How Cells Reproduce

Dividing HeLa Cells

Eukaryotes and Prokaryotes Eukaryotic cells undergo mitosis and/or meiosis Separates duplicated chromosomes of parent cell into two daughter nuclei Another mechanism divides cytoplasm

Mitosis and Meiosis Mitosis Meiosis Basis of growth, cell replacements, and tissue repair in multicelled species Basis of asexual reproduction in many single-celled and multicelled species Meiosis Basis of sexual reproduction Precedes formation of gametes or sexual spores

Cell Division Mechanisms

Chromosome Structure Eukaryotic chromosome Nucleosome Association of DNA, histones, and other proteins Nucleosome Smallest unit of organization Double-stranded DNA looped twice around a spool of histones

Structure of a Condensed Chromosome

Sister Chromatids A duplicated chromosome consists of two sister chromatids Sister chromatids remain attached at their centromere until late in mitosis

Introducing the Cell Cycle Starts when a new cell forms Runs through interphase Ends when cell reproduces by nuclear and cytoplasmic division

Interphase Most cellular activities occur in interphase G1: Cell grows in mass, doubles number of cytoplasmic components S: DNA replication duplicates chromosomes G2: Cell prepares for division

G1 S Interval of cell growth before DNA replication (chromosomes unduplicated) Interval of cell growth when the DNA is replicated (all chromosomes duplicated) cytoplasmic division; each daughter cell enters interphase G2 Interval after DNA replication; the cell prepares to divide Interphase ends for parent cell Fig. 8.5, p.128

Chromosome Number Sum of all chromosomes in cells of a given type In human body cells, chromosome number is 46 Body cells are diploid (have two of each kind of chromosome)

Mitosis and Chromosome Number Mitosis maintains parental chromosome number from one generation to the next Bipolar spindle divides sister chromatids

A Closer Look at Mitosis A nuclear division mechanism that maintains the chromosome number Mitosis proceeds in four stages: Prophase Metaphase Anaphase Telophase

Prophase Duplicated chromosomes become threadlike as they start to condense Microtubules form a bipolar spindle Nuclear envelope starts to break apart

Metaphase All chromosomes become aligned midway between the two spindle poles Chromosomes in most condensed forms

Anaphase Sister chromatids detach from each other Spindles move them toward opposite poles

Telophase Chromosomes reach opposite spindle poles Nuclear envelope forms around each cluster Both new nuclei have the parental chromosome number

Mitosis

Mitosis

Fig. 8.7a, p.130

nuclear envelope chromosome b Early Prophase Mitosis begins. DNA and its associated proteins have started to condense. Two chromosomes (color-coded purple) were inherited from the female parent. The other two (blue) are their counterparts, inherited from the male parent. Fig. 8.7b, p.130

pair of centrioles c Late Prophase The duplicated chromosomes continue to condense. New microtubules move one of two pairs of centrioles to the opposite side of the nucleus. The nuclear envelope starts to break up. Fig. 8.7c, p.130

All of the chromosomes have become lined up midway e Metaphase All of the chromosomes have become lined up midway between the spindle poles. At this stage of mitosis, the chromosomes are in their most tightly condensed form. Fig. 8.7e, p.130

Sister chromatids separate as motor proteins moving along f Anaphase Sister chromatids separate as motor proteins moving along spindle microtubules drag them to opposite spindle poles. Other microtubules push the poles farther apart. Fig. 8.7f, p.130

There are two clusters of chromosomes, which now g Telophase There are two clusters of chromosomes, which now decondense. Patches of new membrane fuse to form a new nuclear envelope. Mitosis is over. Fig. 8.7g, p.130

After cytoplasmic division, there are two daughter h Two Daughter Cells at Interphase After cytoplasmic division, there are two daughter cells. Each is diploid: Its nucleus has two of each type of chromosome, just like the parent cell. Fig. 8.7h, p.130

Cytoplasmic Division Mechanisms Mechanisms of cytoplasmic division differ in plant and animal cells In animal cells A contractile ring of microfilaments (part of cell cortex) contracts and pulls the cell surface inward until the cytoplasm is divided

Cytoplasmic Division in Animal Cells

Cytoplasmic Division in Plant Cells

Results of Cell Division: A Human Embryo

When Control is Lost Mutant checkpoint genes can cause tumors by disrupting normal controls