How Cells Reproduce Chapter 7

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

Division Mechanisms Eukaryotic organisms –Mitosis –Meiosis Prokaryotic organisms –Prokaryotic fission

Roles of Mitosis Multicelled organisms –Growth –Cell replacement Some protistans, fungi, plants, animals –Asexual reproduction

Chromosome A DNA molecule and attached proteins Duplicated in preparation for cell division one chromosome (unduplicated) one chromosome (duplicated)

INTERPHASE S Interval of cell growth, when DNA replication is completed (chromosomes duplicated) Interphase ends for parent cell Cytoplasmic division G1 Interval of cell growth, before DNA replication (chromosomes unduplicated) G2 Interval following DNA replication; cell prepares to divide Each daughter cell starts interphase MITOSIS Telophase Anaphase Metaphase Prophase Fig. 7-2, p.96 The Cell Cycle

The cell cycle The Cell Cycle

Control of the Cycle Once S begins, the cycle usually runs through G2 and mitosis Cycle has a built-in molecular brake in G1 Cancer involves a loss of control over the cycle, malfunction of “brakes”

Interphase Usually the longest part of cycle Cell increases in mass Number of cytoplasmic components doubles DNA is duplicated

Mitosis Period of nuclear division Usually followed by cytoplasmic division Four stages: Prophase Metaphase Anaphase Telophase

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

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

Human Chromosomes

Maintaining Chromosome Number chromosomes (unduplicated) in parent cell at interphase same chromosomes (duplicated) in interphase prior to mitosis mitosis, cytoplasmic division chromosome (unduplicated) in daughter cell at interphase chromosome (unduplicated) in daughter cell at interphase

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

Spindles spindle microtubules chromosomes at spindle equator, midway between spindle poles

Stages of Mitosis Prophase Metaphase Anaphase Telophase

Mitosis step-by-step Mitosis

Cytoplasmic Division Usually occurs between late anaphase and end of telophase Two mechanisms –Cleavage (animals) –Cell plate formation (plants) animal cell pinching in two

Cytoplasmic division Cleavage & Cell Plate Formation

Asexual Reproduction Single parent produces offspring All offspring are genetically identical to one another and to parent

Sexual Reproduction Involves –Meiosis –Gamete production –Fertilization Produces genetic variation among offspring

Homologous Chromosomes Carry Different Alleles Cell has two of each chromosome Chromosome pairs: one from mother, one from father Paternal and maternal chromosomes carry different alleles

Sexual Reproduction Shuffles Alleles Through sexual reproduction, offspring inherit new combinations of alleles, which lead to variations in traits Variation in traits is the basis for evolutionary change

Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells in reproductive organs testes ovaries

Reproductive organs Gamete Formation

Chromosome Number Total number of chromosomes in cell Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number

Diploid To Haploid one chromatid its sister chromatid centromere one chromosome in the duplicated state

each homologue in the cell pairs with its partner (synapsis) then partners separate Stepped Art p.103 two chromosomes (unduplicated) one chromosome (duplicated)

Meiosis: Two Divisions Two consecutive nuclear divisions –Meiosis I –Meiosis II DNA is not duplicated between divisions Four haploid nuclei form

p.102

Meiosis step-by-step Meiosis

Sexual Reproduction and Genetic Variation Two functions of meiosis provide variation in traits: – crossing over – random alignment

Crossing Over Each chromosome attaches to its homologue All four chromatids are closely aligned (synapsis) Nonsister chromatids exchange segments

Crossing over Crossing Over

Effects of Crossing Over After crossing over, each chromosome contains both maternal and paternal segments Creates new allele combinations in offspring

Random Alignment Between prophase I and metaphase I, chromosome pairs align randomly at metaphase plate Initial contact between microtubule and either maternal or paternal chromosome is random

Random alignment Animal Life Cycle

Factors Contributing to Variation among Offspring Crossing over during prophase I Random alignment of chromosomes at metaphase I Random combination of gametes at fertilization

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

Sperm formation Animal Sperm Formation

Egg formation Animal Egg Formation

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

Cancer and metastasis Cancer

Mitosis Functions –Asexual reproduction –Growth, repair Occurs in somatic cells Produces clones Mitosis & Meiosis Compared Meiosis Function –Sexual reproduction Occurs in germ cells Produces variable offspring

Comparing mitosis and meiosis Results of Mitosis and Meiosis