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Bio 211 d11 Chapter 8 – cell division • Why do cells divide? • Reproduc;on – Making the next genera;on – Simple in prokaryotes; complex in eukaryotes • Renewal, growth and development – Eukaryo;c mul;-‐celled organisms only – Produce iden;cal daughter cells – Daughters may then differen'ate, becoming specialized for specific func;ons How do cells divide? • What do all cells need to do? How do cells divide? • What do all cells need to do? – Make more “stuff” – Make more DNA – Separate the stuff and the DNA in half • How does this differ between – Prokaryotes? – Eukaryotes? How do cells divide? • What do all cells need to do? – Make more “stuff” – Make more DNA – Separate the stuff and the DNA in half • How does this differ between – Prokaryotes • Binary fission for reproduc;on – Eukaryotes • Mitosis or meiosis for reproduc;on and other things Mitosis and meiosis • Prokaryotes or eukaryotes or both? – Eukaryotes only! • How is each process different? – Mitosis creates two iden;cal daughters – Meiosis produces four non-‐iden;cal daughters with half as much gene;c material • When is each process used? – Reproduc;on in single celled – mitosis or meiosis – Reproduc;on in mul; celled – usually meiosis – Growth, development, maintenance – mitosis (mul; celled organisms only) Mitosis in mul; celled organisms • Three main cell types – Embryonic cells – rapidly dividing to become adult – Stem cells in adult – divide to replace dying cells – Terminally differen;ated cells – not dividing Mitosis and meiosis for reproduc;on • Asexual reproduc'on– mitosis • Sexual reproduc'on – meiosis • Examples of both in single and mul; celled eukaryotes – eg. plants and starfish can reproduce by mitosis aYer fragmenta;on – eg. single celled yeasts can reproduce either by mitosis OR by meiosis But first • Binary fission -‐ reproduc;on in prokaryotes • The DNA of a prokaryo;c cell is contained in a single, circular chromosome ~1-‐2mm long • Not in a nucleus • Cell cycle has two phases: Long period of growth followed by binary fission (or “spli^ng in two”) Binary fission – the deets 1. At the start of the growth phase, the single prokaryo;c chromosome is usually a_ached at one point to the plasma membrane of the cell 2. During the growth phase, the DNA replicates, producing two iden;cal chromosomes that become a_ached to the plasma membrane at nearby, but separate, sites 3. As the cell increases in size, new plasma membrane is added between the a_achment points, pushing the duplicated chromosomes apart 4. The plasma membrane grows inward between the two chromosome copies 5. Fusion of membrane along the cell equator completes separa;on of the cells, producing two daughter cells, each containing one of the chromosomes 6. The daughter cells are gene;cally iden;cal Figure 9-3 The prokaryotic cell cycle attachment site of chromosome cell wall plasma membrane chromosome The prokaryotic chromosome, a circular DNA double helix, is attached to the plasma membrane at one point. The DNA replicates and the resulting two chromosomes attach to the plasma membrane at nearby points. New plasma membrane is added between the attachment points, pushing the two chromosomes farther apart. cell division by prokaryotic fission cell growth and DNA replication The prokaryotic cell cycle The plasma membrane grows inward at the middle of the cell. The parent cell divides into two daughter cells. Prokaryotic fission Compare eukaryo;c to prokaryo;c DNA § Prokaryo;c chromosome • Circular DNA double helix, ~ 5 million nucleo;des long • DNA is supercoiled • Is in the “nucleoid” region • A_ached to plasma membrane § Eukaryo;c chromosomes • Typical chromosome is 5 to 250 million nucleo;des long • Most eukaryotes have mul;ple chromsomes • DNA in each chromosome is wound around proteins called histones • Histone spools are coiled, the coils are looped, a_ached to protein scaffolding, so that the chromosome is 1,000 ;mes shorter than the extended DNA molecule • During cell division, more proteins fold up the DNA and histones, un;l it is 10 ;mes shorter than during its res;ng state (10,000 ;mes shorter than extended) Figure 9-4 Chromosome structure DNA double helix histone proteins DNA wound around histone proteins Coiled DNA/histone beads Loops attached to a protein scaffold; this stage of partial condensation typically occurs in a nondividing cell protein scaffold Folded chromosome, fully condensed in a dividing cell Then there’s ploidy • Eukaryotes may be haploid or diploid (most diploid) • Eukaryo;c chromosomes in diploid organisms occur in pairs containing similar gene;c informa;on • How do you suppose we know this? – When an en;re set of stained chromosomes from a single cell is examined (its karyotype), we see that most cells contain pairs of chromosomes – Paired chromosomes are the same length, shape, and staining pa_ern – Chromosomes that contain the same genes are called homologous chromosomes, or homologues Are they or aren’t they (diploid)? • Are humans diploid? How do you know? – typical human cell has 23 pairs of chromosomes, total of 46 • Other animals? – Most yes but some tetraploid eg. some frogs, fish, worms • Plants? – Some yes but many are polyploid • Fungi? – examples of both haploid and diploid among unicelled members Human chromosomes • Two kinds – Twenty-‐two out of 23 pairs are called autosomes • Autosomes have similar appearance and DNA sequences, and are paired in diploid cells of both sexes – The twenty-‐third pair is called sex chromosomes and is different in male v female one duplicated chromosome Human karyotype sister chromatids a pair of homologous chromosomes sex chromosomes Male or female karyotype? How do eukaryotes divide? • The mito;c cell cycle consists of two phases: – interphase and mitosis (cell division) – Interphase: acquire nutrients, grow, and replicate