Chromosomes, Mitosis, Meiosis

H. Smith

Terminology

• DNA exists in tightly wound structures called chromosomes.

• All animals have a set # of chromosomes.• Chromosomes existin pairs

Humans

• Humans have 23 pairs, or 46 chromosomes• More chromosomes doesn’t necessarily mean

an organism is more complex.

Why Chromosomes?

Why Chromosomes?

Cell Division

• 2 versions:– Mitosis: division of somatic (body) cells(46)– Meiosis: creation of gametic (sex) cells(23)

– Cells with 46 chromosomes: diploid cells– Cells with 23 chromosomes: haploid cells

The Key Roles of Cell Division

Mitosis:• Development, Growth,

& Repair of multicellular forms

Meiosis: • Creation of sex cells:

Reproduction

The Mitotic Cycle• Interphase 90%– G1 = Grow, normal fx.– S = Synthesis of DNA– G2 = Prepares for cell division

• Mitosis– PMAT

• Cytokinesis –cell pinches in 2

Interphase

• Growth Phase: DNA is replicated

• Nucleus is well defined in a nuclear envelope

• DNA is in the form of loosely packed chromatin fibers

• Accounts for 90% of cell cycle

Prophase• Chromatin condenses into Chromosomes

• The nucleus and nuclear envelope begin to disappear

• Spindle forms• Centrioles

separate

Metaphase

• Chromosomes line up along the Middle• Centrioles send out spindle fibers which attach to

lined up chromosomes.

Anaphase

• Spindle fibers attachAnd pull chromosomes

apart and to opposite sides of cell

Telophase

• Nuclear envelope forms at each pole

• Chromosomes uncoil• Cells begin to divide.

Citokinesis

• Occurs after Mitosis is complete, is the division of the cytoplasm.

• End Product: 2 identical cells

• Occurs within gonads (testes:ovaries)• Meiosis produces sex cells – gametes (sperm:egg)• Gametes have half the chromosomes (23) that somatic

cells do (46)• Meiosis reduces the number of chromosomes by one-half

5.6 Meiosis

Meiosis

• Meiosis I: normal division (like mitosis)• Meiosis II: second division: results in haploid

cells, with only 23 chromosomes: gametes

• There are millions of possible combinations of genes that each parent can produce because of:– Random alignment of

homologous pairs– Crossing over– Random Fertilization

(70 trillion)

Meiosis contributes to Genetic Variation

Crossing Over

• Homologous chromosomes line up and exchange segments of DNA.

*somatic cells*divide once diploid*forms 2 identical cells

*gametes*divide twicehaploid*forms 4 different cells (crossing over)

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html#

Controls in the Cell Cycle

• Checkpoints exist in the cell cycle

• Cell determines if cell is ready to enter next part of cell cycle

5.4 Cell Cycle Controland Mutation

http://highered.mcgraw-hill.com/olc/dl/120082/bio34a.swf

• Cancer begins when the proteins that regulate the cell cycle don’t work, the cell divides uncontrollably– Mutations can be inherited or induced by exposure to U.V. radiation or carcinogens

that damage DNA and chromosomes

5.1 What Is Cancer?

Chromosome terminology

Chromosome terminology

• Chromatin: loose DNA in the nucleus• Unduplicated chromosome: Chromatid: tightly

wound single copy of DNA segment• Duplicated Chromosome: replicated, tightly

wound DNA in nucleus, 2 copies of same DNA segment (made of 2 chromatids: sister chromatids)

Homologous Chromosomes

• You have 2 copies of each chromosome, one from Mother, one from Father. (they make up each “pair”)

• These matching chromosomes are called homologous chromosomes.

• The genes on these chromosomes will code for the same things (hair color) but will code for different versions (blonde vs. brown)

Crossing over

• Crossing over during meiosis (prophase I) occurs between these homologous chromosomes, leading to genetic variation.

Humans