Chapter 9 The Continuity of Life: Cellular Reproduction Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. Why Do Cells Divide? Cells reproduce by cell division, in which a parent cell normally gives rise to two daughter cells Each daughter cell receives a complete set of hereditary information from the parent cell and about half its cytoplasm The hereditary information is usually identical with that of the parent cell Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. Why Do Cells Divide? Cell division transmits hereditary information to each daughter cell
deoxyribonucleic acid (DNA) - hereditary information Polymer of nucleotides Phosphate a sugar (deoxyribose) one of four bases adenine (A), thymine (T), guanine (G), cytosine (C) Chromosome - DNA in a double helix Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. The Structure of DNA phosphate nucleotide base T A sugar C
G G C C C G A A A T C G A
T T A T (a) A single strand of DNA Biology: Life on Earth, 9e (b) The double helix Fig. 9-1 Copyright 2011 Pearson Education Inc. Why Do Cells Divide? Cell division is required for growth and development Mitotic cell division - The cell division of eukaryotic cells by which organisms grow or increase in number After cell division, the daughter cells may differentiate, becoming specialized for specific functions Cell cycle - The repeating pattern of divide, grow, and
differentiate, then divide again Most multicellular organisms have three categories of cells Stem cells Other cells capable of dividing Permanently differentiated cells Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. Why Do Cells Divide? Stem cells Two important characteristics: 1. self-renewal retain the ability to divide one daughter remains a stem cell, thus continuing the line; the other daughter undergoes several divisions 2. the ability to differentiate into a variety of cell types Stem cells include most of the daughter cells formed by the first few cell divisions of a fertilized egg, as well as a few adult cells Biology: Life on Earth, 9e
Copyright 2011 Pearson Education Inc. Why Do Cells Divide? Other cells capable of dividing typically differentiate into only one or two different cell types Dividing liver cells, for example, can only become more liver cells Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. Why Do Cells Divide? Cell division is required for sexual and asexual reproduction Sexual reproduction - eukaryotic organisms occurs when offspring are produced by the fusion of gametes (sperm and eggs) from two adults Asexual reproduction - Reproduction in which offspring are formed from a single parent, without having a sperm fertilize an egg
Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. Cell Division Enables Asexual Reproduction Fig. 9-2 Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. The Prokaryotic Cell Cycle The DNA is contained in a single, circular chromosome about a millimeter or two in circumference Contained in a nucleoid Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. The Prokaryotic Cell Cycle cell division by prokaryotic
fission cell growth and DNA replication 3 New plasma membrane is added between the attachment points, pushing them farther apart. (a) The prokaryotic cell cycle attachment site cell wall circular plasma DNA membrane 1 The circular DNA double helix is attached to the plasma membrane at one point. 4 The plasma membrane grows inward at the middle of the cell. 5 The parent cell divides into two daughter cells.
2 The DNA replicates and the two DNA double helices attach to the plasma membrane at nearby but separate points. (b) Prokaryotic fission Biology: Life on Earth, 9e Fig. 9-3 Copyright 2011 Pearson Education Inc. DNA in Eukaryotic Chromosomes Differ from prokaryotic chromosomes: membrane-bound nucleus always have multiple chromosomes (2-1200) longer and have more DNA (human chromosomes are 10 to 80 times longer and have 10 to 50 times more DNA) Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. DNA in Eukaryotic Chromosomes A linear DNA double helix bound to proteins Each human
chromosome contains a single DNA double helix, about 50 million to 250 million nucleotides long Most of the time, the DNA in each chromosome is wound around proteins called histones 1 DNA double helix histone proteins 2 DNA wound around histone proteins 3 Coiled DNA/histone beads 4 Loops attached to a protein scaffold; this stage of partial condensation typically occurs in a nondividing cell protein scaffold 5 Folded chromosome, fully condensed in a dividing cell
Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. DNA in Eukaryotic Chromosomes Genes are segments of the DNA of a chromosome Units of inheritance Sequences of DNA from hundreds to thousands of nucleotides long Each gene occupies a specific place, or locus (plural, loci) on a chromosome Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. DNA in Eukaryotic Chromosomes In addition to genes, every chromosome has specialized regions that are crucial to its structure and function: 1. Two telomeres Protective caps on each end of a chromosome Essential for chromosome stability 2. One centromere
Temporarily holds two daughter DNA double helices together after DNA replication Is the attachment site for microtubules that move the chromosomes during cell division Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. The Principal Features of a Eukaryotic Chromosome During Cell Division gene loci centromere telomeres (a) A eukaryotic chromosome before DNA replication sister chromatids centromere duplicated chromosome (two DNA double helices)
