Meiosis and Cell Division

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Questions and Answers

What is the number of chromosomes in a normal human somatic cell?

46 (correct)

What is the number of chromosomes in a normal gamete?

23 (correct)

What is the term for any exact multiple of n chromosomes?

Polyploid

Aneuploid

Haploid

Euploid (correct)

What is the term for a chromosome number that is not euploid?

Aneuploid Signup and view all the answers

What is the result of nondisjunction of chromosomes during meiosis?

One cell receives 24 chromosomes and the other receives 22 Signup and view all the answers

What is the term for a condition where an individual has an extra copy of chromosome 21?

Trisomy 21 Signup and view all the answers

How many mature gametes develop from one primary spermatocyte?

Four Signup and view all the answers

How many mature gametes develop from one primary oocyte?

One Signup and view all the answers

What is the main difference between meiosis and mitosis in terms of chromosome alignment?

Homologous chromosomes align themselves in pairs in meiosis. Signup and view all the answers

What is the purpose of crossovers during meiosis I?

To enhance genetic variability. Signup and view all the answers

What is the number of crossovers that occur during each meiotic I division?

1 or 2 per chromosome. Signup and view all the answers

What is the significance of meiosis in terms of genetic variation?

It enhances genetic variation through crossover and random distribution of homologous chromosomes. Signup and view all the answers

What is the number of chromosomes in each gamete after meiosis II?

23 chromosomes. Signup and view all the answers

What is the purpose of meiosis?

To produce gametes. Signup and view all the answers

Why is the diploid number of chromosomes restored after fertilization?

Because meiosis reduces the number of chromosomes to haploid. Signup and view all the answers

What is the estimated total number of germ cells in the ovary by the fifth month of prenatal development?

7 million Signup and view all the answers

What happens to most oogonia by the seventh month of prenatal development?

They become atretic Signup and view all the answers

What is the term for a primary oocyte together with its surrounding flat epithelial cells?

Primordial follicle Signup and view all the answers

During what stage of meiosis do primary oocytes remain arrested before puberty?

Diplotene Signup and view all the answers

What is the estimated total number of primary oocytes at birth?

600,000 to 800,000 Signup and view all the answers

How many follicles are selected from the pool to begin maturation each month during puberty?

15 to 20 Signup and view all the answers

What is the term for the stage of follicular development where surrounding follicular cells change from flat to cuboidal and proliferate?

Primary or preantral Signup and view all the answers

What is the function of oocyte maturation inhibitor (OMI)?

To inhibit oocyte maturation Signup and view all the answers

Where is the first polar body located?

In the perivitelline space between the zona pellucida and the cell membrane of the secondary oocyte Signup and view all the answers

What happens to the oocyte if it is not fertilized?

It degenerates approximately 24 hours after ovulation Signup and view all the answers

At what stage does meiosis II arrest?

Metaphase Signup and view all the answers

What is the function of Sertoli cells?

To support the development of spermatogonia Signup and view all the answers

What type of cells give rise to spermatogonial stem cells?

Primordial germ cells Signup and view all the answers

What happens to the primary spermatocytes during meiosis I?

They undergo a prolonged prophase Signup and view all the answers

What is the significance of cytoplasmic bridges?

They join successive cell generations Signup and view all the answers

At what stage do type A spermatogonia undergo mitotic divisions?

To form type B spermatogonia Signup and view all the answers

What is the term for loosely organized embryonic connective tissue regardless of origin?

Mesenchyme Signup and view all the answers

Where do crest cells from the trunk region migrate to form melanocytes in the skin and hair follicles?

Ectoderm through a dorsal pathway Signup and view all the answers

What is the pathway that crest cells take to become sensory ganglia, sympathetic and enteric neurons, Schwann cells, and cells of the adrenal medulla?

Ventral pathway Signup and view all the answers

What is the term for cells that form and migrate from cranial neural folds, contributing to the craniofacial skeleton and other cell types?

Neural crest cells Signup and view all the answers

What is the result of the development of otic placodes?

