilisation.
- The fusion of membranes activates the egg, and it causes the egg to complete meiosis II.
18
Q
what is the The cortical reaction?
A
- Small vesicles called cortical granules located within the egg near the membrane move towards and fuses with the egg’s plasma membrane where it releases the contents by exocytosis.
- Enzymes from the cortical granules trigger a hardening in the zone pellucida, and this makes it impenetrable to any more sperm.
- so only one sperm cell can fertilise one egg
19
Q
what is cleavage development?
A
- The zygote in the oviduct starts to divide by mitosis around 36 hours post-fertilisation.
- It forms a 2-cell embryo, then a 4-cell embryo and so on until a ball of cells called a blastocyst (~100 cells) is formed.
- This is called cleavage, a series of rapid cell divisions that result in a
multicellular ball (the shape is a result of unequal cell divisions and migration of cells)
20
Q
what is the process of implantation?
A
- By day 7, the blastocyst contains 125 cells is transported down the oviduct, by the cilia of cells in the oviduct wall, to the uterus.
- The zone pellucida protecting the embryo breaks down and the blastocyst has now used up the reserves within the egg cell.
- The blastocyst implants itself into the endometrium (uterine lining) to obtain the external food supply for it to grow and develop
21
Q
what does the trophoblast do?
A
- The outer layer of the blastocyst, also known as the trophoblast, develops finger-like projections allowing it to penetrate the endometrium.
- The trophoblast exchanges materials with the mother’s blood and allow the absorption of food and oxygen, which continues for two weeks until the placenta takes over
22
Q
what is The role of hCG in early pregnancy?
A
- Pregnancy depends on the maintenance of the endometrium, and this process depends on the production of progesterone and estrogen from the corpus luteum.
- These hormones prevent the degeneration of the uterus lining needed to support the developing fetus.
- Following fertilisation, the embryo produces human chorionic gonadotropin (hCG) in early pregnancy.
- After about 10-weeks of pregnancy, the corpus luteum breaks down, but by this stage, cells in the placenta take over the role of secreting estrogen and progesterone.
23
Q
what is hCG?
A
- hCG is a hormone that prevents the corpus luteum degeneration, and ensures the embryo
is supplied with everything it needs.
24
Q
what does the placenta allow mammals to do?
A
- allows a human foetus to remain in the uterus for months longer than non-placental mammals such as marsupials (pouch development) or monotremes (eggs).
- The placenta is needed because the body surface area to volume ratio becomes smaller as the foetus grows larger.
25
Q
what is the structure of the placenta?
A
- made of foetal tissues in close contact with maternal tissues in the uterus wall, and this contains chorionic villi or placental villus, finger-like outgrowths with embryonic blood vessels
- These are closely associated with the blood vessels of the mother’s endometrium.
- The chorionic villi increase in number during pregnancy so it can cope with the increasing demands of the foetus in exchanging materials with the mother.
- Maternal blood flows in the spaces around the villi and foetal blood circulates in blood capillaries close to the surface of each villus.
26
Q
what is the placental barrier and what does it do?
A
- the mother’s blood and the embryo’s blood are not in direct contact and remain separated by cells that form the placental barrier.
- This results in many babies having different blood groups to their mothers.
- The placental barrier is selectively permeable as it allows some substances to pass but not others.
- The placenta allows antibodies to pass from mother to foetus, and most drugs can also cross the placenta.
27
Q
why does the Placenta release estrogen and progesterone?
A
- placenta takes over from the corpus luteum to secrete estrogen and progesterone in large enough quantities to sustain the pregnancy.
- This switch happens at approximately nine weeks, where there is a danger of miscarriage if the switch over from corpus luteum to placenta fails.
- Progesterone is important to maintain pregnancy by reducing coordinated contractions of the uterus.
- It also reduces the immune response so that the mother’s body does not produce antibodies against the baby.
28
Q
what is estrogen pollution?
A
- High levels of estrogen naturally occur in pregnant women, and it inhibits the release of FSH.
- The oral contraceptive pill, which was first introduced in 1943, contains estrogen that mimics pregnancy. This inhibits the development of mature follicles so that pregnancy is prevented.
- so many women took the pill and there was estrogen pollution in the sewers and it affected the fish male count.
29
Q
what are Positive feedback mechanisms needed at birth?
A
- Hormones play a key role throughout pregnancy as well as in inducing labour.
- During pregnancy, progesterone inhibits the secretion of the hormone oxytocin by the pituitary gland, and it also inhibits the contractions of the uterus.
- At the end of pregnancy, hormones produced by the foetus signal to the placenta to stop secreting progesterone.
- Thus, oxytocin is secreted and this stimulate uterine contractions of muscle fibres in the myometrium.
- These contractions are detected by stretch receptors, which signal to the pituitary gland to increase the secretion of oxytocin.
- Increased oxytocin levels makes the contractions more frequent and more vigorous, causing more oxytocin secretion.
30
Q
what happens because of the positive feedback at birth?
A
- Relaxation of the muscles in the cervix causes the cervix to dilate.
- Uterine contraction then bursts the amniotic sac (supports and protects developing foetus) and the amniotic fluid passes out, which is sometimes referring to as the mother’s ‘water’s breaking’.
- Further uterine contractions, usually across a number of hours, finally push the baby out through the cervix and vagina.
- The umbilical cord is broken and the baby takes its first breath, and finally achieves physiological independence from its mother.
31
Q
what are Altricial species?
A
- give birth to relatively helpless young who are often immobile, lacks hair and cannot obtain food on their own.
32
Q
what are Precocial species?
A
- give birth to offspring with open eyes, hair, are immediately mobile and somewhat able to defend themselves against predators.
- Mammals with a large body size are more likely to be precocial.
- This is correlated with a long gestation period, which is needed for the extra development of the foetus in utero
