The life cycle of a cell Cell cycle consists of 2 major phases Interphase, where chromosomes duplicate and cell parts are made The mitotic phase, when nuclear division occurs Figure 8.5

Metabolic activity is very high Most of the life of a cell is spent in Interphase Cell does most of its’ growth during interphase During interphase a cell performs all of its regular functions and gets ready to divide Metabolic activity is very high Figure 8.5

Untwisting and replication of DNA Figure 10.4B

Before a cell starts dividing, the chromosomes are duplicated Sister chromatids This process produces sister chromatids EM of human chromosome that has duplicated Centromere Figure 8.4B

Structure of Chromosomes Homologous chromosomes are identical pairs of chromosomes. One inherited from mother and one from father made up of sister chromatids joined at the centromere. Copyright © McGraw-Hill Companies Permission required for reproduction or display

G2 Phase This phase spans the time from the completion of DNA synthesis to the onset of cell division Following DNA replication, the cell spends about 2-5 hours making proteins prior to entering the M phase Figure 8.5

Centrosomes (with centriole pairs) Early mitotic spindle Centrosome INTERPHASE PROPHASE Centrosomes (with centriole pairs) Early mitotic spindle Centrosome Fragments of nuclear envelope Kinetochore Chromatin Centrosome Spindle microtubules Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Figure 8.6

TELOPHASE AND CYTOKINESIS METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Cleavage furrow Nucleolus forming Metaphase plate Nuclear envelope forming Spindle Daughter chromosomes Figure 8.6 (continued)

Cytokinesis differs for plant and animal cells In animals, cytokinesis occurs by cleavage This process pinches the cell apart The first sign of cleavage is the appearance of a cleavage furrow Cleavage furrow Cleavage furrow Contracting ring of microfilaments Figure 8.7A Daughter cells

Cytokinesis differs for plant and animal cells As the daughter chormosomes move to opposite poles The cytoplasm constricts along the plane of the metaphase plate The process of cytokinesis divides the cell into two genetically identical cells Cleavage furrow Cleavage furrow Contracting ring of microfilaments Figure 8.7A Daughter cells

Chromosome distribution to daughter cells When the cell divides, the sister chromatids separate Chromosome duplication Two daughter cells are produced Each has a complete and identical set of chromosomes Sister chromatids Centromere Chromosome distribution to daughter cells Figure 8.4C

Multicellular diploid adults (2n = 46) Mitosis and development The human life cycle Meiosis is a special form of cell division that produces gametes Haploid gametes (n = 23) Egg cell haploid Sperm cell haploid MEIOSIS FERTILIZATION Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46) Mitosis and development Figure 8.13

Multicellular diploid adults (2n = 46) Mitosis and development There is a special mechanism to produce gametes Each gamete has a single set of chromosomes 22 autosomes and a single sex chromosome Haploid gametes (n = 23) Egg cell haploid Sperm cell haploid MEIOSIS FERTILIZATION Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46) Mitosis and development Figure 8.13

Gametes have a single set of chromosomes Haploid gametes keeps the chromosome number from doubling in each succeeding generation Haploid gametes are produced by a special sort of cell division called meiosis Which occurs only in reproductive organs, ovaries and testes Purpose of meiosis is to produce sperm and egg

MEIOSIS Meiosis involves 2 cell divisions Meiosis produces 4 cells from 1 parental cell Each of the 4 daughter cells has 23 individual chromosomes rather than 23 pairs of chromosomes Meiosis reduces the chromosome number from diploid to haploid Meiosis, like mitosis, is preceded by chromosome duplication However, in meiosis the cell divides twice to form four daughter cells

PARENT CELL (before chromosome replication) Site of crossing over MITOSIS MEIOSIS PARENT CELL (before chromosome replication) Site of crossing over MEIOSIS I PROPHASE PROPHASE I Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication 2n = 4 Chromosomes align at the metaphase plate Tetrads align at the Metaphase plate METAPHASE METAPHASE I ANAPHASE I TELOPHASE I ANAPHASE TELOPHASE Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together Haploid n = 2 Daughter cells of meiosis I 2n 2n No further chromosomal replication; sister chromatids separate during anaphase II MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Figure 8.15

MEIOSIS I: Homologous chromosomes separate INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Microtubules attached to kinetochore Sites of crossing over Metaphase plate Sister chromatids remain attached Spindle Nuclear envelope Chromatin Sister chromatids Tetrad Centromere (with kinetochore) Homologous chromosomes separate Figure 8.14, part 1

MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavage furrow Sister chromatids separate Haploid daughter cells forming Figure 8.14, part 2

Two equally probable arrangements of chromosomes at metaphase I POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 Figure 8.16

Chromosomes are matched in homologous pairs MEIOSIS AND CROSSING OVER Chromosomes are matched in homologous pairs Each synapsis is made up of 2 pairs of sister chromatids This matched set of 4 chromatids is called a tetrad Chromosomes Centromere Sister chromatids Figure 8.12

Crossing over further increases genetic variability Crossing over is the exchange of corresponding segments between two non-sister chromatids of homologous chromosomes Genetic recombination results from crossing over during prophase I of meiosis This increases variation further

How crossing over leads to genetic recombination Coat-color genes Eye-color genes How crossing over leads to genetic recombination Nonsister chromatids break in two at the same spot The 2 broken chromatids join together in a new way Tetrad (homologous pair of chromosomes in synapsis) 1 Breakage of homologous chromatids 2 Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase I 3 Separation of chromatids at anaphase II and completion of meiosis 4 Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Figure 8.18B Gametes of four genetic types

Coat-color genes Eye-color genes A segment of one chromatid has changed places with the equivalent segment of its nonsister homologue If there were no crossing over meiosis could only produce 2 types of gametes Tetrad (homologous pair of chromosomes in synapsis) 1 Breakage of homologous chromatids 2 Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase I 3 Separation of chromatids at anaphase II and completion of meiosis 4 Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Figure 8.18B Gametes of four genetic types