DNA – Mitosis: DNA and organelles are divided out into the two daughter cells • Meio;c cell division … – We’ll get to that next ;me Interphase • Grow in size, replicate DNA, may differen;ate • Most eukaryo;c cells spend the majority of their ;me in interphase • Interphase is divided into three phases 1. G1 (growth phase 1) acquire nutrients and grow to proper size 2. S (synthesis phase) DNA synthesis (every chromosome is replicated) 3. G2 (growth phase 2) more growth, and prepara;on for division into daughters Figure 9-8 The eukaryotic cell cycle ase telopha se and cytokinesis anaph G1: cell growth and differentiation G2: cell growth and preparation for cell division Which is longer? Interphase or mitosis? S: synthesis of DNA; duplication of chromosomes Mitosis is next! • Mito's turns one parent cell into 2 iden;cal daughter cells • First 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 • Next in cytokinesis (cytoplasmic division), the cytoplasm is divided equally between two daughter cells Fun facts about mitosis • Mito;c cell division takes place in all eukaryo;c organisms • It is the mechanism of asexual reproduc;on • Mitosis plus differen;a;on allows a fer;lized egg to grow into an adult with perhaps trillions of specialized cells • It allows organisms to maintain, repair, and even regenerate body parts Phases in mitosis – Is DNA replicated in mitosis? – NO! this happens in S phase of interphase – So at the start of mitosis, each chromosome is two sister iden;cal chroma;ds a_ached at the centromere – Mitosis has four main phases based on appearance and behavior of the chromosomes 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase Prophase – “the stage before” – Five major events occur during prophase 1. Duplicated chromosomes condense 2. Spindle microtubules form 3. Nuclear envelope breaks down 4. Chromosome condensa;on causes nucleolus to dissipate 5. Chromosomes are captured by the spindle microtubules Figure 9-9-1 Mitotic cell division in an animal cell INTERPHASE nuclear envelope MITOSIS chromatin spindle pole condensing chromosomes nucleolus spindle microtubules kinetochore beginning of spindle formation centriole pairs Late Interphase Duplicated chromosomes are in the relaxed uncondensed state; duplicated centrioles remain clustered. Early Prophase Chromosomes condense and shorten; spindle microtubules begin to form between separating centriole pairs. spindle pole Late Prophase (also called Prometaphase) The nucleolus disappears; the nuclear envelope breaks down; some spindle microtubules attach to the kinetochore (blue) located at the centromere of each sister chromatid. kinetochore microtubules Metaphase Kinetochore microtubules line up the chromosomes at the cell’s equator. Metaphase • During metaphase, the chromosomes line up along the equator of the cell – At the end of prophase, the kinetochores of each duplicated chromosome are connected spindle microtubules leading to opposite poles of the cell – Metaphase is a result of duplicated chromosomes connec;ng to opposite spindle poles – A kinetochore microtubule from one pole that is a_ached to a chroma;d’s kinetochore lengthens or shortens, as necessary, to draw the chromosome to the cell’s equator, in a line perpendicular to the spindle Anaphase • During anaphase, sister chroma;ds separate and are pulled to opposite poles of the cell – At the beginning of anaphase, the sister chroma;ds separate, becoming independent daughter chromosomes – Motor proteins in kinetochores pull chroma;ds apart along the kinetochore microtubules and toward opposite poles – At about the same ;me, polar microtubules from opposite poles a_ach to one another where they overlap at the equator – These polar microtubules then simultaneously lengthen and push on one another, which forces the poles of the cell apart, so that the cell assumes an oval shape Telophase • Telophase is the end stage of mito;c cell division – The spindle microtubules disintegrate – A nuclear membrane forms around each group of chromosomes at the pole – Chromosomes unwind and revert to their extended state – The nucleoli (which disappeared in prophase) reappear cytokinesis • During cytokinesis, the cytoplasm is divided between two daughter cells – Cytokinesis differs greatly between animal cells and plant cells • ANIMAL: – microfilaments a_ached to the membrane form a contrac;le ring at the cell’s equator – The ring contracts in half leaving two daughter cells, each with a iden;cal nucleus • PLANTS – s;ff plant cell walls can’t “pinch off” in cytokinesis like in animal cells, WHY? – Instead, sugar-‐filled vesicles line up at the cell’s equator, fuse together forming a cell plate Figure 9-9-2 Mitotic cell division in an animal cell MITOSIS polar microtubules INTERPHASE chromosomes extending microfilaments Anaphase Sister chromatids separate and move to opposite poles of the cell; polar microtubules push the poles apart. Telophase nuclear envelope re-forming nucleolus reappearing One set of chromosomes reaches each pole and begins to decondense; nuclear envelopes start to form; nucleoli begin to reappear; spindle microtubules begin to disappear; microfilaments form rings around the equator. Cytokinesis The ring of microfilaments contracts, dividing the cell in two; each daughter cell receives one nucleus and about half of the cytoplasm. Interphase of daughter cells Spindles disappear, intact nuclear envelopes form, and the chromosomes extend completely. Figure 9-10 Cytokinesis in a plant cell Do plants do it the same way? cell plate forming a new cell wall Golgi apparatus cell wall plasma membrane carbohydratefilled vesicles Carbohydrate-filled vesicles bud off the Golgi apparatus and move to the equator of the cell. The vesicles fuse to form a new cell wall (red) and plasma membrane (yellow) between the daughter cells. Complete separation of the daughter cells.