(b) A eukaryotic chromosome after DNA replication independent daughter chromosomes, each with one identical DNA double helix Biology: Life on Earth, 9e Fig. 9-5 (c) Separated sister chromatids become independent Copyright 2011 Pearson Education Inc. chromosomes DNA in Eukaryotic Chromosomes Eukaryotic chromosomes usually occur in pairs with similar genetic information Karyotype - an entire set of stained chromosomes from a single cell sex chromosomes
Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. DNA in Eukaryotic Chromosomes These similarities occur because each chromosome in a pair carries the same genes arranged in the same order Chromosomes that contain the same genes are called homologous chromosomes, or homologues Cells with pairs of homologous chromosomes are called diploid, which means double 2n Egg and sperm cells dont have both pairs of chromosomes: called haploid n Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. DNA in Eukaryotic Chromosomes Homologous chromosomes are usually not identical Diploid = 23 pairs of chromosomes, for a total of 46
Autosomes - 22 pairs of chromosomes Sex chromosomes - 1 pair and are different in the male and the female Female - two X chromosomes that usually look similar Male - an X and a Y chromosome that appear very different However, in a male, the X and Y chromosomes behave as a pair during meiotic cell division Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. The Eukaryotic Cell Cycle Biology: Life on Earth, 9e o pr Three phases: 1. G1 (growth phase 1) Growth Differentiates
Decides to divide (growth factors) 2. S (synthesis phase) 3. G2 (growth phase 2) telophas e and cytokinesis ase anaph se ha e tap me as ph Interphase, a cell grows in size, replicates its DNA, and often differentiates G : cell growth and cell 1
c i ot mit vision differentiation di G2: cell growth and preparation for cell division interphase S: synthesis of DNA; chromosomes are duplicated Copyright 2011 Pearson Education Inc. The Eukaryotic Cell Cycle During interphase, a cell grows in size, replicates
its DNA, and often differentiates Permanently differentiated cells are stuck in interphase. Without enough cell divisions at the right time and in the right organs, development falters or body parts fail to replace worn-out or damaged cells With too many cell divisions, cancers may form Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. The Eukaryotic Cell Cycle There are two types of cell division in eukaryotic cells Mitotic cell division (mitosis) Meiotic cell division (meiosis) Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. The Eukaryotic Cell Cycle Mitotic cell division
During mitosis (nuclear division), the nucleus of the cell and the chromosomes divide Each daughter nucleus receives one copy of each of the replicated chromosomes of the parent cell During cytokinesis (cytoplasmic division), the cytoplasm is divided roughly equally between the two daughter cells, and one daughter nucleus enters each of the daughter cells Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. Mitotic Cell Division Mitotic cell division takes place in all types of eukaryotic organisms It is the mechanism of asexual reproduction Mitotic cell division followed by differentiation of the daughter cells allows a fertilized egg to grow into an adult with perhaps trillions of specialized cells It allows organisms to maintain, repair, and even regenerate body parts It is the mechanism whereby stem cells reproduce
Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. Mitotic Cell Division sister chromatids centromere duplicated chromosome (two DNA double helices) (b) A eukaryotic chromosome after DNA replication Four phases followed by cytokinesis Prophase Metaphase Anaphase Telophase Cytokinesis Biology: Life on Earth, 9e
Copyright 2011 Pearson Education Inc. Mitotic Cell Division Prophase Spindle microtubules form Pairs of centrioles, which serve as loci from which spindle microtubules form, begin to migrate to opposite sides of the cell, to regions called spindle poles The spindle microtubules radiate from the poles, both toward the nucleus, forming a basket around it and outward toward the plasma membrane The nuclear envelop disintegrates, releasing the duplicated chromosomes Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. Mitotic Cell Division in an Animal Cell INTERPHASE nuclear envelope
MITOSIS chromatin nucleolus condensing chromosomes spindle pole spindle microtubules kinetochore centriole pairs (a) Late Interphase Duplicated chromosomes are in the relaxed uncondensed state; duplicated centrioles remain clustered. Biology: Life on Earth, 9e
beginning of spindle formation (b) Early Prophase Chromosomes condense and shorten; spindle microtubules begin to form between separating centriole pairs. spindle pole (c) Late Prophase The nucleolus disappears; nuclear envelope breaks down; some spindle microtubules attach to the kinetochore (blue) of each sister chromatid. kinetochore microtubules
(d) Metaphase Kinetochore microtubules line up the chromosomes at the cell's equator. Copyright 2011 Pearson Education Inc. Mitotic Cell Division in an Animal Cell INTERPHASE polar microtubules chromosomes extending nuclear envelope re-forming nucleolus reappearing (e) Anaphase Sister chromatids separate & move to opposite poles
of the cell; polar microtubules push the poles apart. Biology: Life on Earth, 9e (f) Telophase One set of chromosomes reaches each pole & begins to decondense; nuclear envelopes start to form; nucleoli begin to reappear; spindle microtubules begin to disappear; microfilaments form rings around the equator. (g) Cytokinesis The ring of microfilaments contracts, dividing the cell in two; each daughter cell receives one nucleus and about half of the cytoplasm.
