Chapter 10 and 11.4 NOTES – Mitosis and Meiosis

A. Overview of Mitosis & Meiosis – two types of NUCLEAR division (DIVISION OF the CELL NUCLEUS)

MITOSIS

·  Produces TWO nuclei with the SAME number of chromosomes as parent cell

·  Cells are genetically IDENTICAL to parent cell and each other

·  SOMATIC (body) cells create new SOMATIC cells

·  In unicellular organisms, mitosis makes NEW ORGANISMS (PROKARYOTES = BINARY FISSION)

·  In multicellular organisms, mitosis used for REPAIR, GROWTH, and REPRODUCTION

MEIOSIS

·  Produces FOUR nuclei with HALF the number of chromosomes as parent cell

·  Cells are genetically DIFFERENT from parent cell and from each other

·  GERM cells produce GAMETES (sex cells – sperm & egg; plants = spores)

·  Vital to SEXUAL reproduction & leads to genetic variety w/in the species

B. Eukaryotic Chromosomes

·  Chromatin is composed of DNA and PROTEIN, contain genetic information

o  DNA is wound around HISTONE proteins, the histone molecules and DNA form beadlike structures called NUCLEOSOMES

·  Chromosomes – super coiled CHROMATIN, can only be seen during cell division

o  Before division, chromosomes replicate and become attached to their identical copy – called SISTER CHROMATIDS (two identical chromatids attached by a CENTROMERE)

·  Humans have 23 pairs of chromosomes = 46 total; one from each pair from your MOM and one from your DAD.

o  Autosomes: NON-SEX CHROMOSOMES, # CHROMOSOMES (22 PAIR = 44 TOTAL)

o  Sex Chromosomes: LAST PAIR

Female = Two X CHROMOSOMES (XX)

Male = One X and One Y (XY)

·  Ploidy = number of sets of chromosomes

Haploid (n)= CELL CONTAINING A SINGLE SET OF CHROMOSOMES (SEX CELLS)

Diploid (2n)= CELL CONTAINING TWO SETS OF HOMOLOGOUS CHROMOSOMES (BODY CELLS)

C. The Cell Cycle = INTERPHASE (G1,S,G2) + MITOTIC PHASE (MITOSIS AND CYTOKINESIS)

D. STAGES OF MITOSIS (PMAT – NUCLEAR DIVISION)

·  PROPHASE: Longest Phase

o  CHROMOSOMES become visible (sister chromatids held together by CENTROMERE)

o  CENTROSOMES (containing centrioles in animal cells) move towards opposite poles

o  SPINDLE FIBERS (made of microtubules) appear and begin to STRETCH across the cell

o  KINETOCHORE PROTEINS (attached to centromere) attach to spindle fibers

o  Nuclear membrane and nucleolus BREAK DOWN (DISAPPEAR)

·  METAPHASE: SHORTEST PHASE – Chromosomes are MOST VISIBLE

o  Sister chromatids line up at the center of the cell (MEET IN THE MIDDLE)

o  Microtubules connect the centromere (kinetochore) of each chromosome

to the two poles of the spindle

·  ANAPHASE:

o  CENTROMERE DIVIDES and liberate sister chromatids (now considered individual chromosomes

o  Microtubules shorten and pull chromosomes towards opposite poles (AWAY/APART from one another)

o  Animal cell begins to elongate

·  TELOPHASE: REVERSE OF PROPHASE

o  Chromosomes uncoil à CHROMATIN

o  Nuclear membrane and nucleoli REFORM, REAPPEAR

o  Spindle breaks apart

o  FOLLOWED BY CYTOKINESIS, mitosis is complete – each side of the cell will become a new cell with the same number of chromosomes as the original/parent cell

E. CYTOKENESIS – CYTOPLASMIC DIVISION

·  Animal Cell – cell membrane pinches in forming a “CLEAVAGE FURROW” eventually the cell will split into 2 identical cells called daughter cells

·  Plant Cell – CELL PLATE forms in the middle and extends outward; eventually becomes new cell wall and cell membrane

F. MEIOSIS

·  Creates 4 haploid (N) nuclei from one DIPLOID (2N) cell.

