1 WHAT CELL REPRODUCTION ACCOMPLISHES Reproduction may result in the birth of new organisms but more commonly involves the production of new cells. When a cell undergoes reproduction, or cell division, two daughter cells are produced that are genetically identical to each other and to the parent cell.
2 Before a parent cell splits into two, it duplicates its chromosomes, the structures that contain most of the cell s DNA. During cell division, each daughter cell receives one identical set of chromosomes from the lone, original parent cell.
3 Cell division plays important roles in the lives of organisms. Cell division replaces damaged or lost cells, permits growth, and allows for reproduction.
4 Division Mechanisms Eukaryotic organisms Mitosis Meiosis Prokaryotic organisms Prokaryotic fission
5 Henrietta s Immortal Cells HeLa cells Derived from cervical cancer that killed Henrietta Lacks First human cells to grow and divide in culture Used in research throughout the world
6 The Eukaryotic Cell Cycle Consists of Interphase and Cell Division A cell cycle starts when a new cell forms, and ends when the cell reproduces Cell cycle A series of events from the time a cell forms until its cytoplasm divides Includes three phases: interphase, mitosis, and cytoplasmic division
7 The Cell Cycle Most of a cell cycle is spent in interphase. During interphase, a cell performs its normal functions, doubles everything in its cytoplasm, and grows in size.
8 Control of the Cycle Once S begins, the cycle usually runs through G2 and mitosis The cycle has a built-in molecular brake in G1 Cancer involves a loss of control over the cycle, malfunction of the brakes
9 The Cell Cycle The mitotic (M) phase includes two overlapping processes: 1. mitosis, in which the nucleus and its contents divide evenly into two daughter nuclei and 2. cytokinesis, in which the cytoplasm is divided in two.
10 Mitosis Period of nuclear division Usually followed by cytoplasmic division Four stages: Prophase Metaphase Anaphase Telophase
11 Roles of Mitosis Multicelled organisms Growth Cell replacement Some protistans, fungi, plants, animals Asexual reproduction
12 Chromosome Number Total number of chromosomes in a cell Somatic cells Chromosome number is diploid (2n) Two of each type of chromosome Gametes Chromosome number is haploid (n) One of each chromosome type
13 Human Chromosome Number Diploid chromosome number (n) = 46 Two sets of 23 chromosomes One set from father One set from mother Mitosis produces cells with 46 chromosomes: two of each type
14 The Spindle Apparatus Consists of two distinct sets of microtubules Each set extends from one of the cell poles Two sets overlap at spindle equator Moves chromosomes during mitosis
15 Stages of Mitosis Prophase Metaphase Anaphase Telophase
16 Results of Mitosis Two daughter nuclei Each with same chromosome number as parent cell Chromosomes are in unduplicated form
17 Cytoplasmic Division Usually occurs between late anaphase and end of telophase Two mechanisms Cell plate formation (plants) Cleavage (animals)
18 In animal cells, cytokinesis is known as cleavage and begins with the appearance of a cleavage furrow, an indentation at the equator of the cell.
19 Asexual Reproduction Mitosis Single parent produces offspring All offspring are genetically identical to one another and to parent
20 Sexual Reproduction Meiosis Involves Meiosis Gamete production Fertilization Produces genetic variation among offspring
21 Eukaryotic Chromosomes Usually Occur in Homologous Pairs with Similar Genetic Information
22 Homologous Chromosomes Different individuals of a single species have the same number and types of chromosomes. A human somatic cell is a typical body cell and has 46 chromosomes.
23 Homologous Chromosomes Carry Different Alleles Cell has two of each chromosome One chromosome in each pair from mother, other from father Paternal and maternal chromosomes carry different alleles
24 Homologous Chromosomes A karyotype is an image that reveals an orderly arrangement of chromosomes. Homologous chromosomes are matching pairs of chromosomes that can possess different versions of the same genes.
25 Meiosis Separates Homologous Chromosomes, Producing Haploid Daughter Nuclei Meiotic Cell Division Followed by Fusion of Gametes Keeps the Chromosome Number Constant from Generation to Generation
26 Homologous Chromosomes Humans have two different sex chromosomes, X and Y, and 22 pairs of matching chromosomes, called autosomes.
27 Gametes and the Life Cycle of a Sexual Organism The life cycle of a multicellular organism is the sequence of stages leading from the adults of one generation to the adults of the next.