(h) Interphase of daughter cells Spindles disappear, intact nuclear envelopes form, and the chromosomes extend completely. Copyright 2011 Pearson Education Inc. Cytokinesis in a Plant Cell Fig. 9-10 Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. How Is the Cell Cycle Controlled? The cells of some tissues, such as skin and intestines, divide frequently throughout the lifespan of an organism Cell division occurs rarely or not at all in other tissues, such as brain, heart, and skeletal muscles Cell division in eukaryotes is driven by enzymes
and controlled at specific checkpoints Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. Why Do So Many Organisms Reproduce Sexually? Sexual reproduction is the prevalent form of reproduction Asexual reproduction by mitosis produces genetically identical offspring Sexual reproduction by meiosis shuffles the genes to produce genetically unique offspring Two parents, each with a different advantageous trait (allele), can combine those traits in one individual (their offspring) through sexual reproduction Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. Why Do So Many Organisms Reproduce Sexually? Genetic variability among organisms is essential for survival in a changing environment
Mutations produce new variation but are relatively rare occurrences The genetic variability that occurs from one generation to the next results almost entirely from meiosis and sexual reproduction Gametes from two humans could produce about 64 trillion different combinations Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. The Eukaryotic Cell Cycle Meiotic cell division occurs in animal ovaries and testes prerequisite for sexual reproduction in all eukaryotic organisms Meiotic cell division involves a specialized nuclear division called meiosis, and two rounds of cytokinesis Two divisional steps produce four daughter cells that can become haploid gametes Each gamete receives one homologue of each pair of chromosomes
Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. Meiosis Is a Reduction Division That Halves the Number of Chromosomes sister chromatids homologous chromosomes (a) Replicated homologues prior to meiosis Biology: Life on Earth, 9e (b) After meiosis I (c) After meiosis II Fig. 9-13 Copyright 2011 Pearson Education Inc.
Meiotic Cell Division Fusion of gametes keeps the chromosome number constant between generations n 2n meiotic cell division 2n 2n n fertilization diploid parental cells Biology: Life on Earth, 9e haploid gametes diploid
fertilized egg Copyright 2011 Pearson Education Inc. Meiotic Cell Division Prophase I, homologous chromosomes pair up and exchange DNA Crossing over If the exchanged segments carry different traits, genetic recombination has occurred Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc. Meiotic Cell Division in an Animal Cell MEIOSIS I chiasma Fig. 9-15a, b, c, d paired homologous chromosomes
recombined chromatids spindle microtubule a) Prophase I Duplicated chromosomes condense. Homologous chromosomes pair up and chromatids of homologues exchange parts by crossing over. The nuclear envelope disintegrates, and spindle microtubules form. Biology: Life on Earth, 9e kinetochores (b) Metaphase I Paired homologous chromosomes line up along
the equator of the cell. One homologue of each pair faces each pole of the cell and attaches to the spindle Microtubules. (c) Anaphase I Homologues separate, one member of each pair going to each pole of the cell. Sister chromatids do not separate. (d) Telophase I Spindle microtubules disappear. Two clusters of chromosomes have formed. Cytokinesis commonly occurs at this stage. There is little or no interphase between meiosis I and meiosis II. Copyright 2011 Pearson Education Inc. Meiotic Cell Division in an Animal Cell
Fig. 9-15e, f, g, h, i MEIOSIS II (e) Prophase II Spindle microtubules re-form and attach to the sister chromatids. Biology: Life on Earth, 9e (f) Metaphase II The chromosomes line up along the equator. (g) Anaphase II The chromatids separate into independent daughter chromosomes, one chromatid moving toward each pole. (h) Telophase II(i) Four haploid cells
result from Cytokinesis Nuclear envelopes re-form, and theeach containing one chromosomes member of each pair decondense. of homologous chromosomes. Copyright 2011 Pearson Education Inc. The Three Types of Eukaryotic Life Cycles mitotic cell division and growth or asexual reproduction n n mitotic cell division, differentiation, and growth multicellular diploid 2n adult n
n multicellular diploid adults 2n meiotic cell division 2n n zygote fusion of gametes n gametes (a) Haploid life cycle (protists, algae, fungi) haploid (n) stages zygote fusion of gametes
n n mitotic cell division, differentiation, and growth n 2n meiotic cell division 2n meiotic cell division n gametes
(b) Diploid life cycle (animals) mitotic cell division, differentiation, and growth multicellular haploid n adult 2n zygote fusion of gametes spore n n gametes (c) Alternation of generations (plants)
Fig. 9-17 diploid (2n) stages Biology: Life on Earth, 9e Copyright 2011 Pearson Education Inc.
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