The Cellular Basis of Reproduction and Inheritance Chapter 8 The Cellular Basis of Reproduction and Inheritance

Rain Forest Rescue Scientists in Hawaii Have attempted to “rescue” endangered species from extinction

The goals of these scientists were To promote reproduction to produce more individuals of specific endangered plants Cyanea kuhihewa Kauai, Hawaii

In sexual reproduction Fertilization of sperm and egg produces offspring In asexual reproduction Offspring are produced by a single parent, without the participation of sperm and egg

CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION 8.1 Like begets like, more or less Some organisms reproduce asexually And their offspring are genetic copies of the parent and of each other LM 340 Figure 8.1A

Other organisms reproduce sexually Creating a variety of offspring Figure 8.1B

8.2 Cells arise only from preexisting cells Cell division is at the heart of the reproduction of cells and organisms Because cells come only from preexisting cells

8.3 Prokaryotes reproduce by binary fission Prokaryotic cells Reproduce asexually by cell division Colorized TEM 32,500 Prokaryotic chromosomes Figure 8.3B

Division into two daughter cells As the cell replicates its single chromosome, the copies move apart And the growing membrane then divides the cells Plasma membrane Prokaryotic chromosome Cell wall Duplication of chromosome and separation of copies 1 Continued elongation of the cell and movement of copies 2 Division into two daughter cells 3 Figure 8.3A

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 And they are grouped into multiple chromosomes in the nucleus

And are visible only when the cell is in the process of dividing Individual chromosomes contain a very long DNA molecule associated with proteins And are visible only when the cell is in the process of dividing If a cell is not undergoing division Chromosomes occur in the form of thin, loosely packed chromatin fibers LM 600 Figure 8.4A

Before a cell starts dividing, the chromosomes replicate Producing sister chromatids joined together at the centromere TEM 36,000 Centromere Sister chromatids Figure 8.4B

Chromosome distribution to daughter cells Cell division involves the separation of sister chromatids And results in two daughter cells, each containing a complete and identical set of chromosomes Centromere Chromosome duplication Sister chromatids Chromosome distribution to daughter cells Figure 8.4C

8.5 The cell cycle multiplies cells The cell cycle consists of two major phases INTERPHASE S (DNA synthesis) G1 G2 Cytokinesis Mitosis MITOTIC PHASE (M) Figure 8.5

During interphase Chromosomes duplicate and cell parts are made During the mitotic phase Duplicated chromosomes are evenly distributed into two daughter nuclei

8.6 Cell division is a continuum of dynamic changes In mitosis, after the chromosomes coil up A mitotic spindle moves them to the middle of the cell

The sister chromatids then separate And move to opposite poles of the cell, where two nuclei form Cytokinesis, in which the cell divides in two Overlaps the end of mitosis

The stages of cell division INTERPHASE PROPHASE PROMETAPHASE LM 250 Chromatin Centrosomes (with centriole pairs) Nucleolus Nuclear envelope Plasma membrane Early mitotic spindle Centrosome Centromere Chromosome, consisting ot two sister chromatids Spindle microtubules Kinetochore Fragments of nuclear envelope Figure 8.6 (Part 1)

TELOPHASE AND CYTOKINESIS METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Spindle Metaphase plate Daughter chromosomes Nuclear envelope forming Cleavage furrow Nucleolus forming Figure 8.6 (Part 2)

8.7 Cytokinesis differs for plant and animal cells In animals Cytokinesis occurs by a constriction of the cell (cleavage) Cleavage furrow SEM 140 Daughter cells Contracting ring of microfilaments Figure 8.7A

A membranous cell plate splits the cell in two In plants A membranous cell plate splits the cell in two TEM 7,500 Cell plate forming Wall of parent cell Daughter nucleus Cell wall New cell wall Vesicles containing cell wall material Cell plate Daughter cells Figure 8.7B

8.8 Anchorage, cell density, and chemical growth factors affect cell division Most animal cells divide Only when stimulated, and some not at all

