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Lecture 7 Mitosis & Meiosis

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1 Lecture 7 Mitosis & Meiosis

2 Cell Division Essential for body growth and tissue repair Interphase
G1 phase Primary cell growth phase S phase DNA replication G2 phase Microtubule synthesis Mitosis Nuclear division Cytokinesis Cytoplasmic division

3 Chromosomes: Carriers of Inherited Traits
Chromosomes were first observed by the German embryologist Walther Fleming in 1882 Chromosomes exist in somatic cells as pairs Homologous chromosomes or homologues The number of chromosomes varies enormously from species to species The Australian ant Myrmecia spp. has only 1 pair Some ferns have more than 500 pairs

4 Human Chromosomes Humans have 46 chromosomes
The 23 pairs of homologous chromosomes can be organized by size This display is termed a karyotype

5 Basic Terminology Diploid cells have two copies of each chromosome
Replicated chromosomes consist of two sister chromatids These are held together at the centromere

6 Chromosomes Chromosomes are composed of chromatin
Complex of DNA (~ 40%) and proteins (~ 60%) A typical human chromosome contains about 140 million nucleotides in its DNA This is equivalent to About 5 cm in stretched length 2,000 printed books of 1,000 pages each! In the cell, however, the DNA is coiled

7 Chromosomes: Packaged DNA
The DNA helix is wrapped around positively-charged proteins, called histones 200 nucleotides of DNA coil around a core of eight histones, forming a nucleosome The nucleosomes coil into solenoids Solenoids are then organized into looped domains The looped domains appear to form rosettes on scaffolds

8 Mitosis The phases of mitosis are: Cytokinesis
Prophase Xm Xf Condense Metaphase Xm Xf Line Up Anaphase Im If Separate Telophase Im If Divide Cytokinesis Cleavage furrow formed in late anaphase by contractile ring Cytoplasm is pinched into two parts after mitosis ends

9 Early Prophase Asters are seen as chromatin condenses into chromosomes
Early mitotic spindle Pair of centrioles Centromere Aster Chromosome, consisting of two sister chromatids Asters are seen as chromatin condenses into chromosomes PLAY Prophase Figure

10 Late Prophase Nucleoli disappear
Fragments of nuclear envelope Polar microtubules Kinetochore Kinetochore microtubule Spindle pole Nucleoli disappear Centriole pairs separate and the mitotic spindle is formed PLAY Prometaphase Figure

11 Metaphase Chromosomes cluster at the middle of the cell with their centromeres aligned at the exact center, or equator, of the cell This arrangement of chromosomes along a plane midway between the poles is called the metaphase plate Metaphase Metaphase plate Spindle PLAY Metaphase Figure

12 Anaphase Centromeres of the chromosomes split
Daughter chromosomes Anaphase Centromeres of the chromosomes split Motor proteins in kinetochores pull chromosomes toward poles PLAY Anaphase Figure

13 Telophase and Cytokinesis
New sets of chromosomes extend into chromatin New nuclear membrane is formed from the rough ER Nucleoli reappear Generally cytokinesis completes cell division Telophase and cytokinesis Nucleolus forming Contractile ring at cleavage furrow Nuclear envelope forming PLAY Telophase

14 Controlling the Cell Cycle
G0 is an extended rest period The eukaryotic cell cycle is controlled by feedback at three checkpoints Cell growth is assessed at the G1 checkpoint DNA replication is assessed at the G2 checkpoint Mitosis is assessed at the M checkpoint PLAY Control of the cell cycle

15 Fingers and toes form from these paddle-like hands and feet
Cell Death Fingers and toes form from these paddle-like hands and feet During fetal development, many cells are programmed to die Programmed cell death Human cells appear to be programmed to undergo only so many cell divisions About 50 in cell cultures Only cancer cells can divide endlessly

16 What is Cancer? Cancer is unrestrained cell growth and division
The result is a cluster of cells termed a tumor Benign tumors Encapsulated and noninvasive Malignant tumors Not encapsulated and invasive Can undergo metastasis Leave the tumor and spread throughout the body

17 What is Cancer? Most cancers result from mutations in growth-regulating genes There are two general classes of these genes 1. Proto-oncogenes Encode proteins that stimulate cell division If mutated, they become oncogenes 2. Tumor-suppressor genes Encode proteins that inhibit cell division Cancer can be caused by chemicals, radiation or even some viruses

