Lecture 6 - Meiosis germ line cells and Feriltization

Question 1

What is the difference b/w diploid and haploid?

Answer 1

1. Diploid cells contain two sets of chromosomes from each parent
2. Haploid cells contain 1 set of chromosomes

Question 2

What is a gamete?

Answer 2

1. Specialized reproductive cells

Sperm or eggs generated through meiosis

Question 3

What is fertilization?

Answer 3

1. Haploid gametes fuse to form a diploid cell (zygote)
2. Sperm and Egg fuse

Question 4

What is a zygote?

Answer 4

1. Divides by mitosis to become a multicellular organism.
2. Diploid
3. formed form fusion of egg and sperm

Question 5

1. How did germline cells compare and contrast w/ somatic ?

Answer 5

1. Germline cells include gametes and precursors
2. Somatic Cells form the rest of the body and leave no progeny

Question 6

1. What is the difference b/w an autosome and sex chromosome?

Answer 6

1. Autosome – chromosomes common to both sexes: one from each parent in each diploid nucleus
2. Sex chromosome – are sex determining chromosomes X and Y

Question 7

1. How does meiosis create genetic diversity?

Answer 7

1. Random segregation of paternal and maternal homologs during meiosis
2. Crossing over

Question 8

1. What is a short description of meiosis?

Answer 8

1. Two cell divisions but one round of DNA replication (Synthesis) that results information of a haploid cells from 1 diploid cell

Question 9

What are the steps of meiosis?

Answer 9

1. Chromosomes replicate in S
   
   1. 2 sister chromatids bound by cohesion complexes.
2. Homologs pair, recombine, and separate in meiosis 1.
3. No further DNA replication and sister chromatids are divide to produce 4 daughter haploid cells (meiosis II)

Question 10

1. What is the structure homologs for in meiosis 1?

Answer 10

1. 4 chromatid Structure is called bivalent
2. Stable pairing requires crossing over (genetic diversity)
   
   1. Formation of a chiasma
3. Homologous are joined by a protein structure called synaptonemal complex.

Question 11

1. Our 22 autosomes pair up using their similar homology, how do our sex chromosome pair?

Answer 11

Essential the same way. There is a small region of homology b/w X and Y called the Pseudoautosomal Region which allows them to pair.

Question 12

1. What are the five phase of prophase?

Answer 12

1. Leptotene
   
   1. Homologs being to condense and pair up
2. Zygotene
   
   1. Homologs pair and synaptonemal complexes begin to form
3. Pachytene
   
   1. Synapsis is complete; crossing over occurs
4. Diplotene
   
   1. Synaptonemal complex begins to break down homologs begin to separate but remain attached to chiasmata
5. DIakinesis
   
   1. Reach maximum condensation; separation of homologs and transition stage into metaphase

Question 13

What components b/w homologs form synaptonemal complex and are important for crossing over?

Answer 13

1. Transverse Filaments
   
   1. P.S homologues should be as close as possible for HR

Question 14

1. _____________ assemble on DNA and during S phase and bind sister chromatids

Answer 14

1. Cohesion Complexes
   
   1. Major components of axial core of each homolog
   2. Play important role in segregating homologs in meiosis 1.

Question 15

1. Meiosis one = separating of homologues and meiosis II is separation of chomratids

Answer 15

1. Both are done at their respective anaphase and via kinetochores

Question 16

1. Any step in Meiosis that has an error is __________? Verbal

Answer 16

1. Affects germline cells and affects next generation

Question 17

1. When are Meiosis 1 and 2 complete in females?

Answer 17

1. Meiosis beings in the fetal ovary and then arrest at meiosis 1
2. Meiosis 1 completed only at ovulation
3. Meiosis II only after fertilization

Question 18

1. What is nondisjunction? “name of mainprocess”

Answer 18

1. Homologies fail to separate properly
2. Errors are very common during egg development and increased w/ advancing maternal age
   
   1. Spontaneous absorptions and mental retardation

Question 19

1. What are cells with abnormal chromosome number called and give one example of it?

Answer 19

1. Aneuploidy
   
   1. Trisomy 21 (down syndrome) most common cause of mental retardation

Question 20

1. In trisomy 21, how many of which chromosome is inherited and which sex does it occur most in?

Answer 20

1. Three 21 chromosomes
2. 70% female and 30% male

Question 21

1. What is an Euploid cell?

Answer 21

1. A cell with a normal number of chromosomes

Question 22

FUN STATS

Answer 22

1. 20% of eggs are aneuploidy while 3 to 4% are aneuploidy for sperm
2. 25% of all human conceptions aneuploidy
   
