Meiosis- general and in humans

Question 1

What do “haploid” and “diploid” mean?

Answer 1

Hapoid- having a single set of unpaired chromosomes

Diploid- having two complete sets of chromosomes, one from each parent

Question 2

How is the “n-x” nomenclature used?

Answer 2

n = chromosome variants; homologs

x = copies of each homolog

For example, meiosis is 2n-2x –>–> 1n-1x

Question 3

What are germline cells? What type of cell division do they undergo?

Answer 3

Cells which result in the formation of gametes; they undergo meiosis

Question 4

By the end of meiosis, what has happened to the number of chromosome variants in the resulting daughter cells? How many cells are produced? What are the specific names of the male and female meiotic events in humans?

Answer 4

The amount of chromosome variants/chromatids has been split in half to create four daughter cells with half the amount of genetic material as a somatic cell.

In females, the formation of gametes is called “oogenesis,” is males it is called “spermatogenesis.”

Question 5

How does the interphase of meiosis compare to that of mitosis? What chromosomal separation occur during meiosis I and meiosis II?

Answer 5

Meiotic interphase is the same as mitotic interphase, the genetic material is copied.

In meiosis I, the homologous chromosomes are separated, halving the number of chromosomal variants

Meiosis II is comparable to mitosis, the sister chromatids are separated

Question 6

What events occur during prophase Iand prometaphase I? What is unique to meiosis and what is shared with mitosis?

Answer 6

Like in mitosis, chromatin condenses, the nuclear envelope dissapears, and the centrioles migrate.

Unlike in mitosis, the homologous chromosomes pair up forming tetrads and “cross-over” certain genes between them, increasing genetic variety

Question 7

What are tetrads and synapsis?

Answer 7

Synapsis is the pairing together of homologous chromosomes, the tetrad is the name of the resulting structure

Question 8

How does crossing over work? What is a chiasmata? What is recombination?

Answer 8

Crossing over works by -recombining- the allelic material between the chromosmes, the chiasmata is the site where this recombination occurs.

Question 9

What occurs during metaphase I? How are the chromosomes organized in the metaphase plate and how is it different from mitosis? What happens to further increase genetic diversity? What is Independent assortment?

Answer 9

Similar to mitosis, the chromosomes align to be separated, however, the homologous chromosomes align across from one another, 2-by-2 and the microtubules connect to the kinetochores of the homologous chromosomes, rather than the individual chromatids. The homologous chromosomes pairs also align themselves at random to increase genetic diversity, this is independent assortment.

Question 10

What occurs during anaphase? What specific chromosome structures are being split during anaphase I?

Answer 10

Meiotic anaphase I is very similar to mitotic anaphase, in the sense that the microtubules pull the genetic material to opposite ends of the cell. However, in meiosis, homologous chromosomes are pulled while the sister chromatids of each homolog remain attached throughout the process so that each pole will receive a complete haploid set of chromosomes with one member of each homologous pair.

Question 11

what occurs during telophase I? What is the ploidy of cells that complete meiosis I?

Answer 11

Telophase I is very similar to mitotic telophase, however, the end result is non-identical haploid cells

Question 12

What occurs during meiosis II? How many daughter cells have been created? Are they genetically identical, if not, why?

Answer 12

Meiosis II is the exact same process as mitosis, but there are two cells to start out, and four daughter cells with half the genetic materical as the original germline cell result- these cells are not genetically identical due to recombination and independent assortment.

Question 13

What is spermatogenesis?

Answer 13

The sequence of events that produces sperm in the seminiferous tubules of the testes.

Question 14

What stage of meiosis and what ploidy do each of the following refer:
Spermatogonia
Primary spermocytes
Secondary spermocytes
Spermatids
Spermatozoa

Answer 14

Spermatogonia- the original 2n germline cell that ultimately gives rise to sperm

Primary spermocytes- the spermatogonia after mitotic division (2n –> 2n)

Secondary spermocytes- primaries that have undergone meiosis I to fomr two haploid cells (2n –> 1n)

Spermatids- secondaries that undergo meiosis II (1n –> 1n)

Spermatazoa- haploid and non-motile (no tail) spermatids that undergo spermiogenesis to form a tail and mature in the epididymis, forming the gametic sperm cells we are familiar with

Question 15

What is oogenesis?

