 Sexual reproduction  Fertilization of sperm and egg produces offspring  Asexual reproduction  Offspring are produced by a single parent, without the participation of sperm and egg

 Asexual reproduction  Chromosomes are duplicated and cell divides  Each daughter cell is genetically identical to the parent and the other daughter  Sexual reproduction  Each offspring inherits a unique combination of genes from both parents  Offspring can show great variation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 8.2 Cells arise only from preexisting cells  "Every cell from a cell" is at the heart of the perpetuation of life  Can reproduce an entire unicellular organism  Is the basis of sperm and egg formation  Allows for development from a single fertilized egg to an adult organism  Functions in an organism's renewal and repair

 8.3 Prokaryotes reproduce by binary fission  Prokaryotic cells reproduce asexually by a type of cell division called binary fission  Genes are on one circular DNA molecule  The cell replicates its single chromosome  The chromosome copies move apart  The cell elongates  The plasma membrane grows inward, dividing the parent into two daughter cells

Continued elongation of the cell and movement of copies Duplication of chromosome and separation of copies Plasma membrane Cell wall Prokaryotic chromosome Division into two daughter cells

Prokaryotic chromosomes Colorized TEM 32,500 

 8.4 The large, complex chromosomes of eukaryotes duplicate with each cell division  Eukaryotic genes  Many more than in prokaryotes  Grouped into multiple chromosomes in the nucleus

 Eukaryotic chromosomes  Contain a very long DNA molecule associated with proteins  Most of the time occur in the form of thin, loosely packed chromatin fibers  Condense into visible chromosomes just before cell division

 Eukaryotic cell division  Chromosomes replicate  Sister chromatids joined together at the centromere  Sister chromatids separate  Now called chromosomes  Cell divides into two daughter cells  Each with a complete and identical set of chromosomes

Sister chromatids Centromere TEM 36,600 

Centromere Chromosome duplication Sister chromatids Chromosome distribution to daughter cells

 8.5 The cell cycle multiplies cells  The cell cycle is an ordered series of events from the time a cell is formed until it divides  Most of the cell cycle is in interphase  G1(Growth 1): cell grows in size  S (Synthesis): DNA synthesis (replication) occurs  G2 (Growth 2): Cell continues to grow and prepare for division

 The cell actually divides in mitotic (M) phase  Mitosis: nuclear division  Cytokinesis: cytoplasmic division  Duplicated chromosomes evenly distributed into two daughter nuclei

I NTERPHASE G1G1 G2G2 S (DNA synthesis) Cytokinesis Mitosis M ITOTIC PHASE (M)

 8.6 Cell division is a continuum of dynamic changes  Interphase: Duplication of the genetic material ends when chromosomes become visible  Prophase (the first stage of mitosis): The mitotic spindle is forming. Centrosomes migrate to opposite ends of the cell  Prometaphase: Chromatins completely coil into chromosomes; nucleoli and nuclear membrane disperse

 Metaphase: The spindle is fully formed; chromosomes are aligned on the metaphase plate (equator)  Anaphase: Chromosomes separate from the centromere, dividing to arrive at poles  Telophase: Cell elongation continues, a nuclear envelope forms around chromosomes, chromosomes uncoil, and nucleoli reappear  Cytokinesis: The cytoplasm divides

I NTERPHASE P ROPHASE P ROMETAPHASE Kinetochore Fragments of nuclear envelope Centrosome Early mitotic spindle Chromatin Centrosomes (with centriole pairs) LM 250  Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Centromere Spindle microtubules

M ETAPHASE A NAPHASETELOPHASE AND C YTOKINESIS Metaphase plate Spindle Daughter chromosomes Nuclear envelope forming Cleavage furrow Nucleolus forming

 Cytokinesis  Animals  Ring of microfilaments contracts into cleavage furrow  Cleavage occurs  Plants  Vesicles fuse into a cell plate  Cell plate develops between two daughter cells