   1. mostly due o nondisjunction in oocytes at meiosis 1
3. Male gametes are more prone to DNA mutations as a result of undergoing more mitotic cell divisions
   
   1. Increased male age = increased risk of point mutations

Question 23

1. What cells are destined to germ cells and where do they migrate

Answer 23

1. Primordial Germ cells (PGCs)
2. Gonad: ovaries/Testes
   
   1. They first proliferate to increase number (mitosis) then they go through mitosis to produce gametes
   2. They aren’t committed to germ line fate until migration and genital ridge is form.
      
      1. Pluripotent (can be come most other cells)
      2. Genital ridge presence testis

Question 24

1. How does the cells distinguish which cells become germ cells.

Answer 24

1. Some animals contain specific molecules (p. Granules) to help determine which cells are germ cells
2. Others rely on neighboring cell signaling to dictate which become PGCs
   
   1. In mammals dividing egg is totipotent (become any cell) for first few rounds of cell division and then differentiate into PGCS

Question 25

1. What is the sex determining region of Y?

Answer 25

1. Can cause sex-reversal if mutations but progeny will be female
   
   1. XX +sry = male or XY –SRY = female
2. Directs differentiation into Sertoli cells
   
   1. Secrete anti-Mullerian hormone
      
      1. Suppresses female development
         
         1. Causes Mullerian duct to regress
   2. Induce leydig cell differentiation in other somatic cells
      
      1. Produces testosterone = secondary male characteristics

Question 26

1. What happens in absence of SRY?

Answer 26

1. Genital ridge becomes ovary
   
   1. PGC becomes an Egg
   2. Somatic cell differentiates into
      
      1. Follicle cells – support cells
      2. Theca cells – estrogen procuring cells

Question 27

1. Egg activation is triggered by?

Answer 27

1. Fertilization

Question 28

1. Why cant pathogenic activation ( activation w/ o sperm) occur in mammals

Answer 28

1. Imprinting
   
   1. Paternal version of gene is off the cell cant survive

Question 29

1. What is the structure of the Egg

Answer 29

1. Yolk
   
   1. Nutritional stockpile and initial development.
      
      1. Rich In lipids, proteins, carbs
2. Egg coat
   
   1. Is a glycoprotein rich extracellular matrix
      
      1. Protects from mechanical damage
      2. Acts as a species-specific barrier to sperm
         
         1. Vitelline layer in non mammalian eggs

Zona Pellucida in mammal

1. Cortical granules
   
   1. Once egg is activated, it enters egg coat and blocks other sperm from coming in,

Question 30

1. How do eggs achieve a larger size?

Answer 30

1. Extra gene copies in cell
   
   1. Growth after DNA replication
   2. Some species produce rRNA
2. Import yolk proteins through receptor mediated endocytosis
3. Accessory cells
   
   1. Nurse cells -> make ribosomes, mRNA, proteins and transport into oocyte (insect)
4. Follicle Cells (vertebrates and invertebrates) connect to each other and oocyte
5. Communicate wia gap junctions

Question 31

1. How does oogenesis occurs

Answer 31

1. PGC enters Gonad
2. Diploid oogonia undergo mitotic divisions before meiosis
3. Primary oocytes are formed during gestation of human embryo
4. Primary oocytes arrest at prophase I of Meiosis 1
5. “Primordial follicle: oocyte surrounded by single layer of follicles
6. “developing follicles: multiple layers of follicle cells (granulosa cells) surround growing oocyte
7. “Some developing follicles acquire fluid filled cavity called the antrum
8. “Puberty: FSH surge induced 10-12 antral follicles; one becomes dominant
9. “FSH _ LH in Middle of menstrual cycle triggers ovulation
10. Resume progression at ovulation and primary oocyte completes M1
11. Cytoplasm divides asymmetrically
    
    1. Small polar body
    2. Large secondary oocyte
12. Secondary oocyte is arrested at metaphase II
13. “Follicle enlarges and ruptures to release oocyte (still surrounded by granulosa cells
14. Completes meiosis only after fertilization