Answer 15

The sequence of events that produces female sex cells by meiosis in the ovaries

Question 16

What stage of meiosis and what ploidy do each of the following refer. When during the female lifespan is each stage completed:
Oogonia
Primary oocytes
Secondary oocytes
Ova

Answer 16

Oogonia- in the ovary of a female fetus, germ cells divide mitotically to produce large numbers of oogonia (2n)

Primary oocytes- still in utero, oogonia enter the first phase of meiosis are are arrested in prophase I (2n)

Secondary oocytes- after decades of arrested development, the primaries re-enter the meiotic cycle after puberty with one primary oocyte completing meiosis I per month during ovulation (1n)

Ova- only if the secondary is fertilized will it complete meiosis II (1n –> 1n), upon fertilization, the fusion of sperm and oocyte nuclei only occurs after meiosis II finishes, creating an ovum (singular of ova)

Question 17

What are polar bodies? How many are produced by the end of meiosis?

Answer 17

During meiotic division, the cell sequesters all of the internal structures- except for the nucleus- into one cell to ensure that the eventual secondary oocyte has ample resources to begin cell division upon fertilization. This extra nucleus is cast off as a polar body.
by the end, three polar bodies will be produced and one ovum will contain the cell resources of four cells.

Question 18

How does the age of male and female gametes affect the probability of abnormalities during development?

Answer 18

Male gametes- mutations are accrued over time due to the germline cells’ expansion by mitosis; these mutations tend to lead to greater incidences of neurological abnormalities such as schizophrenia and autism

Female gametes- mutations arise from from spindle fiber failure caused by the long dormant periods; mutations lead to an increase of chromosomal abnormalities such as aneuploies: ex. trisomy and monomy, which are usually lethal before or shortly after birth

Question 19

What are nondisjunction and aneuploidy? How do they occur? To what do the terms trisomy and monomy refer? How common are human aneuploidies?

Answer 19

Nondisjunction- the chromosomes of the cell so do not separate properly during anaphase I, resulting in cells with either both or neither homologous chromosome

Aneuploidy- exhibited by cells with incorrect chromosome numbers

Trisomy and monomy- the two most common types of aneuploidy–> trisomy = 3 chromosome variants present; monomy = only 1 of a pair of chromosomes

10% of sperm and 50% of eggs contain chromosomal abnormalities; nondisjunctions and aneuploidies account or around 35% of all spontaneous abortions

Question 20

How do each of the following sex aneuploidies occur and what are their genotypes?
Turner syndrome 
Klinefelter's syndrome
Poly-X syndrome
Jacob's syndrome

Answer 20

Turner- only one X chromosome present, nondisjunction resulting in monomy

Klinefelter’s- XXY or XXXY- at least two X chromosomes and at least one Y chromosome; trisomy

Poly-X - XXX, XXXX, XXXXX- at least three X chromosomes; trisomy

Jacob’s- XYY- an X and two Y chromosomes; trisomy

Question 21

What are the phenotypes of the following sex aneuploidies?
Turner syndrome
Klinefelter's syndrom
Poly-X syndrome
Jacob's syndrome

Answer 21

Turner’s- some physical development issues (shorter stature), typically no mental disabilities- on average typically more intelligent than average population, 1/10000 births, present female

Klinefelter’s- some physical development issues (potentially intersex), may be slower mentally but not to the point of disability; xxy is 1/500, xxxy is 1/1000; physical presentation varies, usually self-id as male

Poly-X- no distinct phenotype except tall and thin, therefore commonly found in models, can be mentally slower but not to the point of disability, 1/1500, present female

Jacob’s- no physical development issues, tend to be larger and potentially more agressive, commonly found in prison populations, 1/1000, present male