Chapter 11 The Continuity of Life: Cellular Reproduction Chapter 11 Outline • 1 Cellular Reproduction in the Lives of Individual Cells and Entire Organisms • 2 DNA Organization in Eukaryotes • 3 Mitosis • 4 Control of Cell Cycle • 5 Sexual Reproduction • 6 Meiosis • 7 Eukaryotic Life Cycle • 8 Genetic Variability 1 What Is the Role of Cellular Reproduction in the Lives of Individual Cells and Entire Organisms? – The Cell Cycle and Cellular Reproduction – The Prokaryotic Cell Cycle Consists of Growth and Binary Fission – The Eukaryotic Cell Cycle Consists of Interphase and Cell Division – Eukaryotic Cells Grow and Replicate DNA in Interphase – The Process of Mitosis: Asexual Reproduction – The Process of Meiosis: Prerequisite for Sexual Reproduction Cellular Reproduction • Intracellular activity between one cell division to the next is the cell cycle – Some activities involve growth (enlargement) of the cell – Some activities involve duplication of genetic material and cellular division (reproduction) cell division cell growth and DNA replication Cellular Reproduction • Reproduction from a single parent is asexual reproduction – Some organisms reproduce asexually Cellular Reproduction • Multicellular organisms grow by asexual reproduction; some reproduce The Prokaryotic Cell Cycle Cell cycle in prokaryotes 1. Long growth phase • • Replication of circular DNA chromosome Duplicate chromosomes anchored to membrane 2. Cell increases in size, pulling duplicated chromosomes apart… The Prokaryotic Cell Cycle Cell cycle in prokaryotes 3. Plasma membrane grows inward between chromosome copies 4. Fusion of membrane along cell equator completes separation (binary fission or “splitting in two”)… The Prokaryotic Cell Cycle Cell cycle in prokaryotes 5. Daughter cells are genetically identical – Under ideal conditions Escherichia coli bacteria complete a cell cycle every 20 minutes The Eukaryotic Cell Cycle • Progression through cell cycle in multicellular organisms is variable – Cells may exit the cell cycle and never divide again – Cells may enter or continue through the cell cycle and divide in response to growth hormones The Eukaryotic Cell Cycle Eukaryotic cell cycle divided into two phases • Interphase. Eukaryotic cells spend most time in interphase - • Acquisition of nutrients, growth, chromosome duplication Cell division - One copy of every chromosome and half of cytoplasm and organelles parceled out into two daughter cells The Eukaryotic Cell Cycle Interphase is divided into three phases – G1 (growth phase 1) – Acquisition of nutrients and growth to proper size – S (synthesis) phase – DNA synthesis occurs, replicating every chromosome – G2 (growth phase 2) – Completion of growth and readying for division The Eukaryotic Cell Cycle Decision to proceed or exit the cell cycle in G1 – – Internal and external signals in G1 stimulate cells to proceed through cycle and divide Cells may exit cycle to nondividing G0 phase – Cells remain alive and metabolically active in G0 – Specialization (differentiation) occurs » Unique features of cell type develop Mitosis and Meiosis • There are two types of cell division in eukaryotes – Mitotic cell division (mitosis) – Meiotic cell division (meiosis) Mitosis and Meiosis • Mitosis is the mechanism of asexual reproduction in eukaryotic cells – Used in the reproduction of unicellular organisms – Used in growth of fertilized egg into adult – Used in cloning and stem cell research • Mitotic cell division involves two steps – Nuclear division – Cytokinesis (cytoplasmic separation) mitosis, differentiation, and growth embryo mitosis, baby differentiation, and growth adu Mitosis and Meiosis • Meiotic division occurs in animal ovaries and testes – Two divisional steps produce four daughter cells that can become gametes – Daughter cells are genetically different from parent cell and each other – Daughter cells have half the genetic material of the parent cell meiosis in ovaries egg fertilized egg sperm fertilization meiosis in testes The Eukaryotic Chromosome • • DNA must be condensed (coiled) to fit into nucleus for easy manipulation in cell division Each chromosome consists of a DNA double helix wound around spool proteins The Eukaryotic Chromosome • A chromosome contains hundreds of DNA sequences called genes found at specific locations (loci) • Each chromosome contains – A central centromere – Telomeres The Eukaryotic Chromosome • Centromere (“middle body”) is region where chromosome can attach to a sister chromatid – Two sister chromatids bound at their centromeres comprise a duplicated chromosome – Sister chromatids separate at their centromeres during mitosis The Eukaryotic Chromosome • Telomeres (“end bodies”) are the two ends of a chromosome – Essential in maintaining chromosome stability Homologous Pairs of Chromosomes • Duplicated chromosomes are tightly coiled “X” shapes