Module 4 - Cellular Division and Prenatal Development

Question 1

What is fertilization?

Answer 1

Union of male and female gametes

Question 2

Describe how the sperm gets to the uterus

Answer 2

• First sperm get to the oviduct within 30 min of ejaculation
• Sperm can live for up to 5 days inside the female respiratory tract
• Near ovulation, cervical fluid thins to allow for passage of sperm
• Contraction of myometrium churn to spread sperm
• Female immune systems will kill many sperm
• ½ sperm go the wrong fallopian tube
• Propelled up the oviduct by contraction of smooth muscle
• Some sperm get trapped in cilia
• Of the several hundred million, only a few thousand make it to the oviduct

Question 3

Describe how the egg gets to the meeting point with the sperm

Answer 3

• Ovum released at ovulation from ovary
• Ovum guided into oviduct by
o Fimbriae, finger like projections of the oviduct, contract in a sweeping motion
o Cilia on the fimbriae beat in waves towards the interior of the oviduct
• Mature egg releases allurin
o Sperm have olfactory receptors for alluring (literally sniffing the egg)
o Allurin thought to trigger a second messenger pathway causing intracellular Ca2+ release turning on the microtubule sliding that brings about tail movement

Question 4

Describe what happens when the sperm meets the egg

Answer 4

• Corona radiata contains molecules that cause the acrosome to discharge its contents which are enzymes
• Sperm binds to specific binding sites on the zona pellucida allowing it to penetrate this layer
o Protein called fertilin located on sperm plasma membrane
o Glycoproteins called ZP3 located on the outer layer of the zona pellucida
o Binding releases enzymes that digest the zona pellucida
• Inside zona pellucida there is a space between the egg
• A single cell attaches to the egg membrane

Question 5

Describe what happens after fertilization

Answer 5

• Block to polyspermy
o Release of intracellular Ca2+ into the ovum cytosol
o Triggers release of cortical granules which release enzymes that diffuse into the zona pellucida
o Enzymes inactivate ZP3 receptors preventing other cells from binding
o Enzymes also harden up zona pellucida preventing other sperm from advancing
o Sperm that were in the zona pellucida become trapped
• Ca2+ released into cytosol triggers second meiotic division of the egg
• Egg and sperm nuclei fuse
• Cilia sweep fertilized cell to the uterus

Question 6

Describe the journey to implantation the morula takes

Answer 6

• Rapid division of the zygote after fertilization called cleavage
• Cell does not change in size as one cytoplasm splits into smaller cells
o These are referred to as blastomeres
• Cell size doesn’t change because zona pellucida is hard and the cells grow within them
• Glycogen is released from the endometrium in response to progesterone from corpus luteum
• For the first 3-4 days, morula stays in ampulla due to constriction of the oviduct
o After this, increase in progesterone causes relaxation of the constriction
• If it arrives too early there is not enough nutrients released into the uterus and it dies
• Spends another 3-4 days floating in the uterus as uterus prepares for implantation

Question 7

Describe the blastocyst

Answer 7

• Within a week, morula is now considered a blastocyst that is ready for implantation
• Zona pellucida sheds in a process called hatching
• Has inner cells mass
o Will develop into the embryo
• Outer layer called trophoblast
o Will develop into the chorion
o Secretes enzymes to help implantation

Question 8

Describe what happens during implantation

Answer 8

• Implantation begins when trophoblast overlying the inner cell mass releases enzymes that digests pathways between endometrial cells
o Finger like extensions of trophoblast cells penetrate depths of endometrium
o The breakdown provides nutrients to the embryo
o Creates embryo side of the placenta

Question 9

What is the decidua and how is it formed?

Answer 9

• Modified endometrial tissue at the implantation site
• Stimulated by invading trophoblast endometrial cells release prostaglandins increasing vascularization to the area
• Layer of endometrial cells covers the surface of the hold enveloping the embryo

Question 10

Describe the components of the embryo and mother that prevent her immune system from rejecting the embryo

Answer 10

• Fas ligand
o Released by trophoblast
o Binds with Fas, a receptor on cytotoxic T cells resulting in their apoptosis
• Indoleamine 2, 3-dioygenase (IDO)
o Protein released by fetal side of placenta
o Destroys tryptophan, an amino acid that activates cytotoxic T cells
• Regulatory T cells
o Amount released by mother is doubled or tripled during pregnancy
o Supress cytotoxic T cells

Question 11

Describe the chorion and its formation

Answer 11

• Occurs about day 12 and contributes to the formation of the placenta
• Made of trophoblastic layer that has now increased to 2 layers thick
• Enzymes continuously release to expand into the decidua creating cavities
• Erosion into the mother’s capillaries leak blood into the cavities
o Anticoagulant secreted by chorion prevents clotting

Question 12

Describe the placenta and its formation

Answer 12

• Combination of maternal and embryonic tissues
• Transports nutrients, gases, and wastes between the mother and the embryo/fetus
• Placental villi extend into blood filled cavities
o Consists of embryonic/fetal capillaries surrounded by chorionic tissue
o Chorionic tissue acts as barrier between mother and fetal blood
• By week 5 placenta is well established

Question 13

Describe the umbilical cord

Answer 13

• Has 2 arteries
o Low oxygen blood from embryo/fetus to mother
• Has 1 vein
o Oxygen rich blood to embryo/fetus from mother

Question 14

Describe the amniotic sac

Answer 14

• Created by inner cell mass
• Contains amniotic fluid similar to ECF
• Forms between chorion and the part of the inner cell mass that will form the embryo
• Amnion is the layer of epithelial cells enclosing the sac
• Sac breaks during early stage of labour

Question 15

What is mitosis?

