CHAPTER 8 The Cellular Basis of Reproduction and Inheritance Modules 8.1 – 8.3

How to Make a Sea Star — With and Without Sex The life cycle of a multicellular organism includes development reproduction This sea star embryo (morula) shows one stage in the development of a fertilized egg The cluster of cells will continue to divide as development proceeds

Some organisms can also reproduce asexually This sea star is regenerating a lost arm Regeneration results from repeated cell divisions

CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION Cell division is at the heart of the reproduction of cells and organisms Organisms can reproduce sexually or asexually

8.1 Like begets like, more or less Some organisms make exact copies of themselves, asexual reproduction Figure 8.1A

Other organisms make similar copies of themselves in a more complex process, sexual reproduction Figure 8.1B

8.2 Cells arise only from preexisting cells All cells come from cells Cellular reproduction is called cell division Cell division allows an embryo to develop into an adult It also ensures the continuity of life from one generation to the next

8.3 Prokaryotes reproduce by binary fission Prokaryotic cells divide asexually These cells possess a single chromosome, containing genes The chromosome is replicated The cell then divides into two cells, a process called binary fission Prokaryotic chromosomes Figure 8.3B

Binary fission of a prokaryotic cell Plasma membrane Prokaryotic chromosome Cell wall Duplication of chromosome and separation of copies Continued growth of the cell and movement of copies Division into two cells Figure 8.3A

CHAPTER 8 The Cellular Basis of Reproduction and Inheritance Modules 8.4 – 8.11

THE EUKARYOTIC CELL CYCLE AND MITOSIS 8.4 The large, complex chromosomes of eukaryotes duplicate with each cell division A eukaryotic cell has many more genes than a prokaryotic cell The genes are grouped into multiple chromosomes, found in the nucleus The chromosomes of this plant cell are stained dark purple Figure 8.4A

Chromosomes contain a very long DNA molecule with thousands of genes Individual chromosomes are only visible during cell division They are packaged as chromatin

Before a cell starts dividing, the chromosomes are duplicated Sister chromatids This process produces sister chromatids Centromere Figure 8.4B

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

8.5 The cell cycle multiplies cells The cell cycle consists of two major phases: Interphase, where chromosomes duplicate and cell parts are made The mitotic phase, when cell division occurs Figure 8.5

8.6 Cell division is a continuum of dynamic changes Eukaryotic cell division consists of two stages: Mitosis Cytokinesis

In mitosis, the duplicated chromosomes are distributed into two daughter nuclei After the chromosomes coil up, a mitotic spindle moves them to the middle of the cell

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

The sister chromatids then separate and move to opposite poles of the cell The process of cytokinesis divides the cell into two genetically identical cells

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

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

In plants, a membranous cell plate splits the cell in two Cell plate forming Wall of parent cell Daughter nucleus In plants, a membranous cell plate splits the cell in two Cell wall New cell wall Vesicles containing cell wall material Cell plate Daughter cells Figure 8.7B

Most animal cells divide only when stimulated, and others not at all 8.8 Anchorage, cell density, and chemical growth factors affect cell division Most animal cells divide only when stimulated, and others not at all In laboratory cultures, most normal cells divide only when attached to a surface They are anchorage dependent

Cells continue dividing until they touch one another This is called density-dependent inhibition 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). Figure 8.8A

Growth factors are proteins secreted by cells that stimulate other cells to divide After forming a single layer, cells have stopped dividing. Providing an additional supply of growth factors stimulates further cell division. Figure 8.8B

8.9 Growth factors signal the cell cycle control system Proteins within the cell control the cell cycle Signals affecting critical checkpoints determine whether the cell will go through a complete cycle and divide G1 checkpoint Control system M checkpoint G2 checkpoint Figure 8.9A

Cell cycle control system The binding of growth factors to specific receptors on the plasma membrane is usually necessary for cell division Growth factor Plasma membrane Relay proteins Receptor protein G1 checkpoint Signal transduction pathway Cell cycle control system Figure 8.8B

Cancer cells have abnormal cell cycles 8.10 Connection: Growing out of control, cancer cells produce malignant tumors Cancer cells have abnormal cell cycles They divide excessively and can form abnormal masses called tumors Radiation and chemotherapy are effective as cancer treatments because they interfere with cell division

Malignant tumors can invade other tissues and may kill the organism Lymph vessels Tumor Glandular tissue Metastasis 1 A tumor grows from a single cancer cell. 2 Cancer cells invade neighboring tissue. 3 Cancer cells spread through lymph and blood vessels to other parts of the body. Figure 8.10

8.11 Review of the functions of mitosis: Growth, cell replacement, and asexual reproduction When the cell cycle operates normally, mitotic cell division functions in: Growth (seen here in an onion root) Figure 8.11A

Cell replacement (seen here in skin) Dead cells Epidermis, the outer layer of the skin Dividing cells Dermis Figure 8.11B

