The Cell Unit 3: Mitosis and Meiosis Chapter 8: How Cells Reproduce

Cells: from largest to smallest Cell – smallest unit of life Nucleus – protects genetic material DNA – macromolecule that contains the genetic code Chromosome – strand of DNA Gene – segment of a chromosome that codes for a protein or trait

Three Questions to Keep in Mind Understanding cell division starts with three questions: 1) What kind of information guides inheritance? 2) How is that information copied inside a parent cell before being distributed to each of its daughter cells? 3) What kinds of mechanisms parcel out the information to daughter cells?

Mechanisms of Cell Division Reproduction produces a generation of individuals like their parents Cell division is the bridge between generations Each daughter cell receives: Therefore, before cells are able to reproduce, there has to be a division of the nucleus and its DNA.

Eukaryotes and Prokaryotes Eukaryotic cells undergo _________ and/or __________ These lead to the distribution of DNA to new nuclei in forthcoming daughter cells Another mechanism divides cytoplasm Prokaryotic cells reproduce asexually by a different mechanism called prokaryotic fission

Mitosis and Meiosis Both mitosis and meiosis are eukaryotic nuclear division mechanisms Mitosis is used by multicelled organisms for growth, which occurs through the repeated division of somatic cells Meiosis occurs only in reproductive cells that divide to form gametes or spores Gametes in humans/other mammals: egg and sperm Spores: fungi, plants, and many kind of protists Somatic cells: body cells Gamete: reproductive cell

Cell Division Mechanisms

Chromosome Structure Chromosome: a molecule of DNA complexed with proteins Prior to cell division, each threadlike chromosome is duplicated to form two ________________ held together by a ____________ ___________: proteins that tightly bind to DNA and cause spooling into structural units called nucleosomes, which prevent tangling ______________ are the smallest unit of organization

Structure of Condensed Chromosomes Figure 8.4, pg 127

a One duplicated human chromosome in its most condensed form. centromere Fig. 8.4a, p.127

Fig. 8.4b, p.127

Fig. 8.4d, p.127

Chromosome Structure The constricted region where two sister chromatids attach is the _______________ Remain attached until late in mitosis (or meiosis) During cell division, a ________________ forms at the centromere Kinetochore: the binding sites for microtubules This is the region where duplicated chromosomes will attach to the microtubules of the spindle during nuclear division

Figure 8.3, pg 126

Chromosomes Important things to remember about chromsomes: Individuals of a species have a characteristic number of chromosomes in each cell Chromosomes differ in length and shape, carry different portions of cell’s hereditary information Mechanisms divide information between daughter cells, along with enough cytoplasm for each cell to operate on its own

The Cell Cycle Life cycle: the sequence of stages through which individuals of a species pass during their lifetime The cell cycle is a recurring sequence of events that extends from the time of a cell’s formation until each division is completed Begins when a new daughter cell forms by mitosis and cytoplasmic division Ends when that cell divides Includes __________________, _________________________, and ______________

Eukaryotic Cell Cycle Figure 8.5, pg 128

______________ Most cellular activities occur in interphase About 90% of a cell’s existence is spent in interphase Mitosis occupies only a small portion During interphase: The cell’s mass increases Cytoplasmic components approximately double in number AND the DNA is doubled Some cells are arrested in interphase and usually never divide again

Stages of Interphase The control of cell division resides in these three stages of interphase: G1: (“gap”) Interval during which cell grows in mass, doubles the number of cytoplasmic components, and assembles most of the carbs, lipids, and proteins that are needed by the cell and for export S: (“synthesis”) the DNA and histones are copied through the process of DNA replication G2: (“gap”) Interval during which further protein synthesis drives the cell toward mitosis as it prepares for division

Chromosome Number All somatic cells of a particular species have the same number of chromosomes Human cells have ______chromosomes Chromosomes come in pairs – one from each parent These pairs carry genes for the same trait Chromosome number (n) tells how many of each type of chromosome is present in a cell Body cells are _____________(2n), which means they have two of each kind of chromosome

Human Chromosomes: ________

Mitosis and Chromosome Number Mitosis maintains parental chromosome number from one generation to the next __________ __________ composed of two sets of microtubules divides sister chromatids Figure 8.6, pg 129

Mitosis Mitosis: a nuclear division mechanism that maintains chromosome number Mitosis proceeds in four stages: 1) 2) 3) 4)

Centrosomes Most animal cells have a _____________, a region near the nucleus that will organize microtubules while they are forming Called a Microtubule Organizing Center (MTOC) Usually include two pairs of barrel-shaped centrioles The centromsome is duplicated just before prophase Plants and fungi use other structures as MTOCs

____________ Early Prophase: Chromosomes start to condense and become visible as rodlike units, each consisting of two sister chromatids Late Prophase: Chromosomes continue to condense Microtubules move one centrosome, with one pair of centrioles, to opposite poles of the cell Nuclear envelope begins to disintegrate Microtubules form a bipolar spindle

Figure 8.7b and c, pg 130 Prophase:

Transition from ________to _________ Microtubules of the spindle extend from the centrioles and attach to the centromeres (kinetochore area) of the duplicated chromosomes The transition to metaphase occurs as chromosomes begin an orderly arrangement Sister chromatids are attached separately to microtubules from opposite poles

Transition to Metaphase

______________ The cell is in metaphase when all the chromosomes are aligned at the cell’s equator halfway between the poles Chromosomes are in their most tightly condensed form Figure 8.7e, pg 130 e Metaphase: chromosomes line up in the middle

________________ During anaphase, the cells migrate to opposite poles The kinetochores of sister chromatids separate and move toward opposite poles Microtubules attached to the centromeres _______ and pull the chromosomes toward the poles Other microtubules that overlap at the spindle’s midpoint __________ past each other to push the two spindle poles apart __________ _____________drive movements Once separated, each chromatid is now an independent chromosome

Anaphase: Figure 8.7f, pg 130

______________ Telophase begins when the chromosomes reach the two poles Each half of the cell now contains two clusters of chromosomes that are identical and equal in number The chromosomes decondense and return to the threadlike form typical of interphase The nuclear envelope forms from the fusion of small vesicles We now have two new nuclei that each have the parental chromosome number Mitosis is complete!

Telophase: Figure 8.7g, pg 130

After mitosis and cytoplasmic division: _______________________ Mitosis divides the ___________, NOT the _________________ Figure 8.7h, pg 130