Cleavage furrow Cleavage furrow Daughter cells Cleavage furrow Contracting ring of microfilaments SEM 140 

Cell plate forming Wall of parent cell Daughter nucleus New cell wallCell wall TEM 7,500  Daughter cells Vesicles containing cell wall material Cell plate

 Density-dependent inhibition  Cells form a single layer  Cells stop dividing when they touch one another  Inadequate supply of growth factor causes division to stop

Cells anchor to dish surface and divide. When cells have formed a complete single layer, they stop dividing (density-dependent Inhibition). If some cells are scraped away, the remaining cells divide to fill the dish with a single layer and then stop (density-dependent inhibition).

After forming a single layer, cells have stopped dividing. Providing an additional supply of growth factors stimulates further cell division.

 Growth factors signal the cell cycle control system  The cell cycle control system regulates the events of the cell cycle  If a growth factor is not released at three major checkpoints, the cell cycle will stop  G 1  G 2  M phase

G 1 checkpoint G0G0 G1G1 G2G2 G 2 checkpoint M checkpoint M S Control system

 How a growth factor might affect the cell cycle control system  Cell has receptor protein in plasma membrane  Binding of growth factor to receptor triggers a signal transduction pathway  Molecules induce changes in other molecules  Signal finally overrides brakes on the cell cycle control system

G 1 checkpoint G1G1 G2G2 M S Control system Growth factor Plasma membrane Relay proteins Signal transduction pathway Receptor protein

 Growing out of control, cancer cells produce malignant tumors  Cancer cells do not respond normally to the cell cycle control system  Divide excessively  Can invade other tissues  May kill the organism

 If an abnormal cell avoids destruction by the immune system, it may form a tumor  Benign: abnormal cells remain at original site  Malignant: abnormal cells can spread to other tissues and parts of the body  Metastasis: spread of cancer cells through the circulatory system

Tumor Glandular tissue Lymph vessels Blood vessel A tumor grows from a single cancer cell. Cancer cells invade Neighboring tissue. Cancer cells spread through lymph and blood vessels to other parts of the body.

 Cancers are named according to location of origin  Carcinoma: external or internal body coverings  Sarcoma: tissues that support the body  Leukemia and lymphoma: blood-forming tissues  Radiation and chemotherapy are effective as cancer treatments because they interfere with cell division

 Review of the functions of mitosis: growth, cell replacement, and asexual reproduction  When the cell cycle operates normally, mitotic cell division functions in  Growth  Replacement of damaged or lost cells  Asexual reproduction

 Chromosomes are matched in homologous pairs  The somatic (body) cells of each species contain a specific number of chromosomes  Humans and most other organisms have pairs of homologous chromosomes  Carry genes for the same characteristics at the same place, or locus  Except sex chromosomes  One chromosome is inherited from the female parent, one from the male

Chromosomes Centromere Sister chromatids

 Gametes have a single set of chromosomes  Diploid cells have two sets of chromosomes (2n)  Somatic cells  Haploid cells have one set of chromosomes (n)  Gametes (egg and sperm cells)  Sexual life cycles involve the alternation of haploid and diploid stages  Fusion of haploid gametes in fertilization forms a diploid zygote

 Meiosis reduces the chromosome number from diploid to haploid  Meiosis  Like mitosis, is preceded by chromosome duplication  Unlike mitosis, cell divides twice to form four haploid daughter cells

 The process of meiosis includes two consecutive divisions  Meiosis I  In synapsis, homologous chromosomes are paired  In crossing over, homologous chromosomes exchange corresponding segments  Each homologous pair divides into two daughter cells, each with one set of chromosomes consisting of two chromatids

 Meiosis II  Essentially the same as mitosis  Sister chromatids of each chromosome separate  Result is four cells, each with half as many chromosomes as the parent