Asexual reproduction (seen here in a hydra) Figure 8.11C

CHAPTER 8 The Cellular Basis of Reproduction and Inheritance Modules 8.12 – 8.18

8.12 Chromosomes are matched in homologous pairs MEIOSIS AND CROSSING OVER 8.12 Chromosomes are matched in homologous pairs Somatic cells of each species contain a specific number of chromosomes Human cells have 46, making up 23 pairs of homologous chromosomes Chromosomes Centromere Sister chromatids Figure 8.12

8.13 Gametes have a single set of chromosomes Cells with two sets of chromosomes are said to be diploid Gametes are haploid, with only one set of chromosomes

At fertilization, a sperm fuses with an egg, forming a diploid zygote Repeated mitotic divisions lead to the development of a mature adult The adult makes haploid gametes by meiosis All of these processes make up the sexual life cycle of organisms

Multicellular diploid adults (2n = 46) Mitosis and development The human life cycle Haploid gametes (n = 23) Egg cell Sperm cell MEIOSIS FERTILIZATION Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46) Mitosis and development Figure 8.13

8.14 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

In the first division, meiosis I, homologous chromosomes are paired While they are paired, they cross over and exchange genetic information The homologous pairs are then separated, and two daughter cells are produced

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 is essentially the same as mitosis The sister chromatids of each chromosome separate The result is four haploid daughter cells

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

8.15 Review: A comparison of mitosis and meiosis For both processes, chromosomes replicate only once, during interphase

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

Each chromosome of a homologous pair comes from a different parent 8.16 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Each chromosome of a homologous pair comes from a different parent Each chromosome thus differs at many points from the other member of the pair

The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes Random fertilization also increases variation in offspring

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

8.17 Homologous chromosomes carry different versions of genes The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci

C E C E C E c e c e c e Coat-color genes Eye-color genes Brown Black White Pink Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes Figure 8.17A, B

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

Tetrad Chaisma Centromere Figure 8.18A

How crossing over leads to genetic recombination Coat-color genes Eye-color genes How crossing over leads to genetic recombination 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

CHAPTER 8 The Cellular Basis of Reproduction and Inheritance Modules 8.19 – 8.23

ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE 8.19 A karyotype is a photographic inventory of an individual’s chromosomes To study human chromosomes microscopically, researchers stain and display them as a karyotype A karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes

Preparation of a karyotype Fixative Packed red And white blood cells Hypotonic solution Blood culture Stain White Blood cells Centrifuge 3 2 1 Fluid Centromere Sister chromatids Pair of homologous chromosomes 4 5 Figure 8.19

8.20 Connection: An extra copy of chromosome 21 causes Down syndrome This karyotype shows three number 21 chromosomes An extra copy of chromosome 21 causes Down syndrome Figure 8.20A, B

The chance of having a Down syndrome child goes up with maternal age Figure 8.20C

8.21 Accidents during meiosis can alter chromosome number Abnormal chromosome count is a result of nondisjunction Either homologous pairs fail to separate during meiosis I Nondisjunction in meiosis I Normal meiosis II Gametes n + 1 n + 1 n – 1 n – 1 Number of chromosomes Figure 8.21A

Or sister chromatids fail to separate during meiosis II Normal meiosis I Nondisjunction in meiosis II Gametes n + 1 n – 1 n n Number of chromosomes Figure 8.21B

Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Egg cell n + 1 Zygote 2n + 1 Sperm cell n (normal) Figure 8.21C

8.22 Connection: Abnormal numbers of sex chromosomes do not usually affect survival Nondisjunction can also produce gametes with extra or missing sex chromosomes Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes

Table 8.22

A man with Klinefelter syndrome has an extra X chromosome Poor beard growth Breast development Under- developed testes Figure 8.22A

A woman with Turner syndrome lacks an X chromosome Characteristic facial features Web of skin Constriction of aorta Poor breast development Under- developed ovaries Figure 8.22B

8.23 Connection: Alterations of chromosome structure can cause birth defects and cancer Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer Four types of rearrangement are deletion, duplication, inversion, and translocation

Homologous chromosomes Deletion Duplication Homologous chromosomes Inversion Reciprocal translocation Nonhomologous chromosomes Figure 8.23A, B

Chromosomal changes in a somatic cell can cause cancer A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia Chromosome 9 Reciprocal translocation Chromosome 22 “Philadelphia chromosome” Activated cancer-causing gene Figure 8.23C

CHAPTER 8 Extra Photographs

Sea urchin development Figure 8.0x

E. coli dividing Figure 8.3x

Cell cycle collage Figure 8.5x

Mitosis collage, light micrographs Figure 8.6x1

Mitotic spindle Figure 8.6x2

Fibroblast growth Figure 8.8x

Breast cancer cell Figure 8.10x1

Mammograms Figure 8.10x2

Human female bands Figure 8.19x1

Human female karyotype Figure 8.19x2

Human male bands Figure 8.19x3

Human male karyotype Figure 8.19x4

Down syndrome karyotype Figure 8.20Ax

Klinefelter’s karyotype Figure 8.22Ax

XYY karyotype Figure 8.22x

Translocation Figure 8.23Bx