Homologous Pairs of Chromosomes Chart showing entire set of stained chromosomes (karyotype) shows pairs – – Every chromosome in a non-reproductive cell has a “partner” or homologous chromosome Homologues contain the same kinds of genes and have the same size, shape, and banding pattern Homologous Pairs of Chromosomes Human cells have 23 homologous pairs of chromosomes – Chromosome pairs 1-22 are autosomes with similar appearance between homologues Chromosome pair 23 are sex chromosomes which may have similar or different appearances – – Females have two X chromosomes of similar appearance Males have an X and a Y chromosome (the Y is much smaller) Homologous Pairs of Chromosomes • • A karyotype showing two chromosomes for each pair comes from a diploid (meaning “double”) cell Cells with only one chromosome “per pair” are haploid (containing half the diploid number) – Meiosis (in sexual reproduction) produces haploid cells from one diploid cell Homologous Pairs of Chromosomes • Diploid and haploid numbers – Number of haploid chromosomes in a cell designated “n” – Number of diploid chromosomes in a cell designated “2n” Mitosis Consists of Four Phases • Cells prepare for mitotic division during interphase – Chromosomes are replicated in S phase – Necessary proteins are synthesized in G1 and G2 Mitosis Consists of Four Phases • Four phases of mitosis – Prophase – Metaphase – Anaphase – Telophase Mitosis • Prophase – Nuclear Envelope Breaks. – Chromosomes condense – Nucleolus dissappears – Spindle apparatus assembles – Microtubules connect kinetochores on each pair of sister chromatids to the spindle poles. pole kinetoc pole Mitosis • Metaphase – Chromosomes align in cell’s center (equator). • Metaphase plate. Mitosis • Anaphase "free" spindle fibers – Spindle microtubules shorten and pull daughter chromosomes (formerly sister chromatids) towards each spindle pole – Pole-pole microtubules push cell poles apart. ANAPHASE Mitosis • Telophase chromosomes extending nuclear envelop re-forming – Spindle disassembles. – Nuclear envelope forms around each set of daughter chromosomes. – Nucleoli reappear TELOPHASE One set of chromosomes reaches each pole and relaxes into extended state; nuclear envelopes start to form around each set; spindle microtubules begin to disappear. Cytokinesis • Cleavage of cell into two halves. • Animal Cells: – Actin filaments form a “belt” around the cell’s equator. – The belt contracts, pinching in the cell’s “waist”, forming two daughter cells. INTERPHASE OF DAUGHTER CELLS CYTOKINESIS Cell divides in two; each daughter cell receives one nucleus and about half of the cytoplasm. Spindles disappear, intact nuclear envelopes form, chromosomes extend completely, and the nucleolus reappears. INTERPHASE nuclear envelope MITOSIS chromatin nucleolus centriole pairs LATE INTERPHASE Duplicated chromosomes in relaxed state; duplicated centrioles remain clustered. condensing chromosomes pole beginning of spindle formation pole EARLY PROPHASE Chromosomes condense and shorten; spindle microtubules begin to form between separating centriole pairs. spindle microtubules kinetochore LATE PROPHASE Nucleolus disappears; nuclear envelope breaks down; spindle microtubules attach to the kinetochore of each sister chromatid. METAPHASE Kinetochores interact; spindle microtubules line up chromosomes at cell's equator. INTERPHASE "free" spindle fibers chromosomes extending ANAPHASE Sister chromatids separate and move to opposite poles of the cell; spindle microtubules push poles apart. nuclear envelope re-forming TELOPHASE One set of chromosomes reaches each pole and relaxes into extended state; nuclear envelopes start to form around each set; spindle microtubules begin to disappear. CYTOKINESIS Cell divides in two; each daughter cell receives one nucleus and about half of the cytoplasm. INTERPHASE OF DAUGHTER CELLS Spindles disappear, intact nuclear envelopes form, chromosomes extend completely, and the nucleolus reappears. Cytokinesis • Plant Cells: Cell plate Golgi complex cell wall plasma membrane carbohydratefilled vesicles Carbohydratefilled vesicles bud off the Golgi and move to the equator of the cell. Vesicles fuse to form a new cell wall and plasma membrane between daughter cells. Complete separation of daughter cells. Mitosis in Onion Roots • Interphase early Prophase late Prophase • Metaphase Anaphase late Anaphase • Telophase Daughter cells Control of Cell Cycle • The cells of some tissues divide frequently throughout lifespan • e.g. skin, intestine • Cell division occurs rarely or not at all in other tissues • e.g. brain, heart, skeletal muscles • Cell division in eukaryotes is driven by enzymes and controlled at specific checkpoints Enzymes Drive the Cell Cycle – The cell cycle is driven by proteins called Cyclindependent kinases, or Cdk’s – Kinases are enzymes that phosphorylate (add a phosphate