Formation of the otic vesicles and structures needed for hearing and maintaining equilibrium Signup and view all the answers

During what week do the lens placodes form the lenses of the eyes?

Fifth week Signup and view all the answers

What is the significance of neural crest cells in vertebrate development?

They formed the basis for vertebrate features Signup and view all the answers

What percentage of birth defects are associated with neural crest cells?

One-third Signup and view all the answers

Study Notes

Meiosis and Gametogenesis

Meiosis is a type of cell division that occurs in germ cells to produce male and female gametes (sperm and egg cells).
Meiosis requires two cell divisions, meiosis I and meiosis II, to reduce the number of chromosomes from 46 (diploid) to 23 (haploid).
In meiosis I, homologous chromosomes align themselves in pairs, and crossovers occur, resulting in the exchange of chromatid segments between paired homologous chromosomes.
Meiosis II separates sister chromatids, resulting in four haploid cells, each with 23 chromosomes.

Crossover and Genetic Variability

Crossovers are critical events in meiosis I that increase genetic variability by interchanging chromatid segments between paired homologous chromosomes.
The approximately 30-40 crossovers per meiotic I division occur most frequently between genes that are far apart on a chromosome.

Gametogenesis

Gametogenesis begins with spermatogonial (male) and oogonia (female) cells.
Primary spermatocytes and primary oocytes undergo meiosis to produce haploid gametes.
In females, one primary oocyte gives rise to four daughter cells, each with 22 plus 1 X chromosomes, but only one develops into a mature gamete (oocyte).
In males, one primary spermatocyte gives rise to four daughter cells, two with 22 plus 1 X chromosomes and two with 22 plus 1 Y chromosomes, all of which develop into mature gametes (sperm).

Chromosomal Abnormalities

Chromosomal abnormalities can occur due to errors in meiosis or mitosis, leading to abnormal numbers of chromosomes.
Aneuploidy refers to any chromosome number that is not euploid (exact multiple of n).
Trisomy occurs when an extra copy of a chromosome is present, while monosomy occurs when one is missing.
Down syndrome is an example of a chromosomal abnormality caused by an extra copy of chromosome 21 (trisomy 21).

Oogenesis

Oogenesis is the process of formation of female gametes (oocytes).
Primary oocytes are formed in the ovary during fetal development and enter prophase of meiosis I, but instead of proceeding into metaphase, they enter the diplotene stage (a resting stage during prophase).
Primary oocytes remain arrested in prophase and do not finish their first meiotic division before puberty is reached.
At puberty, a pool of growing follicles is established, and each month, 15-20 follicles are selected to begin maturing.
Maturation of oocytes continues at puberty, and only a few hundred of the original 600,000-800,000 primary oocytes present at birth will be ovulated.

Spermatogenesis

Spermatogenesis is the process of formation of male gametes (spermatozoa).
Spermatogenesis begins at puberty and includes all the events by which spermatogonia are transformed into spermatozoa.
At birth, germ cells in the male infant can be recognized in the sex cords of the testis surrounded by supporting cells (Sertoli cells).
Shortly before puberty, the sex cords acquire a lumen and become the seminiferous tubules.
Type A spermatogonia undergo a limited number of mitotic divisions to produce type B spermatogonia, which then divide to form primary spermatocytes.

Mesenchyme and Neural Crest Cells

Mesenchyme refers to loosely organized embryonic connective tissue regardless of origin.
Neural crest cells (NCCs) are formed from the neural tube and migrate to various parts of the embryo to form a wide range of cell types, including neurons, glial cells, melanocytes, and cells of the adrenal medulla.
NCCs contribute to the craniofacial skeleton and sensory ganglia, and are involved in at least one-third of all birth defects and many cancers.
Evolutionarily, NCCs appeared at the dawn of vertebrate development and formed the basis for vertebrate features, including sensory ganglia and craniofacial structures.

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Description

This quiz covers the process of meiosis, a type of cell division that occurs in germ cells to produce gametes, including the reduction of chromosomes to the haploid number.