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Flashcards in Meiosis Deck (37):

How many chromosomes in each somatic cell (not egg or sperm)

46, arranged in 23 pairs

22 autosomes with one pair of sex chromosomes


Let's review some basic DNA structure

DNA coils around histones to make nucleosomes. Chains of nucleosomes fold to form chromatin fibers. Chromatin fibers hypercoil to form chromosomes



Abnormal number in general


Missing chromosome is called what?



How do we name complete duplications of chromosomes

Triploidy, Tetraploidy, etc.


Discuss the general difference between meiosis 1 and meiosis 2

1: Reductional division - Reduces the chromosome number from diploid to haploid.

2: Equational division - Begins with haploid but processes like mitosis with that number.


What is pairing?

Homologous chromosomes, after duplicating themselves into two chromatids held together by a single chromosome, are matched by homologue alignment.


Discuss crossing over, an event unique to meiosis when compared to mitosis

Homologue pairs are locked together through recombination of chromosome.


What is required tor successful crossing over

Preceded by double stranded DNA breaks. Followed by adherence of the arm from one homologous pair to the site of the break on the other pair. Pieces then cross over.


How many exchanges usually occur in crossing over and how random are they?

Usually only one exchange per chromosome arm.

Events are not random. Occur in euchromatin (stretched out chromatin material) Not in heterochromatin (condensed chromatin material)


When do translocations occur? How dangerous are they?

If pairing occurs between non homologous chromosomes, then exchange can place pieces of one chromosome inside another.

However, this is not usually a problem, as long as no material is lost. We call this a balanced translocation. If material is lost, this is unbalanced, and the consequences are SEVERE.


Primary spermatocytes progress steadily through meiosis 1 to meiosis 2 in how many days? What is the result?

64 - 70 days

Yields 50% 23X, 50% 23Y sperm. Minimal cytoplasm and mitochondria are maintained.


Discuss what happens in interphase of meiosis I

The DNA in the cell is copied resulting in two identical full sets of chromosomes.

During interphase, microtubules extend from centrosomes, which each contain a pair of centrioles.


Discuss prophase I

The copied chromosomes condense into X-shaped structures that can be easily seen under a microscope.

Each chromosome is composed of two sister chromatids containing identical genetic information.

The chromosomes pair up so that both copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on. Crossing over can occur at this point.

At the end of Prophase I the membrane around the nucleus in the cell dissolves away, releasing the chromosomes.

The meiotic spindle, consisting of microtubules and other proteins, extends across the cell between the centrioles.


Discuss Metaphase I

Centrioles now at opposite ends and the chromosome pairs line up next to each other along the center of the cell. The meiotic spindle fibres attach to one chromosome of each pair.

The pair of chromosomes are then pulled apart by the meiotic spindle, which pulls one chromosome to one pole of the cell and the other chromosome to the opposite pole.

In meiosis I the sister chromatids stay together. This is different to what happens in mitosis and meiosis II.


What happens before Prophase II and after Anaphase I?

Cytokinesis and telophase to split this stuff up into two cells


So we just finished Meiosis I, what do we have?

Now there are two daughter cells, each with 23 chromosomes (23 pairs of chromatids). They are HAPLOID (n)


Discuss prophase II

In each of the two daughter cells the chromosomes condense again into visible X-shaped structures that can be easily seen under a microscope.

The membrane around the nucleus in each daughter cell dissolves away releasing the chromosomes.

The centrioles duplicate.

Meiotic spindle forms again


Discuss metaphase II

In each of the two daughter cells the chromosomes (pair of sister chromatids) line up end-to-end along the equator of the cell.

The centrioles are now at opposites poles in each of the daughter cells.

Meiotic spindle fibres at each pole of the cell attach to each of the sister chromatids.


Discuss Anaphase II

The sister chromatids are then pulled to opposite poles due to the action of the meiotic spindle.

Key point: The separated chromatids are now considered their own individual chromosomes.


Discuss what the result of the last cytokinesis and telophase will be

Once cytokinesis is complete there are four granddaughter cells, each with half a set of chromosomes (haploid)

In males, these four cells are all sperm cells

In females, one of the cells is an egg cell while the other three are polar bodies (small cells that do not develop into eggs).


So egg cells always are undergoing meiosis only right?


Primordial germ cells originate from the yolk sac at 4 weeks, migrating to gonadal ridges to undergo MITOSIS to create 7 million oogonia by 20 weeks gestational age. Cells then lose the ability to undergo mitosis and enter meiosis I.


What is dictyotene?

Initial arrest in egg development at meiosis 1, prior to metaphase. Chiasmata (arms overlapping) and crossing over happen here, and it may last for 12-50 years depending on when the egg is ovuated/if it ovulates.


What triggers completion of meiosis 1 ?

LH surge in menstrual cycle.

Leads to unequal cytokinesis, extruding only nuclear material as the first polar body


What happens after completion of meiosis 1?

Another arrest at the beginning of meiosis 2, which won't be completed until fertilization, after penetration of sperm into the oocyte.


What happens after fertilization for the meiotic cycle?

With fertilization, the nucleus completes meiosis 2, generating another polar body. The first PB may also replicate to make another PB.

Result is one large ovum and 3 polar bodies.


What occurs after completion of meiosis 2 in the oocyte?

The sperm during this completion of meiosis 2 has shed its tail, forming the male pronucleus. The remaining material in the oocyte after completion of meiosis 2 forms the female pronucleus. These pronuclei fuse to form the zygote.


What does age affect as far as meiotic processes in the oocyte?

1. Greater chance for errors in segregation machinery (spindle apparatus more likely to break down).

2. Impairment of chromosome exchange process (reduction in exchanges with exchanges also occurring closer to the telomeres)

3. Reduced exchanges leads to chromatids less likely to separate.

In general, the longer the egg remains in meiosis 1, i.e., the older the mother is, the greater the likelihood it will divide unevenly.


What mutation causes Wolf-Hirshhorn syndrome?

4p- (q deletion)


What mutation causes Cri du chat?

5p- (q deletion)


What mutation causes Williams syndrome?

Deletion at 7q


What causes Angelman syndrome? Prader Willi?

15q11 deletion maternal for angelman and paternal for prader willi


What causes velocardiofacial syndrome?

22q11 deletion


What causes Smith Magenis syndrome?

17p11.2 deletion


47 xxy




Turner syndrome


How much cell-free DNA in maternal plasma is fetal in origin?

Approximately 5% of cell free DNA in maternal plasma is of fetal origin.

It is already being utilized for testing of fetal Rh and sex. 100% sensitive for XY and 99.6% for XX