A&P Lab - Class 4 : Mitosis and Meiosis

Human cells with 46 chromosomes actually contain 23 pairs of

Homologous chromosomes

In meiosis, the original number or chromosomes (46 in humans) is restored after:

Fertilization

Homologous chromosomes

A pair of chromosomes with 1 chromosome provided by the maternal side (female parent) and 1 chromosome provided by the paternal side (male parent).
These have identical gene loci and identical centromere positions.

In what phase of mitosis are chromosomes not visible through the microscope?
Why is this?

Interphase.
Because genetic information isn't in the form of chromosomes, it's chromatin (resemble little spaghetti strings through the microscope)

What happens to a cell immediately after cell division?

It enters the G1 phase of interphase

What happens during the G1 phase of interphase?

A cell grows and performs all its routine metabolic functions (duplication of organelles, protein synthesis, etc.)

Why might a cell enter the G0 phase even if it has undergone many divisions and is able to, theoretically, go undergo another?

They may have reached maturity (ex. mature nerve cell), or they are simply destined to never divide again.

What happens during the S phase of interphase?

DNA replication (copying).
The helical molecule of DNA partially uncoils and the two strands separate at the point where the hydrogen bonds connect the nitrogen base pairs.
Each exposed base pair of the old DNA strand then pairs with a complementary base of a newly synthesized nucleotide and the hydrogen bond re-forms.

What does a nucleotide of DNA consist of?

A deoxyribose sugar bound to a phosphate group, and to one of four nitrogenous bases (adenine, thymine, guanine, cytosine).

What is each arm/leg of a replicated chromosome called?

A chromatid (or sister chromatids).

What region holds sister chromatids together?

The centromere

At what stage of interphase does a cell commit to undergoing mitosis?

The S (synthesis) phase

What happens during the G2 phase of interphase?

The cell prepares for actual division. There may be some additional growth as well as an accumulation of energy reserves, and synthesis of enzymes and other proteins.

Where is the centrosome located?

Directly outside the nuclear membrane

What is the centrosome?

A subcellular region which functions through the cell cycle to organize the cell's microtubules forming the spindle apparatus.

What makes up a centrosome?

Pericentriolar material (complexes composed of tubulin protein) and a pair of structures known as centrioles.

What makes up the centriole pair?

2 cylindrical bodies (also composed of microtubules) which lie at right angles to each other.

What happens to centrosomes during interphase?

The single centrosome duplicates, forming two centrosomes which remain together near the nucleus.
(Not visible through a compound microscope).

What is a kinetochore microtubule?

A microtubule that grabs chromosomes in order to pull them towards each pole of the cell (towards the centrosomes).

What is a non-kinetochore microtubule?

A microtubule that does not grab onto a chromosome, but rather reaches to the opposite pole of the cell and push away, causing the cell to grow and eventually separate (cytokinesis)

What is the role of astral rays?
Where are they found in a cell?

Their role is to anchor centrioles to poles.
They're found around the centrioles.

Mitosis vs. cytokinesis

Mitosis: The division of the nucleus.
Cytokinesis: The division of the cell itself (division of the cytoplasm).

Important steps for prophase

- Chromosomes coil and shorten, becoming visible on the compound microscope.
- The mitotic spindle begins forming.
- Asters extend from each centrosome towards the plasma membrane.

What is an aster?

A radial array of short microtubules

At the outside of each centromere is a protein complex known as a:

Kinetochore

What marks the end of prophase?

The disappearance of the nuclear membrane and the nucleolus

Important steps for metaphase

- The duplicated chromosomes line up along the metaphase plate.
- The arms of chromatids extend freely in any direction in the cytoplasm.
- For each chromosome, the kinetochore region attaches to kinetochore microtubules coming from opposite poles.

Important steps for anaphase

- Centromere of each chromatid pair splits, and the chromatids separate.
- Each chromatid is now a separate and functional daughter chromosome.
- Centromere (of each daughter chromosome) is pulled to opposite poles of the cell as kinetochore microtubules shorten.
- The cell elongates.

What is the end of anaphase marked by?

When each daughter chromosome has arrived to their respective poles.

How does a cell elongate (particularly during anaphase?

By non-kinetochore microtubules pushing away from their opposite pole.

Important steps for telophase

- A new nuclear membrane is assembled around each two groups of daughter chromosomes.
- Nucleoli is reformed.
- Chromosomes gradually uncoil and lengthen (returning to the form of chromatin).

What is the end of telophase marked by?

The end of the division of the cytoplasm (cytokinesis).

How does cytokinesis occur in animal cells?

With the aid of a cleavage furrow ("pinching in")

What is used to form the cleavage furrow of an animal cell (during telophase)?

Actin microfilaments forming a contractile ring.

What is a blastula?

An embryological stage in the development of most animals.

Where in the body does meiosis occur?

In the gonads (ovaries and testes)

How is genetic variability introduced in meiosis?

- Independent assortment
- Crossing-over

Sister chromatids are held together at the:

Centromere

Unique features of prophase I

- Pairing of homologous chromosomes
- Synapsis (via tetrads)
- Crossing-over

What are homologous chromosomes?

The same numbered chromosome finding its pair.
Ex. Chromosome 1 (paternal) pairs with chromosome 1 (maternal).

What is synapsis?
What does this form?

Homologous chromosomes pair up side-by-side.
This forms a tetrad.

When does crossing-over occur?

During prophase I when homologous chromosomes have formed tetrads.

During meiosis, how long do homologous chromosome pairs stay together?

Throughout late prophase I and metaphase I.

When does independent assortment occur?

Metaphase I

What does "independent assortment" mean?

Tetrads line up on the metaphase plate randomly (all maternal chromosomes could even be on one side or vice versa).

True or False:
DNA replication occurs in interkinesis because it is identical to interphase.

False.
Each chromosome is still comprised of two sister chromatids.

True or False:
Chromosomes coil and condense during prophase II.

True.
Chromosomes have uncoiled during telophase I and must re-condense.

What is the role of mitosis in the body?

- Growth
- Repair of old/damaged cells

What's the function of meiosis in the human body?

- Producing sex cells
- Passing of genes with genetic information

Interkinesis vs. interphase

During interkinesis, there is no replication of DNA

Prophase of mitosis vs. prophase II of meiosis

- Mitosis: No pairing of homologous chromosomes
- Mitosis: No synapsis, recombination, or crossing-over

Anaphase of mitosis vs. anaphase I of meiosis

- Mitosis: Sister chromatids are being separated.
- Meiosis (I): Chromosomes are being separated from their tetrads/homologous pairs.

Synapsis vs. crossing-over

- Synapsis: Pairing of homologous chromosomes (prophase I).-
- Crossing-over: Exchange of genetic material between tetrads/homologous pairs (during synapsis).