The Cell Cycle, Mitosis and Meiosis
BI211 Fall 2006
•Overview: The Key Roles of Cell Division
•The continuity of life Is based upon the reproduction of cells, or cell division
• Unicellular organisms Reproduce by cell division
•Multicellular organisms
require cell division for Development from a fertilized cell
Growth
Repair
Phases of the Cell Cycle
–Interphase
–The mitotic phase
G1 – Most Growth
S – Replication
G2 – Prep for division
The Cell Cycle Control System
•The sequential events of the cell cycle
•Evidence for Cytoplasmic Signals
directed by a distinct cell cycle control system, like a clock
Molecules present in the cytoplasm regulate progress through the cell cycle
Cell Cycle is Regulated by a Molecular Control System
•Cell division frequency varies with the type of cell
•Cell cycle differences Result from regulation at the molecular level
clock has specific checkpoints
cell cycle stops until a go-ahead signal is received
The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases
–Two types of regulatory proteins are involved in cell cycle control
–Activity fluctuates during the cell cycle
Cell Division
•Cell Cycle
–Interphase
•G1 – Most Growth
•S – Replication - ensures that each daughter cell receives an exact copy of the genetic material, DNA
•G2 – Prep for division
–Cell Division
•Mitosis
–Results in two genetically identical daughter cells
•Meiosis
–Results in four genetically distinct daughter cells
Cellular Organization of the Genetic Material
•A cell’s endowment of DNA, its genetic information Is called its genome
•The DNA molecules in a cell packaged into chromosomes
•Eukaryotic chromosomes consist of chromatin, a complex of DNA & protein; condenses during cell division
•In animals
•Somatic cells have two sets of chromosomes = diploid
•Gametes have one set of chromosomes = haploid
In a cell in which DNA synthesis has occurred
•All chromosomes are duplicated
each chromosome consists of two identical sister chromatids
Distribution of Chromosomes During Cell Division
•In preparation for cell division
DNA is replicated
S of interphase
Each duplicated chromosome has two sister chromatids
•Chromosomes condense
Prophase
•Sister chromatids separate
Anaphase
•Mitosis consists of five distinct phases
–Prophase
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Prometaphase
Metaphase
Anaphase
Telophase
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Comparison of Asexual and Sexual Reproduction
 In asexual reproduction by mitosis
One parent produces genetically identical offspring
•In sexual reproduction
By Meiosis and Fertilization
•Two parents give rise to one offspring that has unique combinations of genes
•One set of genes inherited from each of the two parents
Fertilization & meiosis alternate in sexual life cycles
•A life cycle
Is the generation-to-generation sequence of stages in the reproductive history of an organism
At sexual maturity the ovaries and testes by meiosis produce haploid gametes, sperm and egg cells,
containing only one set of chromosomes = haploid
•During fertilization
gametes, sperm and ovum, fuse, forming a diploid zygote
•The zygote
Develops into an adult organism
•The human life cycle
Overview: Hereditary Similarity and Variation
•Living organisms
reproduce their own kind
•Heredity
transmission of traits from one generation to the next
•Variation
offspring differ somewhat in appearance from parents and siblings
•Genetics
the scientific study of heredity and hereditary variation
offspring acquire genes from parents by inheriting chromosomes
Inheritance of Genes
•Genes
Are the units of heredity
Are segments of DNA
•Each gene in an organism’s DNA has a specific locus on a certain chromosome
•We inherit
–One set of 23 chromosomes from our mother & one set of 23 from our father = 23 pairs of
chromosomes
• Homologous chromosomes
–Are the two chromosomes composing a pair
Have the same genes and physical characteristics
–May also be called autosomes
•Sex chromosomes
–Are distinct from each other in their characteristics
–Are represented as X and Y
Determine the sex of the individual, XX being female, XY being male
•A diploid cell
•A haploid cell
•A karyotype
Has two sets of each of its chromosomes
human = 46 chromosomes (2n = 46)
Has one set of each of its chromosomes
human = 23 chromosomes (n = 23)
Is an ordered, visual representation of the chromosomes in a cell
Meiosis
•reduces the number of chromosome sets from diploid to haploid
•Takes place in two sets of divisions, meiosis I and meiosis II
•Meiosis I
Reduces the number of chromosomes from diploid to haploid
•Meiosis II
Produces four haploid daughter cells
–
Critical Features of Mitosis and Meiosis
I. A chromosome is ALWAYS everything associated with one centromere
II. A chromatid is ALWAYS one double helix of DNA (2 DNA molecules)
III. DNA replicates during S phase
IV. Centromeres split in
mitotic anaphase
NOT in anaphase I
In anaphase II
The cell cycle consists of
–Interphase
G1 – Most Growth
S – Replication
G2 – Prep for division
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–Cell Division
Mitosis
Prophase
Prometaphase
Metaphase
Anaphase Centromeres split
Telophase
Meiosis
PI
PM I
MI
AI
TI
P II
PM II
M II
A II
T II
Homologs pair
Homolog pairs to the center
Centromeres DO NOT split
Centromeres split
A Comparison of Mitosis and Meiosis
•Meiosis distinguished from mitosis by three events in Meiosis l
–Synapsis and crossing over
•Homologous chromosomes physically connect and exchange genetic information
–Tetrads on the metaphase plate
•At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the
metaphase plates
–Separation of homologues
•At anaphase I of meiosis, centromeres DO NOT SPLIT & homologous pairs separate
•In anaphase II of meiosis, centromeres DO SPLIT & sister chromatids separate
•A comparison of mitosis and meiosis
Genetic variation contributes to evolution
•Mutations & Gene Duplication
–The original source of genetic variation => variant genes
•Sexual reproduction
–Produces new combinations of variant genes, adding more genetic diversity
•Genetic Variation produced in sexual life cycles
–by reshuffling of genetic material in meiosis
In species that produce sexually
•Chromosome behavior during meiosis and fertilization causes most of the variation that arises each
generation
–Independent Assortment
Crossing Over
Random Fertilization
Independent Assortment of Chromosomes
•Homologous pairs of chromosomes
–Orient randomly at metaphase I of meiosis
•In independent assortment
–Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of
the other pairs
–
Crossing Over
Random Fertilization
•Crossing over Produces recombinant chromosomes that carry genes derived from two different parents
•Evolutionary Significance of Genetic Variation Within Populations
•Genetic variation is the raw material for evolution by natural selection
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