28 Gametes and the Life Cycle of a Sexual Organism Humans are diploid organisms with body cells containing two sets of chromosomes and haploid gametes that have only one member of each homologous pair of chromosomes. In humans, a haploid sperm fuses with a haploid egg during fertilization to form a diploid zygote.
29 Chromosome Number Total number of chromosomes in cell Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number
30 Gametes and the Life Cycle of a Sexual Organism Sexual life cycles involve an alternation of diploid and haploid stages. Meiosis produces haploid gametes, which keeps the chromosome number from doubling every generation.
31 Sexual Reproduction and Genetic Variation Two functions of meiosis provide variation in traits: crossing over random alignment
32 The Process of Meiosis In meiosis, haploid daughter cells are produced in diploid organisms, interphase is followed by two consecutive divisions, meiosis I and meiosis II, and crossing over occurs.
33 Crossing Over Each chromosome becomes zippered to its homologue All four chromatids are closely aligned Nonsister chromosomes exchange segments
34 Effect of Crossing Over After crossing over, each chromosome contains both maternal and paternal segments Creates new allele combinations in offspring
35 Sexual Reproduction Shuffles Alleles Through sexual reproduction, offspring inherit new combinations of alleles, which lead to variations in traits This variation in traits is the basis for evolutionary change
36 Meiosis: Two Divisions Two consecutive nuclear divisions Meiosis I Meiosis II DNA is not duplicated between divisions Four haploid nuclei form
37 Review: Comparing Mitosis and Meiosis In mitosis and meiosis, the chromosomes duplicate only once, during the preceding interphase. The number of cell divisions varies: Mitosis uses one division and produces two diploid cells. Meiosis uses two divisions and produces four haploid cells. All the events unique to meiosis occur during meiosis I.
39 Mitosis & Meiosis Compared Mitosis Functions Asexual reproduction Growth, repair Occurs in somatic cells Produces clones Function Meiosis Sexual reproduction Occurs in germ cells Produces variable offspring
40 Prophase I Each duplicated chromosome pairs with homologue Homologues swap segments Each chromosome becomes attached to spindle
41 Chromosomes are pushed and pulled into the middle of cell The spindle is fully formed Metaphase I
42 Anaphase I Homologous chromosomes segregate The sister chromatids remain attached
43 Telophase I The chromosomes arrive at opposite poles Usually followed by cytoplasmic division
44 Prophase II Microtubules attach to the kinetochores of the duplicated chromosomes
45 Metaphase II Duplicated chromosomes line up at the spindle equator, midway between the poles
46 Anaphase II Sister chromatids separate to become independent chromosomes
47 Telophase II The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes Four haploid cells
48 Random Alignment During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes Initial contacts between microtubules and chromosomes are random
49 Independent Assortment of Chromosomes When aligned during metaphase I of meiosis, the side-by-side orientation of each homologous pair of chromosomes is a matter of chance. Every chromosome pair orients independently of all of the others at metaphase I. For any species, the total number of chromosome combinations that can appear in the gametes due to independent assortment is 2 n, where n is the haploid number.
50 For a human, n = 23. With n = 23, there are 8,388,608 different chromosome combinations possible in a gamete.
51 Factors Contributing to Variation among Offspring Crossing over during prophase I Random alignment of chromosomes at metaphase I Random combination of gametes at fertilization
52 Random Fertilization A human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote. If each gamete represents one of 8,388,608 different chromosome combinations, at fertilization, humans would have 8,388,608 8,388,608, or more than 70 trillion different possible chromosome combinations. So we see that the random nature of fertilization adds a huge amount of potential variability to the offspring of sexual reproduction.
53 Fertilization Male and female gametes unite and nuclei fuse Fusion of two haploid nuclei produces diploid nucleus in zygote Random chance of sperm fertilizing egg increases variation in offspring
54 Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells in reproductive organs ovaries testes
55 Mechanism of spermatogenesis Spermatogonium (2n): undergoes mitosis Primary spermatocyte (2n): undergoes I meiotic division Secondary spermatocytes (n): II meiotic division Spermatids (n): spermiogenesis Sperm (n)
56 Oogenesis (development of female gametes) Takes place in ovary where oogonia develops into eggs Oogonia undergoes extensive growth which provides the egg with materials important for embryonic development: primary oocyte Undergoes two meiotic divisions providing haploid cells
57 Cancer Characteristics Plasma membrane and cytoplasm altered Cells grow and divide abnormally Weakened capacity for adhesion; cells can move to new tissues Lethal unless eradicated