Lecture # 14 Chapter 7

Mitosis and Meiosis

Fig. 7.1

7.1 Prokaryotes Have a Simple Cell Cycle

• Prokaryotic cells divide asexually– These cells possess a single

circular chromosome, containingabout 1000 genes

– The chromosome is replicated– The cell then divides into two cells,

a process called binary fission

7.2 Eukaryotes Have a Complex Cell Cycle

• Cell division in eukaryotes is more complex than in prokaryotes because– 1. Eukaryotic contain far more DNA– 2. Eukaryotic DNA is packaged differently

• It is in linear chromosomes compacted with proteins

• Some eukaryotes alsomake exact copies ofthemselves via asexualreproduction

amoeba

– G1 phase• Primary growth phase

– S phase• DNA replication

– G2 phase• Microtubule synthesis

– M phase• Chromosomes pull apart

– C phase• Cytoplasm divides

Interphase

Mitosis

Cytokinesis

7.2 Eukaryotes Have a Complex Cell Cycle

Fig. 7.2 How the cell cycle works

Fig. 7.3

• Diploid cells have two similar copies of each chromosomes• Chromosomes exist in somatic cells as pairs

– Homologous chromosomes or homologs• Replicated chromosomes consist of two identical

sister chromatids– These are held together at the centromere

7.3 Chromosomes

• Before a cell starts dividing, the chromosomes duplicate

• This process produces identicalsister chromatids

Sister chromatids

Chromosomeduplication

Chromosomedistribution

todaughter

cells

Centromere

• When the cell divides,the sister chromatids separate

• Each of the two daughter cells has acomplete and identical set of chromosomes

Fig. 7.4

• Humans have 46 chromosomes or 23 pairs of homologs

• The chromosomes can be organized by size into a karyotype

• A typical human chromosome contains about 1000 genes and about 140 million nucleotides in its DNA

• About 5 cm in stretched length• 2,000 printed books of 1,000 pages each!

• In the cell, however, the DNA is coiled into chromatin– Complex of DNA (~ 40%) and proteins (~ 60%)

Levels of eukaryotic chromosome organization

•The DNA helix is wrapped around positively-charged proteins, called histones•200 nucleotides of DNA coil around a core of histones, forming a nucleosome•The nucleosomes coil into solenoids•Solenoids are then organized into looped domains•The looped domains appear to form rosettes on scaffolds

Fig. 7.5

7.2 Eukaryotes Have a Complex Cell Cycle

Eukaryotic cells divide in one of two ways– Meiosis

• Occurs in germ(reproductive) cells

• Results in the production of gametes

– Mitosis• Occurs in

somatic (non-reproductive) cells

MEIOSIS FERTILIZATION

Haploid gametes (n = 23)

Egg cell

Sperm cell

Diploidzygote

(2n = 46)Multicellular

diploid adults(2n = 46)

Mitosis anddevelopment

7.4 MitisosMitotic cell division allows a fertilized egg to develop into an embryo, an adolescent and eventually an adult

sea urchinembryo

• Organisms can regenerate some body partsThese sea stars are regenerating using mitosis.

What can humans regenerate?

- Asexual reproduction, development, growth and cell replacement are mitotic divisions

Deadcells

Dividingcells

Epidermis, the outer layer of the skin

Dermis

skin

Prophase• Chromosomes coil

• Nuclear membrane breaks down

• Spindle fibers form

Metaphase

• Chromosomes line up on the midline

• Spindle fibers attach to centromeres

Anaphase

• Centromeres divide

• Spindle fibers shorten

• Sister chromatids separate and move to opposite poles

Telophase• Cell elongates

• Nuclear membrane reforms•• Chromosomes uncoil

• Spindle disappears

Cytokinesis

• Division of the cytoplasm

• Cleave furrow forms at equator of the cell

• Constriction tightens by contraction of filaments

• Cell is divided into two identical cells

• Cytokinesis

• Animal cells

• Plant cells

Fig. 7.8

– Cleavage furrow forms, pinching the cell in two

– Cell plate forms, dividing the cell in two