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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