“ refers to image below

Question 32

1. What are the two distinct regions of sperm

Answer 32

1. Much smaller than egg (optimize to carry DNA) no other cytoplasmic organelles
2. Head – contains nucleus
   
   1. Acrosomal vesicle
      
      1. Contains hydrolytic enzymes
         
         1. Released by exocytosis in acrosome reaction
         2. Helps penetrate eggs outer coat
3. Tail – propulsion
   
   1. Includes mid piece
      
      1. Pack with mitochondria and Dynein motor proteins

Question 33

1. What are some characteristics of spermatogenesis?

Answer 33

1. Meiosis does not begin until puberty
2. Occurs in seminiferous tubules
   
   1. Maintains close contact w/ Sertoli cells
3. Spermatogonia undergo limited mitotic divisions entering meiosis I
4. Prophase 1 of meiosis 1
   
   1. Paired homologs cross over
5. Primary spermatocytes complete M1 to produce secondary spermatocytes
   
   1. Remember there HAPLOID 22 duplicated autosomes and an X or Y
6. Each secondary Spermatocyte enters MII and produce 4 haploid spermatids
   
   1. Which Differentiate into sperm which escape into lumen of seminiferous tubule
   2. Pass into epididymis, where its stored to undergo further maturation
7. Spermatids don’t complete cytokinesis
   
   1. Remain connected through cytoplasmic bridges
      
      1. Called SYNCYTIUM

Produce in batches

Question 34

What is capacitation

Answer 34

1. Process required for a sperm to acquire capacity to fertilized egg
   
   1. Tales 5 to 6 hrs
   2. Completed in oviduct
   3. Increases motility of sperm flagellum and makes sperm capable of acrosome reaction

Question 35

1. What are the requirements of Capacitation?

Answer 35

1. Albumin
   
   1. Helps extract cholesterol from membrane, increasing ability to fuse with acrosome membrane
2. Ca2+ and HCO3-: activate adenyl cyclase in sperm to produce cAMP
   
   1. Helps to initiate capacitation-associated changes.

Question 37

1. In a more in-depth explanation, what is fertilization?

Answer 37

1. Released egg is surrounded by granulosa cells and an ECM rich in hyaluronic acid
2. Capacitated sperm must penetrate granulosa cells using hyaluronidase
3. Sperm binds to zona Pellucida
   
   1. Acts as species barrier
4. Zona Pellucida induces sperm to undergo acrosome reaction
   
   1. Contents help sperm to tunnel through zona pellucida
   2. Alters sperm so can bind and fuse with plasma membrane of egg
5. Sperm binds egg plasma membrane first by tip and then side
   
   1. Microvilli on egg aid in process
6. Certain membrane proteins are crucial to binding
   
   1. Zp1,2,3
      
      1. Zp2 and 3 form long filaments

Zp1 cross links filaments

1. Fusion with a sperm induces (triggered by increased Ca2+ in cytosol)
   
   1. Cortical reaction
      
      1. Cortical granules release contents
   2. Meiosis resumes

Question 38

1. How can egg activation be triggered?

Answer 38

1. Injecting Ca2+ directly into cell
2. By injection or sperm head or extract into egg.

Question 39

1. What is the cortical reaction

Answer 39

1. Caused by the fusion of sperm which then causes a change in PM of egg
   
   1. Prevents other sperm from fusing (polyspermy)
2. Releases of enzymes that change the zona pellucida so sperm can’t get in
   
   1. Inactivates ZP3 so it no longer binds sperm or induces acrosome reaction
   2. ZP2 is cleaved which make ZP impenetrable

Question 40

1. What happens after fertilization?

Answer 40

1. 2 haploid nuclei fused forming a zygote which DIPLOID w/ a SINGLE nucleus
2. sperm contributes centrosome and centrioles which was not present in eggs
3. centrosome duplicated and assembles the mitotic spindle
   
   1. in polyspermy, extra mitotic spindles are present
   2. leads to faulty segregation of chromosomes and aneuploidy

Question 41

1. What is in vitro fertilization?

Answer 41

1. Eggs harvested form women and fertilized in cultured dish with sperm.
2. 2-3 embryos are transferred to woman’s uterus

Question 42

1. What does IVF and Preimplantation genetic diagnosis (PGD) do?

Answer 42

1. Couple at risk for a genetic condition to conceive healthly children

Question 43

1. What intracytoplasmic sperm injection used for?

Answer 43

1. Allows men with few or non motile sperm to father children
2. Egg is injected with a single sperm
   
   1. Bypasses normal hurdles
      
      1. Capacitation
      2. Swimming to egg
      3. Acrosome reaction
      4. Plasma membrane fusion