Copyright 2003 Pearson Education Inc publishing as Benjamin
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8. 5
• Untwisting and replication of DNA Figure 10. 4 B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Before a cell starts dividing, the chromosomes are duplicated – This process produces sister chromatids Sister chromatids Centromere – EM of human chromosome that has duplicated Figure 8. 4 B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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 © Mc. Graw-Hill Companies Permission required for reproduction or display Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
G 2 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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
INTERPHASE PROPHASE Centrosomes (with centriole pairs) Early mitotic spindle Centrosome Chromatin Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Figure 8. 6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Fragments of nuclear envelope Centrosome Kinetochore Spindle microtubules
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
METAPHASE ANAPHASE Cleavage furrow Metaphase plate Spindle TELOPHASE AND CYTOKINESIS Daughter chromosomes Figure 8. 6 (continued) Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Nuclear envelope forming Nucleolus forming
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Cytokinesis differs for plant and animal cells • In animals, cytokinesis occurs by cleavage Cleavage furrow – This process pinches the cell apart – The first sign of cleavage is the appearance of a cleavage furrow Cleavage furrow Figure 8. 7 A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Contracting ring of microfilaments 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 Figure 8. 7 A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Cleavage furrow Contracting ring of microfilaments Daughter cells
• When the cell divides, the sister chromatids separate – Two daughter cells are produced Chromosome duplication – Each has a complete and identical set of chromosomes Chromosome distribution to daughter cells Figure 8. 4 C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Sister chromatids Centromere
• 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 Figure 8. 13 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings FERTILIZATION Diploid zygote (2 n = 46) Multicellular diploid adults (2 n = 46) Mitosis and development
• There is a special mechanism to produce gametes Haploid gametes (n = 23) Egg cell haploid Sperm cell haploid MEIOSIS • Each gamete has a single set of chromosomes • 22 autosomes and a single sex Figure 8. 13 chromosome Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings FERTILIZATION Diploid zygote (2 n = 46) Multicellular diploid adults (2 n = 46) Mitosis and development
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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
MITOSIS MEIOSIS PARENT CELL (before chromosome replication) Site of crossing over PROPHASE I Tetrad formed by synapsis of homologous chromosomes PROPHASE Duplicated chromosome (two sister chromatids) METAPHASE ANAPHASE TELOPHASE 2 n Chromosome replication 2 n = 4 Chromosomes align at the metaphase plate Tetrads align at the Metaphase plate Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together 2 n Daughter cells of mitosis Figure 8. 15 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSIS I METAPHASE I ANAPHASE I TELOPHASE I Daughter cells of meiosis I Haploid n=2 No further MEIOSIS II chromosomal replication; sister chromatids separate during anaphase II n n Daughter cells of meiosis II
MEIOSIS I: Homologous chromosomes separate INTERPHASE Centrosomes (with centriole pairs) Nuclear envelope PROPHASE I METAPHASE I Microtubules attached to Spindle kinetochore Sites of crossing over Chromatin Sister chromatids Tetrad Figure 8. 14, part 1 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Metaphase plate Centromere (with kinetochore) ANAPHASE I Sister chromatids remain attached Homologous chromosomes separate
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Cleavage furrow Sister chromatids separate Figure 8. 14, part 2 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Haploid daughter cells forming
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Figure 8. 16 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Combination 3 Combination 4
MEIOSIS AND CROSSING OVER Chromosomes are matched in homologous pairs • Each synapsis is made up of 2 pairs of sister chromatids Chromosomes Centromere • This matched set of 4 chromatids is called a tetrad Sister chromatids Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8. 12
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Coat-color genes • How crossing over leads to genetic recombination Eye-color genes Tetrad (homologous pair of chromosomes in synapsis) • Nonsister chromatids break in two at the same spot 1 Breakage of homologous chromatids 2 Joining of homologous chromatids Chiasma • The 2 broken chromatids join together in a new way Figure 8. 18 B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 3 Separation of homologous chromosomes at anaphase I 4 Separation of chromatids at anaphase II and completion of meiosis Parental type of chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types
• 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 Figure 8. 18 B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Coat-color genes Eye-color genes Tetrad (homologous pair of chromosomes in synapsis) 1 Breakage of homologous chromatids 2 Joining of homologous chromatids Chiasma 3 Separation of homologous chromosomes at anaphase I 4 Separation of chromatids at anaphase II and completion of meiosis Parental type of chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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