In laboratory cultures Most normal cells divide only when attached to a surface They continue dividing Until they touch one another 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

Are proteins secreted by cells that stimulate other cells to divide 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 A set of proteins within the cell Controls the cell cycle

Signals affecting critical checkpoints in the cell cycle Determine whether a cell will go through the complete cycle and divide Control system G1 S G2 M G1 checkpoint G2 checkpoint M checkpoint G0 Figure 8.9A

Is usually necessary for cell division. The binding of growth factors to specific receptors on the plasma membrane Is usually necessary for cell division. Control system G1 S G2 M G1 checkpoint Plasma membrane Growth factor Receptor protein Relay proteins Signal transduction pathway Figure 8.9B

8.10 Growing out of control, cancer cells produces malignant tumors CONNECTION 8.10 Growing out of control, cancer cells produces malignant tumors Cancer cells divide excessively to form masses called tumors

Can invade other tissues Malignant tumors Can invade other tissues Tumor Glandular tissue 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. Lymph vessels Blood vessel Figure 8.10

Radiation and chemotherapy Are effective as cancer treatments because they interfere with cell division

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 LM 500 Figure 8.11A

Replacement of damaged or lost cells LM 700 Figure 8.11B

Asexual reproduction LM 10 Figure 8.11C

MEIOSIS AND CROSSING OVER 8.12 Chromosomes are matched in homologous pairs The somatic (body) cells of each species Contain a specific number of chromosomes For example human cells have 46 Making up 23 pairs of homologous chromosomes

The chromosomes of a homologous pair Carry genes for the same characteristics at the same place, or locus 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, eggs and sperm, are haploid With a single set of chromosomes

Involve the alternation of haploid and diploid stages Sexual life cycles Involve the alternation of haploid and diploid stages Mitosis and development Multicellular diploid adults (2n = 46) Diploid zygote (2n = 46) 2n Meiosis Fertilization Egg cell Sperm cell n Haploid gametes (n = 23) Figure 8.13

8.14 Meiosis reduces the chromosome number from diploid to haploid Meiosis, like mitosis Is preceded by chromosome duplication But in meiosis The cell divides twice to form four daughter cells

The first division, meiosis I Starts with synapsis, the pairing of homologous chromosomes In crossing over Homologous chromosomes exchange corresponding segments Meiosis I separates each homologous pair And produce two daughter cells, each with one set of chromosomes

Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is a total of four haploid cells

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

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

8.15 Review: A comparison of mitosis and meiosis Parent cell (before chromosome replication) Chromosome replication 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 No further chromosomal replication; sister chromatids separate during anaphase II Prophase Metaphase Anaphase Telophase Duplicated chromosome (two sister chromatids) Daughter cells of mitosis 2n Daughter cells of meiosis I n 2n = 4 Tetrad formed by synapsis of homologous chromosomes Meiosis i Meiosis ii Prophase I Metaphase I Anaphase I Telophase I Haploid n = 2 Daughter cells of meiosis II Figure 8.15

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

Two equally probable arrangements of chromosomes at metaphase I Random arrangements of chromosome pairs at metaphase I of meiosis Lead to many different combinations of chromosomes in eggs and sperm Combination 1 Combination 2 Combination 3 Combination 4 Gametes Metaphase II Two equally probable arrangements of chromosomes at metaphase I Possibility 1 Possibility 2 Figure 8.16

Random fertilization of eggs by sperm Greatly increases this variation

8.17 Homologous chromosomes carry different versions of genes The differences between homologous chromosomes Are based on the fact that they can bear different versions of a gene at corresponding loci Tetrad in parent cell (homologous pair of duplicated chromosomes) e c E C White Pink Meiosis Black Brown Chromosomes of the four gametes Eye-color genes Coat-color genes Brown coat (C); black eyes (E) White coat (C); pink eyes (e) Figure 8.17B Figure 8.17A