I NTERPHASE P ROPHASE  M ETAPHASE  A NAPHASE  M EIOSIS  Centrosomes (with centriole pairs) Sites of crossing over Spindle Microtubules attached to kinetochore Metaphase plate Sister chromatids remain attached Homologous chromosomes separate Centromere (with kinetochore) Tetrad Sister chromatids Chromatin Nuclear envelope : Homologous chromosome separate

Cleavage furrow T ELOPHASE  P ROPHASE  M ETAPHASE  A NAPHASE  T ELOPHASE  Sister chromatids separate Haploid daughter cells forming M EIOSIS  : Sister chromatids separate  AND C YTOKINESIS

 REVIEW  Mitosis  Provides for growth, tissue repair, and asexual reproduction  Produces daughter cells genetically identical to the parent  Meiosis  Needed for sexual reproduction  Produces daughter cells with one member of each homologous chromosome pair

 Independent Assortment of chromosomes in meiosis and random fertilization lead to varied offspring  Reshuffling of the different versions of genes during sexual reproduction produces genetic variation  Random arrangements of chromosome pairs at metaphase I of meiosis lead to many different combinations of chromosomes  Random fertilization of eggs by sperm greatly increases this variation

Possibility 1 Combination 1 Combination 2 Gametes Metaphase  Two equally probable arrangements of chromosomes at metaphase  Possibility 2 Combination 3Combination 4

Each chromosome of a homologous pair can bear different versions of genes at corresponding loci  Makes gametes (and thus offspring) different from one another  Examples: coat color and eye color in mice

Coat-color genes Eye-color genes Brown Meiosis White Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes Black C E e c C C c c E e E e Pink

Brown coat (C); black eyes (E)White coat (c); pink eyes (e)

 Crossing over further increases genetic variability  Crossing over is a genetic rearrangement between two homologous chromosomes  Homologues pair up into a tetrad during prophase I of meiosis  Maternal and paternal chromatids break at the same place  The two broken chromatids join together in a new way at the chiasma

Tetrad Chiasma Centromere TEM 2,200 

 When homologous chromosomes separate at anaphase I, each contains a new segment  In meiosis II, each sister chromatid goes to a different gamete  Gametes of four genetic types result

 A karyotype is a photographic inventory of an individual's chromosomes  A blood sample is treated with a chemical that stimulates mitosis  After several days, another chemical arrests mitosis at anaphase, when chromosomes are most highly condensed  Chromosomes are photographed and electronically arranged by size and shape into the karyotype  Normal humans have 22 pairs of autosomes and two sex chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 An extra copy of chromosome 21 causes Down syndrome  A person may have an abnormal number of chromosomes  Down syndrome is caused by trisomy 21, an extra copy of chromosome 21  The most common human chromosome number abnormality  Many physical and mental problems  Increased incidence in older mothers

Infants with Down syndrome (per 1,000 births) Age of mother

 Accidents during meiosis can alter chromosome number  Abnormal chromosome count is a result of nondisjunction  The failure of homologous pairs to separate during meiosis I  The failure of sister chromatids to separate during meiosis II

Number of chromosomes Gametes Normal meiosis  Normal meiosis  Nondisjunction in meiosis  n  1 n  1

Number of chromosomes Gametes Normal meiosis  Nondisjunction in meiosis  n  1 n  1 n n

 Fertilization of an egg resulting from nondisjunction with a normal sperm results in a zygote with an abnormal chromosome number  May be involved in trisomy 21

Egg cell Sperm cell n (normal) n  1 Zygote 2n  1

 Abnormal numbers of sex chromosomes do not usually affect survival  Nondisjunction can produce gametes with extra or missing sex chromosomes  Lead to varying degrees of malfunction in humans  Usually do not affect survival

 Alterations of chromosome structure can cause birth defects and cancer  Breakage can lead to rearrangements affecting genes on one chromosome  Deletion: loss of a fragment of chromosome  Duplication: addition of a fragment to sister chromatid  Inversion: reattachment of a fragment in reverse order  Inversions least harmful because all genes are present in normal number

Deletion Duplication Inversion Homologous chromosomes