Topic 4.2Meiosis

Topic 4.2.1Meiosis is a Reduction Division

• Diploid nucleus divides to form haploid nuclei• Form of cell division which results in gametes• Each new resulting cell has only half the number of

chromosomes of a traditional cell• Human somatic cells have 46 chromosomes, but

egg and sperm have only 23 (haploid cells)• Cells with the full number of chromosomes are

diploid cells (somatic cells)• Therefore, meiosis is a reduction division

• During sexual reproduction, each parent contributes 50% of the genetic information

• The four haploid cells formed from meiosis are referred to as daughter cells

• Each sex cell or gamete has a unique mix of half of the genetic information of the parent cell

Topic 4.2.2Homologous Chromosomes

• In humans, 46 chromosomes can be grouped into 23 pairs called homologous chromosomes

• Homologous means similar in shape and size and that the two chromosomes carry the same genes

• The reason there are two of each is because one came from the father and the other from the mother

• Although homologous chromosomes carry the same genes, they are not identical

• Alleles for the genes from each parent could be different

• Homologous chromosomes show the same banding pattern in a karyotype

• The letter n denotes the unique number of chromosomes in an organism • Human somatic cells are 2n• Human sex cells (gametes or egg and sperm) are n• When an egg and sperm unite in fertilization they

are 2n or 46 chromosomes• http://www.youtube.com/watch?v=vXNaTRs83hE

&feature=player_embedded#!

Topic 4.2.3The Process of Meiosis

• Meiosis can be divided into two main stages:– Meiosis I– Meiosis II

• Both Meiosis I and Meiosis II are subdivided into:– Prophase– Metaphase– Anaphase– Telophase

• Homologous chromosomes are paired up in prophase I and split apart in anaphase I

Meiosis I Meiosis II

• Prophase I:– Chromosomes become visible– Crossing over occurs

• exchange of genetic material between non-sister chromatids• The exchange happens between homologous chromosomes (maternal and paternal)• Synapsis = homologous chromosomes and their sister

chromatids pair up during prophase I and form a tetrad or bivalents

• Recombinant chromatids which end up in the egg or sperm cells are a mosaic of the parent cells original chromatids

• The point where homologous chromosomes cross over is called the chiasmata

– Spindle fibers made from microtubules form– Nuclear membrane begins to disappear

• Metaphase I:– Bivalents (tetrads) line up across the cells equator

• Anaphase I:– Spindle fibers from the poles attach to chromosomes

and pull them to opposite poles (homologous c’somes separate…NOT the sister chromatids)

• Telophase I:– Spindles and spindle fibers disintegrate– Chromosomes uncoil and new nuclear membranes form

• The homologous pairs are now separated at the end of Meiosis I

• At the end of Meiosis I, cytokinesis happens and the cell splits into two separate cells• The cells are now haploid; however, each chromatid still has its sister chromatid attached at the __??????_ (fill in this blank)

• These two cells now begin Meiosis II in order to separate the sister chromatids

• Prophase II:– DNA condenses into visible chromosomes again– New meiotic spindle fibers are produced– Nuclear membrane begins to disappear

• Metaphase II:– Sister chromatids line up at the equator in no special

order (random orientation)– Spindle fibers from opposite poles attach to each of the sister chromatids at the centromere

• Anaphase II:– Centromeres split, releasing sister chromatids as individual chromosomes– Spindle fibers pull chromatids to opposite ends of the cell– Because of random orientation the chromatids could be

pulled towards either of the newly forming daughter cells

• Telophase II:– Chromosomes unwind their strands of DNA– Nuclear envelopes form around the four haploid cells preparing them for cytokinesis

• Result = four genetically unique haploid gametes

Meiosis Video Clip

• http://www.myspace.com/video/emo-loser-lol/meiosis-square-dance/3965243