Meiosis

Topics 4.2 and 10.1

Assessment Statements

4.2.1State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei.

4.2.2Define homologous chromosomes.4.2.3Outline the process of meiosis, including pairing of

homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells.

4.2.4Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).

4.2.5State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.

4.2.6State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

4.2.7Analyse a human karyotype to determine gender and whether non-disjunction has occurred.

Meiosis

• Form of cell division which results in gametes (sex cells)

• Each new cell has only half the number of chromosomes that a typical cell in that organism has

• These cells are haploid• Cells with the full number are called diploid cells• Division is called a reduction division because

the number of chromosomes has been reduced• Four cells are formed and are called daughter cells• Each haploid daughter cell has a unique mix of

half of the genetic information of the parent cell

Homologous chromosomes

• Similar in shape and size• Two chromosomes carry the same

genes• Not identical b/c the alleles could be

different• The letter n is used to denote the

number of unique chromosomes• Haploid (n) and diploid (2n)

Phases of meiosis

• Interphase• Prophase I

– Chromosomes become visible– Homologous chromosomes pair up– Crossing over occurs– Spindle fibers form

• Metaphase I– Homologues align across equator– Nuclear membrane disintegrates

• Anaphase I– Spindle fibers attach and pull

chromosomes to poles

• Telophase I– Spindles disintegrate– Chromosomes uncoil– New nuclear membranes form

• Cytokinesis

• Prophase II– DNA condenses– New spindle produced

• Metaphase II– Nuclear membrane disintegrates– Chromosomes align at equator– Spindle fibers attach to sister

chromatids

• Anaphase II– Centromeres split, releasing each sister

chromatid as an individual chromosome– Spindle fibers pull chromatids to poles

• Telophase II– Chromosomes unwind– Nuclear envelopes from around each of

the four haploid cells

Meiosis Square Dance

Down’s syndrome

• Chromosomes do not always separate the way they are expected to do so during meiosis

• Result is an unequal distribution of chromosomes due to non-disjunction

• Non-disjunction of 21st pair results in Down’s syndrome

• A.k.a. Trisomy 21• Effects:

– Malformations of digestive system– Learning difficulties

• Occurs in 1 in 800 births• Risk increases for mothers over 35

Karyotypes

• Photograph of the chromosomes found in a cell

1. Cells stained and prepared on slide2. Photomicrograph images obtained

during mitotic metaphase3. Images cut out and separated4. Images place in order by size and

the position of their centromeres

Obtaining cells for karyotyping

• Amniocentesis– Hypodermic needle used to extract amniotic

fluid around developing baby

• Removing from chorionic villus– Obtained from tissue sample from the

placenta’s projections into the uterine wall

• White blood cells grown in lab• Very expensive and invasive• Recommended if expectant mother is over

35

Analysis of a human karyotype

• Gender?• Non-disjunction?• Karyotyping activity• http://www.biology.arizona.edu/huma

n_bio/activities/karyotyping/karyotyping.html

Meiosis

Topic 10.1

Assessment Statements

• 10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis.

• 10.1.2 Outline the formation of chiasmata in the process of crossing over.

• 10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.

• 10.1.4 State Mendel’s law of independent assortment.

• 10.1.5 Explain the relationship between Mendel’s law of independent assortment and meiosis.

Chromosome behavior during meiosis

Prophase I: chromosomes become more visible, homologous chromosomes pair up (called synapsis), crossing over occurs

Prophase II: DNA condenses

Metaphase I: centromeres have microtubules attached, bivalents line up randomly (random orientation), chromatids no longer identical

Metaphase II: individual chromosomes line up along equator (random orientation), spindle fibers attach to centromeres

Anaphase I: homologous chromosomes separate, pulled to opposite poles resulting in independent assortment

Anaphase II: centromeres split, each sister chromatid becomes an individual chromosome and is pulled to opposite pole of cell

Telophase I: chromosomes surrounded by two new nuclear membranes

Telophase II: chromosomes unwind

Crossing over

• Explains family resemblances• Adds chance for variety in offspring• Mixing genetic material between

non-sister chromatids occurs when chromosomes intertwine and break

• Tips of two chromatids are switched• The place where the two connect to

each other is called a chiasma (pl. chiasmata)

Genetic variety

• Generated by:1. Crossing over during prophase I

• Chromatids which were once identical now have different combinations of maternal and paternal alleles at their various loci

2. Random orientation during metaphase I• Bivalents arrange themselves independent

of chromosomes from same parent

Mendel’s law of independent assortment

• When gametes are formed, the separation of one pair of alleles between the daughter cells is independent of the separation of another pair of alleles

• Just b/c one trait (flower color) is inherited from a parent it does not follow that any specific trait of that parent (such as seed color) must be passed on as well

• But, of course, there are exceptions to every rule…– This rule doesn’t always apply when genes are

on the same chromosome

Independent assortment and meiosis

• The distribution and separation of the chromosomes at the end of metaphase I ensures a random distribution of alleles between the gametes