group to) other proteins, stimulating or inhibiting their activity – Cdk’s are active only when they bind to other proteins called cyclins Enzymes Drive the Cell Cycle • Cell division occurs when growth factors bind to cell surface receptors, which leads to cyclin synthesis • Cyclins then bind to and activate specific Cdk’s Enzymes Drive the Cell Cycle • Activated Cdk’s promote a variety of cell cycle events – Synthesis and activation of proteins required for DNA synthesis – Chromosome condensation – Nuclear membrane breakdown – Spindle formation – Attachment of chromosomes to spindle – Sister chromatid separation and movement Checkpoints Control Cell Cycle • Although Cdk’s drive the cell cycle, multiple checkpoints ensure that – The cell successfully completes DNA synthesis during interphase – Proper chromosome movements occur during mitotic cell division Checkpoints Control Cell Cycle • There are three major checkpoints in the eukaryotic cell cycle, each regulated by protein complexes – G1 to S: – G2 to mitosis – Metaphase to anaphase Checkpoints Control Cell Cycle • G1 to S: Ensures that the cell’s DNA is suitable for replication – p53 protein expressed when DNA is damaged • Inhibits replication • Stimulates synthesis of DNA repair enzymes • Triggers cell death (apoptosis) if damage can’t be repaired Checkpoints Control Cell Cycle • G2 to mitosis: Ensures that DNA has been completely and accurately replicated – p53 protein expression leads to decrease in synthesis and activity of an enzyme that facilitates chromosome condensation – chromosomes remain extended and accessible to DNA repair enzymes, which fix DNA before cell enters mitosis Checkpoints Control Cell Cycle • Metaphase to anaphase: Ensures that the chromosomes are aligned properly at the metaphase plate – a variety of proteins prevent separation of the sister chromatids if there are defects in chromosome alignment or spindle function Why do So Many Organisms Reproduce Sexually? • Meiosis – Sexual reproduction involves production of haploid gametes through meiosis. • Fertilization – A gene might have alternate forms (alleles). Sexual reproduction allows new gene combinations. meiosis in ovaries meiosis in testes egg fertilized egg Copyright © McGraw-Hill Companies Permission required for reproduction or display sperm adults gene 1 same alleles gene 2 different alleles How Does Meiotic Cell Division Produce Haploid Cells? • There are two cell divisions during Meiosis; Meiosis I and II. • Meiosis I Separates Homologous Chromosomes into Two Haploid Daughter Nuclei • The total number of cells produced is 4 Meiosis I Meiosis II n 2n meiotic cell division 2n 2n n fertilization Meiotic Cell Division Followed by Fusion of Gametes Keeps the Chromosome Number Constant from Generation to Generation Unique Features of Meiosis (3) • Synapsis – Homologues pair along their length. • Homologous Recombination – Crossing over occurs between homologous chromosomes. • Reduction Division – 2 Cell divisions: Chromosomes do not replicate between Meiosis I and II. MEIOSIS I Prophase I • Duplicated chromosomes condense. • Homologous chromosomes pair up and chiasmata occur as chromatids of homologues exchange parts. • The nuclear envelope disintegrates, and spindle chiasma microtubules form paired homologous chromosomes spindle microtubule Crossing Over • Presence of chiasmata [chiasma] indicates crossing over has occurred. Metaphase I • Terminal chiasmata holds homologous pair together. • Spindle microtubules attach to kinetochore proteins only on the outside of each centromere. • Metaphase plate: Each joined pair of homologues lines up. – Orientation of each pair is random • Mendel’s “independent assortment” is explained Completing Meiosis • Anaphase I – Spindle fibers begin to shorten and pull whole centromeres towards poles. • Each pole receives a member of each homologous pair. Completing Meiosis • Telophase I – Chromosomes segregated into two clusters at opposite ends of cell • Nuclear membrane re-forms. – Sister chromatids are no longer identical. MEIOSIS I paired homologous chromosomes recombined chromosomes spindle microtubule chiasma Prophase I. Duplicated chromosomes condense. Homologous chromosomes pair up and chiasmata occur as chromatids of homologues exchange parts. The nuclear envelope disintegrates, and spindle microtubules form. 