The Cell Cycle

The Cell Theory: All organisms consist of cells and arise from preexisting cells n Mitosis is the process by which new cells are generated. n Meiosis is the process by which gametes are generated for sexual reproduction.

Flagella Cell wall Cell membrane Chromosome Cytoplasm Ribosomes Cell and Chromosome Structure n Prokaryotic cells have no nucleus n DNA is organized into one circular chromosome Chromosome

Cell and Chromosome Structure n In eukaryotic cells, the genetic material is found within a membrane-bound nucleus n DNA is organized into multiple linear shaped chromosomes Cell membrane Cytoplasm Nuclear membrane Nucleus Chromosome

DNA is packaged into chromosomes

Eukaryotic chromosomes may be in their unreplicated or unreplicated state One chromosome (Unreplicated state)= One-double stranded DNA molecule One chromosome (Replicated state)= Two double stranded DNA molecules Centromere Sister chromatids DNA replication

Chromosomes in cells n All individuals of a particular species have a characteristic number of chromosomes Fruit flies have 8 chromosomes in their nuclei Humans have 46 chromosomes in their nuclei n Chromosomes of most species come in pairs One pair of homologous chromosomes- Unreplicated state One pair of homologous chromosomes- Replicated state

Homologous chromosomes n Each member of a pair of homologous chromosomes: Same size Same shape, centromere location Same genetic loci = physical location of a gene on a chromosome Gene for eye color One pair of homologous chromosomes

Homologous chromosomes n But: Each member of the pair (each “homolog”) may carry different versions of the gene Different versions of the same gene = alleles Eye color gene:  Brown allele  Blue allele Blue eye alleleBrown eye allele

A cell with 2 pairs of homologous chromosomes Bb Eye color gene:B= brown eye allele b= blue eye allele Height gene: T= tall allele t= short allele t T BB bb T T t t DNA replication

Ploidy: chromosomes don’t always come in pairs n Haploid = a cell with only one of each kind of chromosome n Diploid = a cell with two of each kind of chromosome n Triploid = a cell with three of each kind of chromosome 1n = 2 2n = 43n = 6 Number of each type of chromosome Total number of chromosomes How many different kinds of chromosome

Chromosome Number of chromosomes: 4 Unreplicated chromosomes A cell that is 2n = 4

Replicated chromosomes Chromosome Sister chromatids Centromere Number of chromosomes: 4 A cell that is 2n = 4

Humans are diploid 23 pairs of chromosomes: 2n = 46

Parent cell Replicated chromosomes condense at the start of mitosis Mitosis Sister chromatids separate and two daughter cells are formed. Parent cell and daughter cell contain the same complement of chromosomes. Overview of Mitosis:

The cell cycle represents all phases in the life of a cell n Interphase: DNA replication (S phase) must precede mitosis so that all daughter cells receive the same complement of chromosomes as the parent cell. The Gap phases separate mitosis from S phase. This is the time when molecular signals mediate the switch in cellular activity. n Mitosis: division of nucleus n Cytokinesis: division of cytoplasm

The cell cycle:

Functions of Mitosis n Mitosis produces genetically identical cells n Purpose Asexual Reproduction Growth and Development Tissue repair

Asexual Reproduction in Amoeba

Growth and development Sand dollar embryo 1-celled zygote (2n) mitosis Multi-cellular adult (2n)

Tissue renewal Dividing bone marrow cells give rise to new blood cells