Meiosis By: Kirit Sahi

Introduction Word meiosis come from the Greek meioun, meaning to “to make similar”, since it results in a reduction of the chromosome number. Meiosis is the process that allows one diploid cell to divide in a special way to generate haploid cells in eukaryotes.

Different between Mitosis and Meiosis Mitosis is process to make copies of ALL OTHER cells for growth, repair, development, etc. Meiosis is a process of cell division of gametes o Sperm and egg

Overview of Meiosis 1 Copy DNALine Up 1 prophase 1metaphase 1 telophase 1 Divide 1 double stranded 4 chromosomes diploid 2n 2 chromosomes haploid 1n

Meiosis I Purpose: It is the reduction division, so it reduces the number of chromosomes in half, making the daughter cells haploid (when the parent cell was diploid). It is during meiosis I that most of the genetic recombination occurs.

Homologous chromosome Homologous chromosomes: Two chromosomes of a pair that have same length, centromere position, and staining pattern Both chromosomes of each pair carry genes controlling same inherited characters These homologous chromosomes are duplicated at interphase (before Prophase I)

Prophase 1 The cells genetic material condenses into a visible threadlike structures As DNA condensation proceeds and the chromosomes first become visible, they are visible as tetrads (4 chromatids) Tetrads becomes visible during prophase

5 stages of chromosomes in prophase I Leptotene stage:  Individuals chromosomes begins to condense into long strands within the nucleus Zygotene stage:  Chromosomes line up with each other into homologous chromosomes  They are also referred to as tetrad, a reference to the four sister chromatids Pachytene stage:  Crossing over takes place

Crossing Over nonsister chromatids chiasmata: site of crossing over variation Tetrad Prophase 1

Diplotene stage  Homologous chromosomes falls apart and begins to repel each other  The chromosomes themselves uncoil a bit, allowing some transcription of DNA Diakinesis stage  Chromosome re-condense  Site of crossing over entangle together effectively overlapping, making chiasmata clearly visible  Nucleoli disappears and nuclear membrane disintegrates attaching chromosomes to kinetochore

Metaphase I Tetrads line up the equator. The kinetochore microtubules has completely formed. Maternal or paternal homologues may point to either pole

Anaphase I Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart Each chromosomes are pulled toward opposing poles, forming two haploid sets Each chromosome still contains a pair of sister chromatids

Telophase I and Cytokinesis The first meiotic division effectively ends when the centromeres arrive at the poles Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids The microtubules that make up the spindle network disappear and a new nuclear membrane surrounds each haploid set The chromosomes uncoil back into chromatin Two haploid cells are produced

2 chromosomes haploid 1n gametes Line Up 2 Bye Bye 2 telophase 1 metaphase 2 telophase 2 Over view of Meiosis II

Meiosis II Purpose: At the end of meiosis I, each chromosome still had two chromatids. That is double the amount of DNA that a cell should have. So, the entire reason to go through meiosis II is to reduce the amount of DNA back to normal-- basically, to split the chromosomes so that each daughter cell has only one chromatid per chromosome (the normal genetic material).

Prophase II Nucleoli and nuclear membrane again disappears Chromatids shorten and thicken Centrioles move to the polar regions and are arranged by spindle fibers

Metaphase II Centromeres contain two kinetochores, organizing fibers from the centrosomes on each side

Anaphase II Centromeres are cleaved, allowing kinetochores to pull the sister chromatids apart The sister chromatids by convention are now called sister chromosomes and are pulled toward opposing poles Error: nondisjunction can occur during anaphase. o This leads to aneuploidy diseases ( trisomy 21- Down syndrome)

Telophase II and Cytokinesis Uncoiling, lengthening and disappearance of the chromosomes occur as well as the disappearance of the microtubules Nuclear envelopes reform Cleavage or cell wall formation eventually produces a total of four daughter cells Each with a haploid set of chromosomes