DNA and Cell Division

Packaging of DNA

DNA segments are coiled around histones forming a complex called a nucleosome

Non-dividing cells DNA is loosely coiled forming network

called chromatin Cells preparing to divide

DNA becomes tightly coiled forming distinct structures called chromosomes

4-2

Chromosomes Human somatic cells contain 46

chromosomes that exist as 23 pairs of homologous chromosomes

Homologous chromosomes: same size and shape

Homologous Chromosomes

Code for the same traits Cells contain two

genes for every trait

4-5

Cell Cycle

Period of time from cell formation to cell division

Phases: Interphase• Period of cell

growth Mitotic phase• Cell division

4-12

Mitosis

One cell divides into 2 daughter cells with identical copies of DNA

Functions of mitosis embryonic development tissue growth replacement of dead cells repair of injured tissues

4-13

Phases of Mitosis

Prophase Metaphase Anaphase Telophase

4-14

Prophase Chromatin condenses into

chromosomes Spindle fibers grow from centrioles

4-15

Metaphase

Chromosomes line up along equator of cell Spindle fibers attach to chromosomes

4-16

Anaphase

Spindle fibers separate sister chromatids Daughter chromosomes move towards opposite poles

of cells

4-17

Telophase New nuclear envelopes assemble around each set

of daughter chromosomes Chromosomes uncoil into chromatin

4-20

Cytokinesis

Division of cytoplasm into 2 cells Myosin pulls on microfilaments of actin in the

membrane cytoskeleton (animal cells) creates crease around cell equator called

“cleavage furrow” Cell pinches in two

Sexual Reproduction Genetic material from

two parents are combined to form a unique combination of genes

Meiosis

Unique type of cell division that reduces chromosome number by half

Produces gametes or sex cells Spermatogenesis sperm cell production Oogenesis egg cell production

Gametes

Male and female gametes (sex cells) combine genetic material to form a fertilized egg (zygote) containing 46 chromosomes

Formation of Gametes

Gametes contain a single copy of 23 chromosomes (haploid)

Requires two cell divisions Meiosis• Meiosis I separates homologous chromosome pairs into

two haploid cells

• Meiosis II separates duplicated sister chromatids into four haploid cells

Crossing Over

Independent Assortment

Errors in Meiosis

Polypoid gamete Contains extra

complete sets of chromosomes

Nondisjunction Replicated

chromosomes fail to separate

Prokaryotic Cell Division

Asexual Reproduction Daughter cells are

genetically identical to parent cell

Genetic Variability in Asexual Reproduction

Transformation Transduction Conjugation Mutations