The Cell Cycle
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 G1 – Most Growth S – Replication G2 – Prep for division
The mitotic phase
The Cell Cycle Control System
The sequential events of the cell cycle directed by a distinct cell cycle control system, like a clock
Evidence for Cytoplasmic Signals
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 Prometaphase Metaphase Anaphase Telophase
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 Has two sets of each of its chromosomes human = 46 chromosomes (2n = 46)
A haploid cell Has one set of each of its chromosomes human = 23 chromosomes (n = 23)
A karyotype 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
Cell Division
Mitosis Meiosis
Prophase P I Homologs pair
Prometaphase PM I
Metaphase M I Homolog pairs to the center
Anaphase Centromeres split A I Centromeres DO NOT split
Telophase T I
P II
PM II
M II
A II Centromeres split T II
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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