Lecture 6 - Meiosis, Germ Cells, and Fertilization

Question 1

What mechanisms are used in meiosis to create genetic diversity? Briefly describe each.

Answer 1

Crossing over: exhange of DNA between homologous chromosomes (occurs 2-3 times per homolog)

Independent assortement: random seperation of homologous chromosomes and sister chromatids in M1 and M2 respectively. (2²³ possible arrangments)

Question 2

Describe the change in the amount of DNA and its arrangement during meiosis.

Answer 2

1. Beginining of meiosis: DNA is replicated sister chromatids are paired, diploid
2. Metaphase I: homologous chromososmes are paired, diploid
3. Anaphase I: homologous chormosomes seperate, haploid
4. Anaphase II: sister chromatids seperated, haploid

Question 3

What is cohesin?

Answer 3

Protein that binds sister chromatids together.

Question 4

What is meant by bivalent?

Answer 4

It is the arrangement of DNA in starting at prophase I where homologous chromosomes are paired.

Question 5

What is the structure formed between homologous chromosomes that contributes to stable pairing?

Answer 5

Chiasma

Question 6

What are the 5 phases of prophase and what occurs in each step?

Answer 6

1. leptotene: homologs begin to condense
2. zygotene: homologs pair; synaptonemal complexes form
3. pachytene: synapsis is complete; crossing over occurs
4. diplotene: synaptonemal complexes breakdown; homologs remain attached at chiasmata
5. diakinesis: maximum condensation reached; homologs seperate

Question 7

What is the synaptonemal complex? Describe its structure.

Answer 7

Complex that hold homologous chromosomes together.

Cohesin complexs hold sister chromatids together and are are attached to axial cores. Axial cores of homologous chromosomes are held together by transverse filaments.

Question 8

Describe the timing of the maturation of an oocyte through the stages of meiosis.

Answer 8

1. Primary oocyte formed as an embryo and is arrested after diplotene
2. Meiosis I finishes at ovulation
3. Meiosis II finishes after fertilization

Question 9

What is nondisjunction and where is it most frequently seen?

Answer 9

Failure of homologs to seperate resulting in abnormal chromosome number.

It is more commonly seen in oocytes with an increasing frequency with age.

Question 10

What are cells with an abnormal amount of chromosomes called? Normal number?

Answer 10

aneuploid; euploid

Question 11

What are primordial germ cells?

Answer 11

Pluripotent cells found in embryos which give rise to gametes.

Question 12

What is the beginning process of gonadal development?

Answer 12

Primordial germ cell migrates to genital ridge where the sex chromosomes of the genital ridge determine if gonad will be an ovary or testes.

Question 13

What is the Sry gene?

Answer 13

Sex-determining region Y. Encodes DNA binding protein Sox9.

Question 14

What happens in gonadal development when Sry is expressed?

Answer 14

Results in formation of testes.

Sox9 is produced and stimulates differentiation into Serotoli cells and Leydig cells. Seretoli cells secrete anti-Mullerian hormone which supresses female development. Leydig cells secrete testosterone which creates secondary sexual characteristics.

Question 15

What happens in gonadal development when Sry is not expressed?

Answer 15

Results in formation of ovaries.

Cells differentiate into eggs, folicle cells, and theca cells (secrete estrogen)

Question 16

What is the zona pellucida?

Answer 16

Glycoprotien rich coat of egg cell which acts as a species barrier.

Question 17

Describe the process of mature egg formation.

Answer 17

PGCs produce diploid oogonium.

Meiosis I reults in the production of a primary oocyte which arrests in diplotene. Cell grows and develops while arrested. cortical granules are formed.

Meiosis I completes at ovulation resulting in a first polar body and secondary oocyte.

Meiosis II occurs after fertilization resulting in a mature egg and second polar body.

Question 18

Describe the structure of an egg.

Answer 18

Nurse cells produce protiens, ribosomes, and mRNA for the oocyte and to the oocyte via cytoplamsic bridges.

Surrounded by follice cells.

Question 19

Describe the process of sperm formation.

Answer 19

PGCs produce diploid spermatagonium.

Spermatagonium undergo meiosis starting at puberty

Primary spermatocyte is formed at the beginning of meiosis I.

Meiosis I creates two secondary spermatocytes.

Meiosis II creates four spermatids.

Spermatids differentiate into mature sperm cells.

Question 20

What special process occurs allowing progeny of spermatogonium to have access to a full diploid genome?

Answer 20

Progeny do not complete cytokinesis resulting in connections called syncytium.

Question 21

Describe the structure of a mature sperm cell.

Answer 21

Consists of a head and a tail.

Head contains nucleus and acrosomal vesicle which is used to penetrate egg’s outer coat.

Tail consists of a midpiece which contains many mitochondria and a flagellum.

Sperm cell contains no other organelles and minimal cytoplasm.

Question 22

What is capacitation?

Answer 22

Biochemical and functional changes which occur to sperm in the oviduct to give sperm the ability to fertilize an egg cell.

Increases flagellum motility and makes cell capable of undergoing acrosome reaction.

Requires albumin, Ca²⁺ and HCO₃^- which are found in the oviduct.

Question 23

What are the first steps in fertilization?

Answer 23

Sperm binds zona pellucida which initiates acrosomal reaction. Sperm penetrates zona pellucida, the cell membranes fuse, and sperm contents enter the egg.

Question 24

What happens after fusion of the sperm to the egg?

Answer 24

Cortical reaction which prevents polyspermy.

Cortical granules release contents into zona pellucida inactivating ZP3 which prevents sperm binding and acrosomal reaction in addition to cleaving ZP2 making the zona pellucida impentrable.

Question 25

What happens after fertilization?

Answer 25

The haploid protonuclei fuse.

Centrosomes and centrioles contributed by the sperm and were not previously present in the egg allow for cell division to occur.

Question 26

What is in vitro fertilization?

Answer 26

Women take hormones to stimulate the development of multiple ooctyes.

Eggs are harvested and fertilized in a culture dish.

2-3 embryos are later transferred to the uterus.

Question 27

What is ICSI?

Answer 27

Intracytoplasmic sperm injection.

Direct injection of sperm from men with low sperm count or non-motile sperm into egg cell.

Question 28

What is theraputic cloning?

Answer 28

Transfer of a somatic cell nucleus into an egg which contains no nucleus creating embryonic stem cells which are genetically identical to donor.