·  Consists of TWO divisions - Meiosis I and Meiosis II (b/c nucleus can only divide into two cells at a time)

·  DNA replication occurs before Meiosis I

·  In Meiosis I, TETRADS are separated, in Meiosis II, SISTER CHROMATIDS are separated. (meiosis II looks just like mitosis)

MEIOSIS I (M-I)

PROPHASE I *BIGGEST DIFFERENCE

1) Chromosomes condense

2) Synapsis: formation of TETRADS - when HOMOLOGOUS pairs (each existing as a pair of sister chromatids) find one another.

·  Homologous Chromosomes – CHROMOSOMES THAT ARE SIMILAR IN SIZE, SHAPE, AND ARRANGEMENT OF GENES

3) Crossing Over: chromatids of homologues exchange equal portions (usually). CHIASMA = cross-over exchange point. “New” chromatids are called RECOMBINANTS.

METAPHASE I TETRADS LINE UP METAPHASE PLATE (HELD TOGETHER BY CHIASMA)

ANAPAHASE I TETRADS SPEARATE; HOMOLOGOUS PAIRS SEPARATE

TELOPHASE I SAME AS MITOSIS, HOWEVER, SISTER CHROMATIDS ARE STILL TOGETHER, FOLLOWED BY CYTOKINESIS

MEIOSIS I = REDUCTION – CHROMOSOME NUMBER IS REDUCED BY 1/2

BETWEEN M-I AND M-II NO INTERPHASE – NO DNA REPLICATION (some cells have interkenesis where chromosomes become uncoiled and then have to coil again)

MEIOSIS II (M-II)

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II

SAME AS MITOSIS (BUT HAVE 2 CELLS) END WITH 4 N CELLS

G. Genetic Reassortment - leads to greater genetic VARIETY (Why you have characteristics of both parents.)

·  Two sources from Meiosis:

1) GENETIC RECOMBINATION – new combinations of genes on chromosomes created by CROSSING OVER during PROPHSE I

- Results in creation of chromosomes with genes from both parents on them.

- Humans average crossing over 2-3X per homologous pair

2) New assortments of chromosomes (Independent Assortment) – lining up at the equator during METAPHASE I is completely random

·  Third source from FERTILIZATION: random fusion of sperm & egg

ALL THREE LEAD TO GENETIC VARIATION à RAW MATERIAL FOR EVOLUTION

Check Mitosis, Meiosis or Both / Mitosis / Meiosis / Both
No pairing of homologous chromosomes / X
Two divisions / X
Four daughter cells produced / X
Associated with growth and asexual reproduction / X
Associated with sexual reproduction / X
One division / X
Two daughter cells produced / X
Involves duplication of chromosomes / X
Chromosome # maintained / X
Chromosome # is halved / X
Crossing over between homologous may occur / X
Daughter cells are identical to parent cell / X
Daughter cells are not identical to parent cell / X
Produces gametes / X
Synapsis occurs in prophase / X

H. Gamete Formation - Gametogenesis

SPERMATOGENESIS – Production of male gametes: SPERM (TESTES)

SPERMATOGONIA = male germ cells; divide by mitosis to create new germ cells, some will become

PRIMARY SPERMATOCYTES = cells that undergo meiosis

M-I M-II

Spermatid à mature sperm

Secondary spermatocyte Spermatid à mature sperm Primary Spermatocyte

Secondary spermatocyte Spermatid à mature sperm

Spermatid à mature sperm

Anatomy of a sperm:

Head – genetic material

Midpiece - contains lots of mitochondria

Tail – flagella for movement

Acrosome – cap-like structure; produces enzymes for egg penetration

OOGENESIS – Production of female gametes: Eggs (OVA) (largest cells in female body)

OOGONIA = female germ cells; divide by mitosis to create new germ cells, eventually become

PRIMARY OOCYTE = cells that undergo meiosis

Different than spermatogenesis because cytokenesis divides cells unequally, resulting in one cell (EGG) being larger than others (POLAR BODIES) each time.

M-I M-II

EGG

Secondary Oocyte Polar Body

Primary Oocyte (unequal division)

(unequal division) Polar Body Polar Body disintegrate

Polar Body

Interesting note: P-I occurs before birth, resuming at puberty.

Why Split unevenly? Egg cell will absorb nutrients and stockpile materials needed for early embryonic development