Chapter 11 Meiosis

Zygote

A diploid cell formed by the fusion of two haploid gametes (egg and sperm) during fertilization; the first cell of a new organism containing a complete set of chromosomes (2n).

Germ-line cells

Specialized diploid cells in the reproductive organs (testes or ovaries) that undergo meiosis to produce haploid gametes (sperm or eggs).

Fertilization

The process in which a haploid sperm cell fuses with a haploid egg cell to form a diploid zygote, restoring full chromosome number.

Diploid

A cell containing two complete sets of chromosomes, one from each parent (2n).

Haploid

A cell containing only one set of chromosomes, typical of gametes (sperm and egg) (n).

Centromere

The region where sister chromatids are tightly joined and where spindle fibers attach.

Chiasma (plural: chiasmata)

The visible site of crossing over between nonsister chromatids of homologous chromosomes during prophase I of meiosis.

Homolog/Homologous chromosomes

A pair of chromosomes, one maternal and one paternal, that are similar in size, shape, and gene content, pairing up during meiosis.

Kinetochore protein

A specialized protein structure on the centromere that serves as the attachment site of spindle microtubules during chromosome segregation.

Kinetochore microtubules

Spindle fibers that attach to kinetochores and help pull chromosomes toward opposite poles of the cell during division.

Centriole

A cylindrical organelle made of microtubules that helps organize the spindle apparatus during cell division in animal cells.

Crossing over

The exchange of genetic material between nonsister chromatids of homologous chromosomes during prophase I, increasing genetic variation.

Synapsis

The pairing of homologous chromosomes along their lengths during prophase of meiosis, forming tetrads (bivalents).

Synaptonemal complex

A protein structure that forms between homologous chromosomes during synapsis, holding them together and facilitating crossing over.

Nondisjunction

The failure of homologous chromosomes in meiosis or sister chromatids in meiosis II to separate properly, leading to abnormal chromosome numbers in gametes.

Correct order of stages for Meiosis I to Meiosis II (including Interphase phases)

Interphase, prophase, metaphase, anaphase, telophase, cytokinesis, prophase II, metaphase II, anaphase II, telophase II, cytokinesis.

Meiosis II daughter cell chromosome number

A diploid cell with 40 chromosomes undergoes meiosis; each daughter cell at the end of meiosis II will have 20 chromosomes.

Nondisjunction outcome

During meiosis I, if homologous chromosomes fail to separate, half of the gametes will have one extra chromosome and half will be missing one.

Mutation effect on gametes

In an organism where 2n = 12, if a mutation prevents crossing-over, gametes still receive 6 chromosomes, but genetic variation will be reduced.

Gametes receive 6 chromosomes

Gametes still receive 6 chromosomes, but genetic variation will be reduced.

Gametes receive 12 chromosomes

Gametes will receive 12 chromosomes.

Gametes receive fewer than 6 chromosomes

Gametes will receive fewer than 6 chromosomes.

Gametes receive genetically identical chromosomes

Gametes will receive 6 chromosomes but will always be genetically identical.

Diploid cell after meiosis I

A biologist observes that after meiosis I but before meiosis II, a cell is still diploid though each chromosome consists of two chromatids.

Original diploid cell chromosomes

A cell has just completed meiosis II and contains 12 chromosomes. How many chromosomes did the original diploid cell have?

Preventing DNA replication outcome

A scientist prevents DNA replication between meiosis I and meiosis II. Which outcome is most likely?

Sister chromatids pull during meiosis I

If a species exhibits sister chromatids that pull to opposite poles during meiosis I, what would be the consequence for the ploidy of gametes?

Meiotic daughter cells with chromosome error

You observe four meiotic daughter cells from an organism; two have 11 chromosomes, two have 13. Given the species' normal haploid number is 12, what error likely occurred?

Gene therapy targeting somatic cells

If a gene therapy targets somatic cells rather than germ-line cells, what is one major limitation regarding hereditary transmission?

Meiosis fails to suppress DNA replication

In a plant species where 2n = 32, meiosis fails to suppress DNA replication between the divisions, what is the most likely result?

Significant reduction of genetic variation

Consider an organism that shows significant reduction of genetic variation in its offspring. You test and find no chiasmata during meiosis. What is the most plausible explanation?

Difference in attachments of kinetochores

Which of the following best describes the difference in attachments of kinetochores in meiosis I versus meiosis II?

Mutation causing persistent cohesion at centromeres

Sister chromatids fail to segregate, producing gametes with extra chromatids.

Gamete chromosome restoration

Fertilization restores 2n = 18, ensuring each gamete gets exactly nine chromosomes through Meiosis I reductive division.

Scenario reducing genetic diversity

Elimination of independent assortment of chromosomes would reduce a population's genetic diversity the most.

Sister chromatids in prophase II

A cell in prophase II of meiosis with 12 chromosomes has 24 sister chromatids.

Chromosome number in zygote

A male produces sperm with 21 chromosomes due to nondisjunction in meiosis II, resulting in a zygote with 41 chromosomes after fertilization with a normal egg.

Importance of chromosome number reduction

The reduction of chromosome number from diploid to haploid is crucial to avoid doubling chromosome number at fertilization.

Cell types that never divide by meiosis

Only germ line cells undergo meiosis; somatic cells (like skin, muscle, and blood) only divide by mitosis.

Cohesin breakdown phases

Cohesin is broken allowing sister chromatids to separate during anaphase of mitosis and anaphase II of meiosis.

Outcome of mitosis

The main outcome of mitosis is the formation of two genetically identical daughter cells, each with the same diploid number of chromosomes (2n) as the parent cell.

Outcome of meiosis

The main outcome of meiosis is the formation of four genetically unique haploid cells (n), which have half the number of chromosomes as the original diploid cell.