Lecture 19: Dividing the Cell III: Meiosis and Genetic Variation Bio Sci 93

Meiosis

A special type of cell division used in sexual reproduction to form gametes. One round of DNA replication is followed by two cell divisions (meiosis I and II), producing four genetically different haploid cells.

Diploid (2n)

A cell with two complete sets of chromosomes, one maternal and one paternal. Human somatic cells are diploid (2n = 46).

Haploid (n)

A cell with a single set of chromosomes. Human gametes are haploid (n = 23).

Gametes

Haploid sex cells (sperm and egg) formed by meiosis. They fuse at fertilization to form a diploid zygote.

Karyotype

A display of condensed chromosomes arranged in homologous pairs, usually photographed at metaphase. It shows chromosome number, size, and banding pattern, and can reveal abnormalities such as trisomy.

Crossing over

The exchange of segments between non-sister chromatids of homologous chromosomes during prophase I. It produces recombinant chromosomes and increases genetic variability.

Homologous chromosomes

A pair of chromosomes, one inherited from each parent, that have the same genes in the same order and the same size and centromere position. Humans have 23 pairs.

Sister chromatids

The two identical copies of a duplicated chromosome, joined at the centromere. They separate in meiosis II (and in mitosis).

Non-sister chromatids

Chromatids from different chromosomes of a homologous pair (one maternal, one paternal). They exchange segments during crossing over.

Non-disjunction

The failure of chromosomes to separate properly during meiosis. It results in gametes with an extra or missing chromosome (n+1 or n-1).

Trisomy

Having three copies of a chromosome instead of two (2n+1). It is caused by non-disjunction, as in trisomy 21 (Down syndrome).

Independent assortment

The random orientation of homologous pairs at the metaphase I plate, so each pair separates independently of the others. It gives 2^23 (a bit over 8 million) possible chromosome combinations per human gamete.

Random fertilization

Any sperm can fuse with any egg by chance. With 2^23 x 2^23 possible combinations, it gives about 70 trillion possible zygotes.

Genetic variability

Genetic differences among individuals. Meiosis generates it through crossing over, independent assortment of chromosomes, and random fertilization.

Features of a chromosome (karyotype)

A duplicated chromosome has two sister chromatids joined at a centromere. Homologous chromosomes pair up by size, banding, and centromere position. Humans have 46 chromosomes (23 pairs).

Information learned from a karyotype

Chromosome number, size, and structure; sex (XX or XY); and abnormalities such as extra or missing chromosomes (e.g., trisomy 21).

Why are cells stopped at metaphase to make a karyotype?

At metaphase, chromosomes are condensed and present as sister chromatid pairs, so they are easiest to see and count.

Role of growth factor and hypotonic solution in making a karyotype

Growth factor stimulates cells to enter the cell cycle. A hypotonic solution makes cells swell so they rupture more easily and release the condensed chromosomes.

Purpose of meiosis

To produce haploid gametes for sexual reproduction, so fertilization restores the diploid number, and to create genetic variation.

Mitosis vs. meiosis: number of divisions

Mitosis has one division and produces 2 identical diploid cells. Meiosis has two divisions and produces 4 genetically different haploid cells.

Mitosis vs. meiosis: role in the body

Mitosis is used for growth, repair, and asexual reproduction (clones) in somatic cells. Meiosis forms gametes for sexual reproduction.

Mitosis vs. meiosis: synapsis and crossing over

Synapsis and crossing over occur only in meiosis (prophase I), not in mitosis.

DNA replication in meiosis

DNA replicates once, during interphase (S phase), before meiosis I. There is no replication between meiosis I and meiosis II.

Meiosis I

Separates homologous chromosomes. Stages: prophase I (synapsis and crossing over), metaphase I (homologous pairs line up), anaphase I (homologs separate while sister chromatids stay attached), and telophase I and cytokinesis. It results in two haploid cells whose chromosomes still have two sister chromatids.

Meiosis II

Separates sister chromatids. Stages: prophase II, metaphase II, anaphase II, and telophase II and cytokinesis. It results in four haploid cells with unduplicated chromosomes.

How meiosis reduces chromosome number

Meiosis I separates homologous chromosomes, which halves the chromosome number (2n to n). Meiosis II then separates sister chromatids, giving four haploid cells.

Prophase I

Chromosomes condense, homologous chromosomes pair up (synapsis), and crossing over occurs at chiasmata between non-sister chromatids.

Metaphase I vs. metaphase II

In metaphase I, homologous pairs line up at the metaphase plate. In metaphase II, individual chromosomes (sister chromatid pairs) line up.

Anaphase I vs. anaphase II

In anaphase I, homologous chromosomes separate and sister chromatids remain attached. In anaphase II, sister chromatids separate.

Chiasma

The X-shaped site where non-sister chromatids of homologous chromosomes cross over during prophase I.

Non-disjunction in meiosis I

A pair of homologous chromosomes fails to separate. All four resulting gametes are abnormal (two n+1 and two n-1).

Non-disjunction in meiosis II

Sister chromatids fail to separate. Two gametes are abnormal (one n+1 and one n-1) and two are normal (n).

Down syndrome

Trisomy 21: three copies of chromosome 21, caused by non-disjunction. Most cases come from an error in the egg, usually in meiosis I.

Why does the risk of non-disjunction rise with maternal age?

Eggs begin meiosis I before birth and pause in prophase I for 15 to 45+ years. The cohesin holding homologs together is not replaced and slowly wears out, so homologs may separate incorrectly.

Which trisomies are seen at birth?

Trisomy 13 (Patau), 18 (Edwards), and 21 (Down). These chromosomes have the fewest genes among the autosomes.

Why do most trisomies not survive?

An extra chromosome means extra protein from every gene on it (gene dosage). Hundreds of extra genes can be tolerated, but thousands cannot.

Gene dosage

The amount of protein produced from the number of copies of a gene. An extra chromosome increases dosage for every gene on it.

How crossing over increases variation

Non-sister chromatids exchange segments, producing recombinant chromosomes with new combinations of maternal and paternal alleles.

How independent assortment increases variation

Homologous pairs orient randomly at metaphase I, so each gamete gets a random mix of maternal and paternal chromosomes (2^23, a bit over 8 million, combinations in humans).

How random fertilization increases variation

Any sperm can fuse with any egg, multiplying the possible combinations (2^23 x 2^23, about 70 trillion).

Three sources of genetic variability in meiosis

1) Crossing over, 2) Independent assortment of chromosomes, 3) Random fertilization.

Chromosome number: human vs. other species

Human 2n = 46. Fruit fly 2n = 8, house fly 2n = 12, cat 2n = 38, dog 2n = 78.