Cell Division Chapter 5

Why does a cell divide? As a cell absorbs nutrients and gets larger, the volume of the cell increases faster than the surface area. -Therefore, the demands of the cell (the volume) exceed the ability of the cell to bring in nutrients and export wastes. Solution? Divide into two smaller cells

Cell Increase and Decrease Cell division: Mitosis (division of nucleus or karyokinesis) Cytokinesis (division of cytoplasm) Apoptosis (cell death) decreases the number of cells. Both cell increase and apoptosis occur during normal development and growth.

Cell Division, Cell Death Functions of Cell Division Mitosis Increase number of somatic cells Wound healing Asexual reproduction Meiosis Production of gametes (germ cells, sperm and eggs) Apoptosis—Programmed cell death Amoeba

Asexual reproduction 2 Hydra

The cell cycle

Interphase—3 Stages G1 stage – S stage – G2 stage – cell growth cell doubles its organelles accumulates materials for DNA synthesis S stage – DNA synthesis occurs DNA replication results in duplicated chromosomes G2 stage – cell synthesizes proteins needed for cell division

Chromosomes Chromosome – Condensed chromatin

Homologous Chromosomes Same size Same genes in the same order Chromosome pairs, one from mom, one from dad

a. Diploid number (2n) - total number of chromosomes in a cell (somatic cells) b. Haploid number (n) - only one homologue of each chromosome is present (gametes)

The Big Picture

The Cell Cycle Interphase (90% of cycle) • G1 phase~ growth • S phase~ synthesis of DNA • G2 phase~ preparation for cell division Mitotic phase • Mitosis~ nuclear division • Cytokinesis~ cytoplasm division

The Mitotic Stage Following interphase is the M stage, including mitosis and cytokinesis. During mitosis, sister chromatids of each chromosome separate. The cell cycle ends when cytokinesis, the cleaving of the cytoplasm, is complete.

Figure 5.30 The DNA double helix and its replication

In mitosis each chromosome is replicated and then divides

Duplicated Chromosome Sister chromatids attached at centromere Sister chromatids separate during cell division

PLOIDY Ploidy - number of copies of each chromosome - also number of genomes Haploid (N)= one set of unpaired chromosomes Diploid (2N or 2n)= one set of paired chromosomes Triploid = three copies of each chromosome Tetraploid = four copies of each chromosome Hexaploid = six copies of each chromosome Etc, etc, etc

Chloroplasts have their own DNA, separate from plant nuclear DNA, derived from their cyanobacterial origin

Most eukaryotes also have a mitochondrial genome, derived from a eubacterial endosymbiont ancestor Human mtDNA 37 genes, 16 kilobasepairs

Apoptosis Apoptosis - programmed cell death. Function?

Mitosis Overview 1 diploid (2N) cell>>>>2 diploid (2N)cells Genetically identical daughter cells Function---Maintain the somatic cell chromosome number

Mitosis Overview

Chromosome Distribution

Spindle and Fibers Spindle made of? Centrosomes (MTOC)—function? microtubules that disassemble and assemble. Centrosomes (MTOC)—function? Types of Fibers Polar Spindle Fibers Centomeric (kinetochore) fibers Asters

Phases of Mitosis Prophase Metaphase Anaphase Telophase Pretty Men Are Tough Overview

As cell enters mitosis from interphase it has 2 complete sets of chromosomes because of replication in the S phase. Each set must be re-arranged and distributed into the 2 new daughter nuclei. This is mitosis.

Late Interphase

Early Prophase

Late Prophase

Metaphase

Anaphase

Telophase Review

Results of Mitosis Two daughter nuclei Each with same chromosome number as parent cell Genetically identical to each other and the parent cell

CYTOKINESIS – Cytoplasm splits into 2 cells. -Animal cells: Cleavage furrow forms from outside in. - Plant cells: Division plate forms from inside out.  

Cytokinesis in Animal Cells

Cytokinesis in plant cells

REVIEW OF MITOSIS Remember: Mitosis Is Part Of The Cell Cycle!! Parent Cell Remember: Mitosis Is Part Of The Cell Cycle!! DNA Synthesis (“S” Phase) Mitosis & Cytokinesis Two daughter cells Parent cell and daughter cells contain the same complement of chromosomes.

