4.2 Mitosis and Meiosis Big Ideas Mitosis is a process of asexual reproduction that produces genetically identical cells It distributes the diploid number of chromosomes to daughter cells Meiosis occurs in sexual reproduction It distributes an assortment of chromosomes to the gametes

The timing and regulation of cell division are critical for normal growth and development A control system that depends on specific proteins within the cell regulates the sequence of events in the cell cycle If a cell reproduces at the wrong time or in the wrong place it can form a mass of cells (tumour) Cancer is a disease caused by the severe disruption of the mechanisms that regulate the cell cycle Disruption leads to uncontrolled cell division Cancer is so dangerous because of its ability to spread. A tumour displaces normal tissue as it grows. If the tumour is malignant and is not removed it can metastasize meaning it spreads to other tissues or organs in the body. Many different biochemical changes can affect the cell cycle and result in cancer and that’s why there is not single cure but rather there are multiple approaches to controlling the progress of the disease.

Mitosis & Asexual Reproduction Cell division is essential for living things and is the basis of reproduction for every organism Single-celled organisms often reproduce by simple cell division in which a single cell or group of cells splits into two new genetically identical cells The two new cells rapidly grow and develop into organisms that are genetic clones of one another Called asexual reproduction: produces offspring that inherit all their genetic material from one parent

Cells of the body are known as somatic cells Asexual reproduction also enables multicellular organisms to grow and to repair and replace damaged cells Cells of the body are known as somatic cells How often they divide depends on the type of cell and its role in the body When a single-celled organism or somatic cell divides it must go through a precise sequence of steps in order to create daughter cells that are genetically identical to the original parent cell Some specialized cells never divide, while others divide daily

Phases of Mitosis – pages 106/107 After interphase, the cell's DNA has been replicated and the M phase begin Mitosis distributes the duplicated sets of chromosomes to two daughter nuclei and cytokinesis divides the nucleus producing two daughter cells Mitosis involves 4 phases and how long mitosis takes depends on the cell (minutes to days)

During interphase the cell replicates its DNA and makes other new molecules and organelles Chromosomes are not yet visible because they are still loosely packed fibres of chromatin The presence of a nucleolus (a dark-staining region on the nucleus) indicates that a cell is actively making proteins

1. Prophase: The sister chromatids become visible and in animal cells, the centrioles separate and move to opposite sides of the nucleus The sister chromatids attach to the long fibres of the spindle By the end of the stage, the nucleolus disappears and the nuclear envelope breaks down

2. Metaphase: The sister chromatids are fully attached to the spindle fibres The spindle fibres move the chromatids and line them up across the middle of the cell 3. Anaphase: The sister chromatids separate from their partners to become individual chromosomes The spindle fibres pull the chromosomes to opposite poles/ends of the cell

Chromosomes begin to uncoil and take the form of chromatin 4. Telophase: Chromosomes begin to uncoil and take the form of chromatin Spindle fibres disappear and nuclear envelopes re-form around each set of daughter chromosomes At the end of this stage mitosis is complete Cytokinesis completes cell division by dividing the cytoplasm into two daughter cells each with its own nucleus Occurs at the same time as telophase Mitosis video: http://www.youtube.com/watch?v=C6hn3sA0ip0 6 min

Meiosis & Sexual Reproduction Sexual reproduction: the process in which two parents provide genetic material in order to produce offspring With sexual reproduction some of the genetic material from each parent combines producing offspring that differ genetically from both parents Each of the offspring inherits a unique combination of traits

In animals, meiosis occurs in the sex organs Meiosis: is a form of cell division that produces four cells, each containing half the number of chromosomes as the parent cell Sexual reproduction depends on this specialized type of cell division In animals, meiosis occurs in the sex organs Testes in males and ovaries in females Almost all cells in an individual organism have the same number and types of chromosomes Human somatic cells have _______________ of chromosomes for a total of _______ chromosomes 23 pairs, 46

Karyotype: a display of all the chromosomes in a cell or individual Each chromosome has a counterpart that resembles its size and shape One chromosome of each pair is inherited from each of your parents The two chromosomes of each matching pair are called homologous chromosomes Each member of a pair carries the same series of genes controlling the same inherited traits

The total number of chromosomes is called the diploid number Most human cells are diploid meaning they contain two sets of chromosomes The total number of chromosomes is called the diploid number Represented as 2n, in humans 2n=____ An exception to diploid cells are the sex cells (sperm and eggs = gametes) which contain only a single set of chromosomes, one from each homologous pair A cell with a single set of chromosomes is called a haploid number Represented as n, in humans n= _____