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 spindle microtubules via its kinetochore (blue). Anaphase I. Homologues separate, one member of each pair going to each pole of the cell. Sister chromatids do not separate. Telophase I. Spindle microtubules disappear. Two clusters of chromosomes have formed, each containing one member of each pair of homologues. The daughter nuclei are therefore haploid. Cytokinesis commonly occurs at this stage. There is little or no interphase between meiosis I and meiosis II. Second Meiotic Division • Meiosis II resembles normal mitotic division but with few chromosomes – Prophase II - Nuclear envelope breaks down. – Metaphase II - Spindle fibers bind to both sides of centromere – Anaphase II - Spindle fibers contract and sister chromatids move to opposite poles – Telophase II - Nuclear envelope reforms; chromosomes relax. Second Meiotic Division Second Meiotic Division – Cytokinesis results in: • Four non-identical haploid cells MEIOSIS II Prophase II. If chromosomes have relaxed after telophase I, they recondense. Spindle microtubules re-form and attach to the sister chromatids. Metaphase II. Chromosomes line up along the equator, with sister chromatids of each chromosome attached to spindle microtubules that lead to opposite poles. Anaphase II. Chromatids separate into independent daughter chromosomes, one former chromatid moving toward each pole. Telophase II. Chromosomes finish moving to opposite poles. Nuclear envelopes re-form, and the chromosomes become extended again (not shown here). Four haploid cells. Cytokinesis results in four haploid cells, each containing one member of each pair of homologous chromosomes (shown here in condensed state). The mechanism of crossing over sister chromatids of one duplicated homologue protein strands joining duplicated chromosomes direction of “zipper” formation pair of homologous, duplicated chromosomes Duplicated homologous chromosomes pair up side by side. Protein strands “zip” the homologous chromosomes together. recombinatio n enzymes Recombination enzymes bind to the joined chromosomes. chiasm a Recombination enzymes snip chromatids apart and reattach the free ends. Chiasmata (the sites of crossing over) form when one end of the paternal chromatid (yellow) attaches to the other end of a maternal chromatid (purple). chiasma Recombination enzymes and protein zippers leave. Chiasmata remain, helping to hold homologous chromosomes together. duplicated chromosomes spindle microtubules Sexual Reproduction Produces Genetic Variability in Three Ways : • Shuffling of Homologues Creates Novel Combinations of Chromosomes • Crossing Over Creates Chromosomes with Novel Combinations of Genes • Fusion of Gametes Adds Further Genetic Variability to the Offspring meiosi ovaries egg fertilized egg fertilization sper m Evolutionary Consequences of Sex • Evolutionary process is revolutionary and conservative. – Pace of evolutionary change is accelerated by genetic recombination – Evolutionary change not always favored by selection • May act to preserve existing gene combinations • Life Cycles In Haploid Life Cycles, the Majority of the Cycle Consists of Haploid Cells • In Diploid Life • Cycles, the Majority of the Cycle Consists of Diploid Cells In Alternation-ofGeneration Life Cycles, There Are Both Diploid and Haploid Multicellular Stages Haploid Life Cycles • • • Fungi and unicellular algae Most of life cycle is haploid Asexual reproduction by mitotic cell division produces a population of identical, haploid cells Haploid Life Cycles • • Under certain environmental conditions, “sexual” haploid cells are produced Two sexual haploid cells fuse, forming a diploid cell that immediately undergoes meiosis, producing haploid cells again Diploid Life Cycles • • • • • Most animals Most of the cycle is in diploid state Haploid gametes are formed by meiosis Gametes fuse to form a diploid zygote Zygote develops into adult through mitotic cell divisions Alternation-of-Generation Cycles • • • • Plants Includes both multicellular diploid and multicellular haploid body forms Multicellular diploid body gives rise to haploid spores, through meiosis Spores undergo mitosis to produce a multicellular haploid generation Alternation-of-Generation Cycles • • • Eventually, certain haploid cells differentiate into haploid gametes Two gametes fuse to form a diploid zygote The zygote grows by mitotic cell division into a diploid multicellular diploid generation The making of Dolly Finn Dorset ewe donor cell from udder electric pulse fused cells Cells from the udder of a Finn Dorset ewe are grown in culture with low nutrient levels. The starved cells stop dividing and enter the non-dividing G0 phase of the cell cycle. Blackface ewe egg cell nucleus is removed DNA Meanwhile, the nucleus is sucked out of an unfertilized egg cell taken from a Scottish Blackface ewe. This egg will provide cytoplasm and organelles but no chromosomes. The egg cell without a nucleus and the quiescent udder cell are placed side by side in a culture dish. An electric pulse stimulates the cells to fuse and initiates mitotic cell division. The cell divides, forming an The ball of cells is implanted embryo that consists of a hollow ball into the uterus of another of cells. Blackface ewe. The Blackface ewe gives birth to Dolly, a female Finn Dorset lamb, a genetic twin of the Finn Dorset ewe.