Cell Division in Prokaryotes Binary fission---process of asexual reproduction in prokaryotes. Two daughter cells Identical to the original parent cell and each other All contain a single chromosome.

The Big Picture

Overview of Meiosis 1 diploid (2N) cell>>>>4 haploid (1N) cells Genetically unique daughter cells Function---reduce the chromosome number by ½ The main source of genetic diversity in sexually reproducing organisms

MEIOSIS Two Parts: Meiosis I: Meiosis II: Separation of homologous pairs Ploidy Level? Meiosis II: Separation of sister chromatids

COMPARISON-- MITOSIS and MEIOSIS Occurs in somatic (body) cells Growth, development, replacement & repair Produces 2 daughter cells Clones of parent and each other Daughter cells are diploid (2N) Meiosis: Occurs in germ cells (immature reproductive cells) Production of gametes (sex cells) Produce 4 daughter cells Distinct from parent and each other Daughter cells are haploid (1N)

Genetic Recombination Meiosis increases genetic diversity. Two points of genetic recombination. Crossing-over of nonsister chromatids—Prophase 1 Independent assortment of homologous chromosomes during Metaphase 1

Synapsis and crossing-over . Defs

More Terminology Crossing Over Synapsis Tetrad Segments of paternal/maternal chromatids exchange during Meiosis I Synapsis Process of homologs coming together physically during Meiosis I Tetrad Consists of 4 sister chromatids from 2 homologous chromosomes

Independent assortment .

Meiosis in Detail Phases Interkinesis. Replication of DNA?? Prophase Metaphase Anaphase telophase Interkinesis. Replication of DNA??

Meiosis I in an animal cell

Meiosis II

Sources of Genetic Variation Three sources of genetic recombination: Independent alignment – metaphase I Crossing-over---prophase I Fertilization

SUMMARY-- MITOSIS and MEIOSIS Occurs in somatic (body) cells Growth, development, replacement & repair Produces 2 daughter cells Clones of parent and each other Daughter cells are diploid (2N) Meiosis: Occurs in germ cells (immature reproductive cells) Production of gametes (sex cells) Produce 4 daughter cells Distinct from parent and each other Daughter cells are haploid (1N) comparison

Meiosis compared to mitosis

Life cycle of humans

Spermatogenesis

Oogenesis

FERTILIZATION RESTORES FULL SET OF CHROMOSOMES Female gamete Male gamete fertilization Diploid zygote contains homologous pairs of chromosomes

Sources of Genetic Variation Three sources of genetic recombination: Independent assortment – metaphase I Crossing-over---prophase I Fertilization

MEIOSIS & GENETIC VARIATION Independent Assortment Diploid organisms can produce 2n diff. gametes Ea. homologous pair can orient in two different ways Humans: 223 = 8,388,608 (est. 8.4 million) Each homologous pair can orient in either of two ways

HYPOTHETICAL EXAMPLE OF INDEPENDENT ASSORTMENT Eye color Hair color Gene for brown eyes Gene for blue eyes r Gene for black hair Gene for red hair

INDEPENDENT ASSORTMENT During meiosis I, tetrads can line up 2n different ways. OR Meiosis I & II Brown eyes Black hair Blue eyes Red hair Blue eyes Black hair Brown eyes Red hair

MEIOSIS & GENETIC VARIATION Crossing Over: Physical exchange of sections of chromosomes Results in shuffling of genome New combo of genes on chromosomes (recombination) A single crossover in each homologous pair = 5.0 x 10 27 different zygotes possible (4951,760,200,000,000,000,000,000,000)!

MEIOSIS & GENETIC VARIATION Random Fertilization: Each human can produce an nearly 8.4 million different kinds of gametes Crossing over not taken into account!!! Sex yields: 8.4 mill x 8.4 mill = 70.6 x 10 12 genetically distinct offspring can be produced

SUMMARY-- MITOSIS and MEIOSIS Occurs in somatic (body) cells Growth, development, replacement & repair Produces 2 daughter cells Clones of parent and each other Daughter cells are diploid (2N) Meiosis: Occurs in germ cells (immature reproductive cells) Production of gametes (sex cells) Produce 4 daughter cells Distinct from parent and each other Daughter cells are haploid (1N) comparison