Meiosis, Karyotypes, and Chromosomal Abnormalities

Comprehensive practice flashcards covering chromosome structure, karyotypes, stages of Meiosis I and II, crossing over, differences between mitosis and meiosis, and non-disjunction disorders such as Down syndrome.

What are homologous chromosomes?

A pair of chromosomes that carry genes for the same trait at the same loci, have the same centromere position, are equal in length, and consist of one chromosome from the father and one from the mother.

Importance of meiosis

Produces four haploid, genetically diverse daughter cells to ensure genetic variation and maintain stable chromosome numbers across generations.

Introduces genetic variation through crossing over and random arrangement.

Halving effect of meiosis overcomes the doubling effect of fertilisation thus maintaining a constant chromosome number .

What is a karyotype?

A representation of the number, shape, and arrangement of a full set of chromosomes in the nucleus of a somatic cell.

How are human chromosomes classified into autosomes and gonosomes?

Chromosome pairs 1 to 22 are autosomes (totaling 44 autosomes), and chromosome pair 23 are gonosomes or sex chromosomes (totaling 2 gonosomes).

How do sex chromosomes (gonosomes) differ between human females and males?

Females have $XX$ chromosomes ($44\text{ autosomes} + XX\text{ gonosomes}$), while males have $XY$ chromosomes ($44\text{ autosomes} + XY\text{ gonosomes}$). The $X$ chromosome is larger and the $Y$ chromosome is smaller.

What is cytokinesis?

The process of division during which the cytoplasm of a single cell divides into two daughter cells.

What is non-disjunction?

When homologous chromosome pairs fail to separate in meiosis.

What is a chiasma?

The point where two chromatids overlap during crossing over.

What are bivalents?

A pair of homologous chromosomes involved in crossing over.

Where does meiosis occur in plants, and what gametes are produced?

In males, meiosis occurs in the anther to produce pollen grains. In females, meiosis occurs in the ovary / ovule to produce egg cells / ova.

What major events occur during Prophase I of meiosis?

Homologous chromosomes pair up, crossing over occurs, the centrosome splits into two centrioles which move to opposite poles, and the nuclear membrane and nucleolus start to disappear.

What are the sequential steps of crossing over?

1. Occurs in prophase I of meiosis.\n2. Homologous chromosomes pair off.\n3. Chromatids of each homologous chromosome pair overlap at points called chiasmata.\n4. A break occurs at each chiasma.\n5. Each chromosome now has a piece of chromatid from its partner.

What occurs during Metaphase I of meiosis?

Homologous chromosomes arrange in pairs randomly at the equator of the cell, and spindle fibres attach to the centromere of each chromosome.

What occurs during Anaphase I of meiosis?

Homologous chromosomes separate and move to opposite poles when the spindle fibres contract.

What are the results of Telophase I of meiosis?

Chromosomes reach the poles, the nuclear membrane reappears, and two haploid daughter cells are formed.

What occurs during Prophase II of meiosis?

Single chromosomes with two chromatids are present, the centrosome splits into two centrioles that move to opposite poles, and the nuclear membrane and nucleolus start to disappear.

How does chromosome arrangement differ between Metaphase I and Metaphase II?

In Metaphase I, homologous chromosomes are arranged in pairs at the equator, whereas in Metaphase II, single chromosomes are arranged randomly at the equator.

What events occur during Telophase II of meiosis?

Chromatids reach the poles and form new nuclei, the cytoplasm divides in each cell to form 2 daughter cells per cell, resulting in four haploid, genetically different daughter cells.

How do the structures pulled to opposite poles differ between Anaphase I and Anaphase II?

Whole chromosomes are pulled to opposite poles in Anaphase I, whereas chromatids are pulled to opposite poles in Anaphase II.

How does the chromosome number change across Meiosis I versus Meiosis II?

The chromosome number is halved during Meiosis I, whereas the chromosome number does not change during Meiosis II.

What is the three-fold importance of meiosis?

1. It produces haploid gametes.\n2. It introduces genetic variation through crossing over and random arrangement of chromosomes at the equator.\n3. The halving effect of meiosis overcomes the doubling effect of fertilisation, maintaining a constant chromosome number from one generation to the next.

What are four similarities between mitosis and meiosis?

1. DNA replication takes place.\n2. The nucleus divides.\n3. The cytoplasm divides.\n4. New cells are formed.

What are the main differences between mitosis and meiosis?

Mitosis has no crossing over, forms somatic cells, involves one nuclear division, and yields two genetically identical diploid cells. Meiosis features crossing over in Prophase I, forms sex cells, involves two nuclear divisions, and yields four genetically different haploid cells.

What abnormalities can occur during Anaphase I and Anaphase II?

In Anaphase I, one or more homologous pairs of chromosomes may fail to separate. In Anaphase II, sister chromatids of one or more chromosomes may fail to separate.

What is the difference between aneuploidy and polyploidy?

Aneuploidy occurs when gametes have one less (monosomic) or one more (trisomic) chromosome. Polyploidy occurs when gametes have one extra set ($3n$) or two extra sets ($4n$) of chromosomes.

How does Down syndrome develop due to non-disjunction?

Non-disjunction of chromosome pair 21 occurs during Anaphase I in humans, forming an abnormal gamete with 24 chromosomes. The fusion of this abnormal gamete with a normal gamete (23 chromosomes) results in a zygote with three copies of chromosome 21 (totaling 47 chromosomes).

What is the biological significance of crossing over?

1. It introduces genetic variation among offspring through new combinations of alleles from parents.\n2. It gives offspring higher chances of survival under unfavourable environmental conditions.\n3. It ensures that no offspring are genetically identical.