Cell Replication Mitosis & Meiosis

Cell Division Somatic cell production Male adult 2N Female adult Many mitosis divisions Male adult 2N Female adult Female embryo 2N Male embryo Meiosis A single set of chromosomes Egg 1N Sperm Several mitotic divisions Somatic cell production A double set of chromosomes Embryo 2N Many mitotic divisions Somatic cell production Adult 2N Gamete production Fertilization Zygote 2N

Mitosis in Onion Cells If the cells of a growing root tip are examined, a proportion of them are in mitosis. Cells in different stages of division can be seen. Mitosis is the process by which the cell produces two new daughter cells from the original parent cell. Prophase Anaphase Late anaphase Telophase

Cell division for somatic growth and repair. Mitosis Micrographs Cell division for somatic growth and repair. 1. Interphase 2. Prophase 3. Metaphase 5. Telophase 4. Anaphase

Interphase Interphase is the first step of the process Cells prepare for Mitosis by accumulating materials required At this point, individual chromosomes cannot be seen This is the longest phase in which the DNA replicates 1. Interphase

Interphase Chromosomes need to be duplicated into double stranded chromosomes, so each daughter cell can receive an identical copy

Prophase During Prophase, the chromosomes begin to condense, and become visible under a microscope Eventually they can be seen as individual chromosomes The nuclear membrane breaks down Centrioles appear at opposite ends of the cell 2. Prophase

Metaphase During Metaphase, the chromosomes line up through the middle of the cell Spindle fibres extend from the centrioles, and meet up with the chromosomes in the middle The ends of the spindle fibres attach to the chromosomes 3. Metaphase

Anaphase During Anaphase, the spindle fibres begin to contract, pulling the chromosomes. They split apart at the centromere, separating them into 2 chromatids These chromatids are now single stranded chromosomes The spindle fibres draw the chromosomes to either end of the cell, each side having the exact copies 4. Anaphase

Telophase During Telophase, the opposite occurs to prophase. The chromosomes begin to expand. The chromosomes can no longer be seen individually A nuclear membrane begins to form around each set of chromosomes The spindle dismantles and disappears 5. Telophase

Cytokinisis Cytokinesis is not part of Mitosis, although it starts occurring during Anaphase or Telophase It is the separation of the two daughter cells The membrane pinches inwards and a cleavage furrow appears The membrane is formed and 2 daughter cells are born!!

Cell Division haploid (1N) diploid (2N) Somatic cell production Many mitosis divisions Male adult 2N Female adult Female embryo 2N Male embryo Meiosis A single set of chromosomes Egg 1N Sperm Several mitotic divisions Somatic cell production A double set of chromosomes Embryo 2N Many mitotic divisions Somatic cell production Adult 2N Gamete production Fertilization Zygote 2N haploid (1N) diploid (2N) 12

Crossing over may occur at this stage in meiosis Intermediate cell 1N First Division (Reduction division) The purpose of meiosis is to produce haploid sex cells. Haploid sex cells have only one copy of each homologous pair of autosomes plus one sex chromosome. Second Division ('Mitotic' division) 1N Gametes (eggs or sperm) 1N

2N Interphase DNA replication Meiosis I 2N Prophase 1 Synapsis and crossing over The first division of meiosis is called a ‘reduction’ division because it reduces (halves) the number of chromosomes. One chromosome from each homologous pair is donated to each intermediate cell. 2N Metaphase 1 Bivalents line up on the equator Anaphase 1 Telophase 1 1N Intermediate cell

NOTE: Half the full chromosome complement shown Intermediate cell NOTE: Half the full chromosome complement shown Meiosis II 1N Prophase 2 The second division of meiosis is called a ‘mitotic’ division, because it is similar to mitosis. Sister chomatids of each chromosome are pulled apart and are donated to each gamete cell. Metaphase 2 Anaphase 2 1N Gamete (egg or sperm) Telophase 2

Mitosis and Meiosis Compared Homologous chromosomes pair up at the equatorial plate Homologous chromosomes do not pair up at the equatorial plate Reduction division Meiosis I Cell division ‘Mitotic’ division Meiosis II Cell division 1N 2N

Non-disjunction in Meiosis I The meiotic spindle normally distributes chromosomes to daughter cells without error. Sometimes, homologous chromosomes or sister chromatids fail to separate. In these cases, one gamete receives two of the same type of gamete and the other gamete receives no copy. This mishap, called non-disjunction, results in abnormal numbers of chromosomes in the gametes. This example shows non-disjunction in meiosis I; homologous chromosomes fail to separate properly at anaphase during meiosis I. Meiosis II Non-disjunction in Meiosis I n+1 n–1

Non-disjunction in Meiosis II Non-disjunction can also occur in meiosis II, when sister chromatids fail to separate during anaphase of meiosis II. In cases of non-disjunction, if either of the aberrant gametes unites with a normal one at fertilization, the offspring will have an abnormal chromosome number (e.g. 2N+1 or 2N–1). This is known as aneuploidy and it accounts for chromosomal defects such as Down syndrome (trisomy 21) and Edward syndrome (trisomy 18). Meiosis II Non-disjunction in Meiosis II n+1 n–1 n

Overview Mitosis Meiosis Cells undergo mitotic division during which time the genetic material is doubled and divided into two cells. Meiosis Meiosis is a reduction division that results in the formation of haploid (N) cells from diploid (2N) ones. Its purpose is to produce gametes for sexual reproduction. During meiosis, genetic material is exchanged between chromosomes; this introduces genetic variation into the offspring.

Helpful Hints to remember Cell division takes place for the growth, development, repair and replacement of tissues Mitosis replaces cells and meiosis produces gametes for reproduction Males gamete = sperm formed in the testes and female gamete = ovum formed in the ovaries Haploid the daughter cells have half the number of chromosomes of the parent cell. Diploid have double, in other words the daughter and parent cells have the same number of chromosomes Each human body cell has 23 pairs, or 46 chromosomes. Sperm and ova have only 23 chromosomes

Helpful Hints to remember MITOSIS I Inter = Prepare/replicate DNA P Pro = Condense M Meta = Line up A Ana = Split T Telo = expand/2 new cells/diploid