18 Cancer, cell division, and p53 protein
PLAY How tumor suppression genes work

19 New molecular therapies for cancer
Receiving the signal to divide Stopping tumor growth Stepping on the gas Passing the signal via a relay switch Checking that everything is ready Amplifying the signal Releasing the “brake” Potential cancer therapies are being developed to target seven different stages in the cancer process Stages 1-6 Prevent the start of cancer within cells Focus on the decision-making process to divide Stage 7 Act outside cancer cells Prevents tumors from growing and spreading

20 Importance of Generating Diversity
Diversity is what allows gene populations to survive changes in their environment The greater the diversity, the more likely some individuals will have the traits to survive a major change Genetic diversity is the raw material that fuels evolution No genetic process generates diversity more quickly than sexual reproduction

21 Two Types of Reproduction
Asexual reproduction Creates a faithful copy Does not involve fertilization Contain one set of chromosomes Sexual reproduction Maximizes diversity Involves the alternation of meiosis and fertilization Meiosis halves the number of chromosomes from each parent cell Fertilization restores the number of chromosomes Contains two sets of chromosomes

22 The Sexual Life Cycle The life cycles of all sexually-reproducing organisms follows the same basic pattern Haploid cells or organisms alternate with diploid cells or organisms There are three basic types of sexual life cycles Spend most of life in haploid form Spend most of life in diploid form Spend half of life in haploid form and half in diploid form

23 The Sexual Life Cycle in Humans

24 Discovery of Meiosis Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in 1887 Gametes (eggs and sperm) contain half the complement of chromosomes found in other cells The fusion of gametes is called fertilization or syngamy It creates the zygote, which contains two copies of each chromosome

25 Meiosis PLAY Unique features of meiosis Mitosis Meiosis I Meiosis II
Prophase Xm Xf Condense Metaphase Xm Xf Line Up Anaphase Im If Separate Telophase Im If Divide Mitosis Meiosis I mX Xf Xm fX Xm fX mX Xf Xm Xf Xf Xm Xf Xm Im If If Im Meiosis II Xf Xm PLAY Unique features of meiosis

26 Meiotic Cell Division: Meiosis I
Figure 27.7 Tetrads line up at the spindle equator during metaphase I In anaphase I, homologous chromosomes still composed of joined sister chromatids are distributed to opposite ends of the cell At the end of meiosis I each daughter cell has: Two copies of either a maternal or paternal chromosome A 2n amount of DNA and haploid number of chromosomes In telophase I: The nuclear membranes re-form around the chromosomal masses The spindle breaks down The chromatin reappears, forming two daughter cells PLAY Prophase I, Metaphase I, Anaphase I, Telophase I

27 Meiotic Cell Division: Meiosis II
Mirrors mitosis except that chromosomes are not replicated before it begins Meiosis accomplishes two tasks: It reduces the chromosome number by half (2n to n) It introduces genetic variability PLAY Meiosis II

28 Differences Between Mitosis & Meiosis
Purpose Create new cells (faithful copies) Create new individuals (diversity) Where it occurs Body cells Special reproductive organs Number of divisions after one chromatin replication event One Two How chromosomes Line up at metaphase Individually Homologous pairs in Meiosis I, Individually in Meiosis II Number of daughter cells Two Four Homologous chromosomes in daughter cells Two (diploid) One (haploid) Makeup of daughter cells vs. parent cell Same as parent cell Different from parent cell PLAY Differences between meiosis &. mitosis

29 Evolutionary Consequences of Sex
Sexual reproduction increases genetic diversity through three key mechanisms Independent assortment Crossing over Random fertilization

30 Independent assortment
Three chromosome pairs 23 combinations In humans, a gamete receives one homologue of each of the 23 chromosomes Humans have 23 pairs of chromosomes 223 combinations in an egg or sperm 8,388,608 possible kinds of gametes

31 Crossing over DNA exchanges between maternal and paternal chromatid pairs This adds even more recombination to independent assortment that occurs later

32 Random fertilization The zygote is formed by the union of two independently-produced gametes Therefore, the possible combinations in an offspring 8,388,608 X 8,388,608 = 70,368,744,177,664 More than 70 trillion! And this number does not count crossing-over

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