The haploid gametes are produced through the process of meiosis In the life cycle of sexually reproducing organisms, the nucleus of a haploid sperm from the father fuses with the nucleus of a haploid egg cell from the mother The fusion of the two gametes is called fertilization and results in a zygote which is diploid because it contains two pairs of homologous chromosomes, one from each parent (2 x n = 2n)

Autosomes: all other chromosomes Therefore, meiosis enables the zygote to have the correct number of chromosomes Eventually the diploid zygote will develop into a sexually mature adult with trillions of cells produced by mitosis! Sex chromosomes: one set of chromosomes that determine an individuals sex Autosomes: all other chromosomes In humans, the 23rd pair of chromosomes are the sex chromosomes Go back to karyotype on slide 13 Females have two X chromosomes, males have one X and one Y chromosome, an egg cell contains one X chromosomes and a sperm cell contains either an X or a Y chromosome

Phases of Meiosis – pages 110/111 Many phases of meiosis are similar to those of mitosis As with mitosis, DNA replicates before meiosis begins Unlike mitosis, this single round of DNA replication is followed by two distinct cell division stages: meiosis I and meiosis II By the end of meiosis II, the original diploid cell has become four haploid cells

Meiosis I – Homologous Chromosomes Separate 1. Prophase I: Begins with two complete sets of chromosomes in the form of sister chromatids Each duplicated chromosome pairs with its corresponding homologous chromosome forming a tetrad The homologous chromosomes then exchange portions of DNA Genetic Recombination: the exchange of DNA segments

2. Metaphase I: The tetrads move to the middle of the cell and line up 3. Anaphase I: Homologous chromosomes separate from the tetrad and migrate to opposite poles of the spindle Sister chromatids remain attached at their centromeres

The nuclear membrane re-forms around each cluster of chromosomes 4. Telophase I: The nuclear membrane re-forms around each cluster of chromosomes Chromosomes of each daughter cell are present in duplicates as sister chromatids Each cell is considered haploid even though they contain the diploid number of chromosomes, because they only contain half the chromosomes but two of each The cells must go through a second division in order to have the correct number of chromosomes 1. 1 2 3 4 2. 1 2 3 4 1 2 3 4 Draw on board

Meiosis II – Sister Chromatids Separate (in each cell) 1. Prophase II: Not preceded by a round of DNA replication*** In each haploid daughter cell a spindle forms and attaches to the centromeres of the sister chromatids 2. Metaphase II: The spindle attached to the centromeres moves the sister chromatids so that they line up in the middle of the cell

3. Anaphase II: The sister chromatids separate and move to opposite poles of the cell Once separated they are considered individual chromosomes 4. Telophase II: Chromosomes begin to uncoil and take the form of chromatin Spindle fibres disappear and nuclear envelopes re-form around each set of daughter chromosomes Cytokinesis splits the cells one more time, resulting in four haploid cells

New Gene Combinations Offspring that result from sexual reproduction are genetically different from their parents and from one another The genetic variety in offspring arises through the process of meiosis and fertilization Each parent contributes a unique combination of genes to the offspring and this set of genes codes for a unique combination of traits Mitosis and Meiosis video: http://www.youtube.com/watch?v=zGVBAHAsjJM 12 min Mitosis 0-5:50 min

The way in which chromosomes assort or distribute during meiosis contributes to variation in the gametes How the chromosomes in each homologous pair (tetrad) line up and separate in metaphase I occurs by chance Therefore, the assortment of chromosomes in the resulting gametes is random Called random assortment

In addition to the random assortment of chromosomes, in prophase I there is an exchange of genetic material between homologous chromosomes Called crossing over When crossing over takes place, homologous chromosomes are closely paired along their lengths Each gene on one pair of sister chromatids is precisely lined up with the corresponding gene on the adjacent pair of sister chromatids

This close association between homologous chromosomes in early meiosis is called synapsis At regions called chiasmata, chromatids from different pairs are attached and segments of the chromatids are exchanged Genetic recombination results in chromosomes with new combination of genetic information different from those carried by the original chromosomes

One single cross over event can affect many genes on a chromosome Without this process, every chromosome produced by meiosis would be purely of maternal origin or purely of paternal origin One single cross over event can affect many genes on a chromosome Since more than one cross over event can occur in each tetrad, it is not surprising that the resulting gametes and offspring can be so varied Homework: page 115 Mitosis #1,2,14 Meiosis #3-7,9-13,15,16 STUDY PAGE 113