Answer 15

• Division of the nucleus

Question 16

Describe prophase

Answer 16

• Chromatin material condenses to form visible threads
• Each of the 2 sister chromatids of each chromosome are joined at together at a centromere
• Spindle fibres become apparent
• Nucleoli disappear and nuclear envelope dissolves
• Centrioles migrates to opposite sides of the cell
o Plants do not have centrioles as they have cell walls to anchor their spindle fibers to
• Spindle fibres
o Series of radiating microtubules (spindle fibers) that form between the centrioles
o Can be referred to as an aster
o Tubulin
 Protein that form spindle fibers
• Kinetochore
o A region of DNA and protein that attach to the microtubules
o The site the chromatids attach to the spindle fibre
o Contain a motor protein that moves the chromosomes along the cell and pulls them to the middle

Question 17

Describe metaphase

Answer 17

• Begins when all the chromatids are lined up in the center of the cell
• Equatorial Plane
o The center of the cell where all the chromatids move to
o Also called metaphase plate
• In human cells, 92 chromatids in 46 chromosome pairs are lined up here, each attached at the kinetochore to the spindle fibre
• The two chromatids separate from each other at which point they are referred to as chromosomes

Question 18

Describe anaphase

Answer 18

• Shortest phase of mitosis
• Spindle fibres between the poles extend, pushing the poles further away from each other
o They add tubulin subunits to do this
• The spindle fibers attached to the centromeres shorten, pulling the chromosomes to each pole
o They remove tubulin subunits to do this
• Chromosomes move apart from one another as the centromeres are pulled apart
o A total of 46 chromosomes move towards each pole
o The chromosomes take on a V shape as they are being pulled

Question 19

Describe telophase

Answer 19

• Chromosomes arrive at the opposite poles
• Chromosomes uncoil to form masses of chromatin
• The spindle apparatus is dismantled and its amino acids recycled
o The tubulin monomers are used to make the cytoskeletons of daughter cells
• The nucleoli reappear and the nuclear envelope reforms

Question 20

Describe cytokinesis

Answer 20

• Division of the cytoplasm
• Not technically part of mitosis, but generally starts about the same time as telophase
• Begins with the formation of a furrow in the equatorial plane
o Also called a cleavage furrow
o Formed by a constricting belt made up of actin filaments
• Cell cleavage
o The process of the cell membrane pinching into the cytoplasm to create 2 new daughter cells
o The actin filaments slide past one another, making the belt tighter and tighter until it eventually pinches off the two cells
• Plants do not pinch the same way so they grow a cell plate between the 2 daughter cells

Question 21

Describe prophase I in meiosis

Answer 21

•	Homologous chromosomes become closely associated 
•	Synapsis 
o	The fusion of chromosome pairs during meiosis 
o	Tetrad
	Homologues line up side by side 
•	DNA Cross Over
o	Exchange segments of DNA 
o	Increases genetic diversity 
•	Recombination Nodules
o	Locations where crossing over can occur
o	Recombinant pairs have transfers
	Recombinant genotype
o	Non-recombinant pairs do not have transfers 
	Parent genotype 
•	Chiasmata
o	Point at which they cross over

Question 22

Describe metaphase I in meiosis

Answer 22

• Instead of lining up single file, the homologues line up as a pair, one facing each pole
• Spindle fibers are only able to attach to one side of the centromere

Question 23

What is independent assortment?

Answer 23

• Alignment is random, meaning paternal and maternal copies do not necessarily stay on one side or the other
• Further increases genetic diversity

Question 24

What is anaphase I during meiosis?

Answer 24

• The homologues are separated, rather than the sister chromatids due to the spindle fibres only being attached to one side of the centromere
• This results in one set of chromosomes (haploid) at each pole, as the pairs have been split

Question 25

Describe telophase I in meiosis

Answer 25

• Nuclear membrane forms around each daughter nucleus
• Cytokinesis may or may not follow telophase 1
o Sometimes a double cytokinesis occurs after anaphase II

Question 26

Describe metaphase II in meiosis

Answer 26

• Chromosomes align in single file and spindle fibres attach to both sides of the centromeres

Question 27

Describe anaphase II during meiosis

Answer 27

 Sister chromatids are pulled apart resulting in 4 haploid cells