8.18 Crossing over further increases genetic variability Genetic recombination Which results from crossing over during prophase I of meiosis, increases variation still further Chiasma Tetrad Centromere TEM 2,200 Figure 8.18A

Tetrad (homologous pair of chromosomes in synapsis) How crossing over leads to genetic variation Coat-color genes Eye-color genes C E Tetrad (homologous pair of chromosomes in synapsis) c e Breakage of homologous chromatids 1 C E c e Joining of homologous chromatids 2 C E Chiasma c e 3 Separation of homologous chromosomes at anaphase I C E C e c E c e 4 Separation of chromatids at anaphase II and completion of meiosis C E Parental type of chromosome C e Recombinant chromosome c E Recombinant chromosome c e Parental type of chromosome Figure 8.18B Gametes of four genetic types

ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE 8.19 A karyotype is a photographic inventory of an individual’s chromosomes A karyotype Is an ordered arrangement of a cell’s chromosomes

Preparation of a karyotype from a blood sample Blood culture Fluid Centrifuge Packed red and white blood cells Hypotonic solution Fixative White blood cells Stain Centromere Pair of homologous chromosomes Sister chromosomes 2,600X A blood culture is centrifuged to separate the blood cells from the culture fluid. 1 The fluid is discarded, and a hypotonic solution is mixed with the cells. This makes the red blood cells burst. The white blood cells swell but do not burst, and their chromosomes spread out. 2 Another centrifugation step separates the swollen white blood cells. The fluid containing the remnants of the red blood cells is poured off. A fixative (preservative) is mixed with the white blood cells. A drop of the cell suspension is spread on a microscope slide, dried, and stained. 3 The slide is viewed with a microscope equipped with a digital camera. A photograph of the chromosomes is entered into a computer, which electronically arranges them by size and shape. 4 The resulting display is the karyotype. The 46 chromosomes here include 22 pair of autosomes and 2 sex chromosomes, X and Y. Although difficult to discern in the karyotype, each of the chromosomes consists of two sister chromatids lying very close together (see diagram). 5 Figure 8.19

8.20 An extra copy of chromosome 21 causes Down syndrome CONNECTION 8.20 An extra copy of chromosome 21 causes Down syndrome A person may have an abnormal number of chromosomes Which causes problems

Down syndrome is caused by trisomy 21 An extra copy of chromosome 21 5,000 Figure 8.20A Figure 8.20B

Infants with Down syndrome (per 1,000 births) The chance of having a Down syndrome child Goes up with maternal age Age of mother 45 50 35 30 25 40 20 90 10 60 70 80 Infants with Down syndrome (per 1,000 births) Figure 8.20C

8.21 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

Nondisjunction in meiosis I Normal meiosis II Gametes n + 1 n 1 Number of chromosomes Nondisjunction in meiosis II Normal meiosis I n -1 n Number of chromosomes Figure 8.21B Figure 8.21A

Fertilization after nondisjunction in the mother Sperm cell Egg cell n (normal) n + 1 Zygote 2n + 1 Figure 8.21C

CONNECTION 8.22 Abnormal numbers of sex chromosomes do not usually affect survival Nondisjunction can also produce gametes with extra or missing sex chromosomes Leading to varying degrees of malfunction in humans but not usually affecting survival Figure 8.22A Poor beard growth Breast Development Under-developed testes Figure 8.22B Characteristic facial features Web of skin Constriction of aorta Poor breast development Under developed ovaries

Human sex chromosome abnormalities

CONNECTION 8.23 Alterations of chromosome structure can cause birth defects and cancer Chromosome breakage can lead to rearrangements That can produce genetic disorders or, if the changes occur in somatic cells, cancer

Deletions, duplications, inversions, and translocations Reciprocal translocation Nonhomologous chromosomes Deletion Duplication Inversion Homologous chromosomes Figure 8.23B “Philadelphia chromosome” Chromosome 9 Chromosome 22 Reciprocal translocation Activated cancer-causing gene Figure 8.23C Figure 8.23A

5,000