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 theSAME 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 FOURnuclei 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
  • 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
  • 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.
  • Autosomes:NON-SEX CHROMOSOMES, # CHROMOSOMES (22 PAIR = 44 TOTAL)
  • 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
  • CHROMOSOMES become visible (sister chromatids held together by CENTROMERE)
  • CENTROSOMES (containing centrioles in animal cells) move towards opposite poles
  • SPINDLE FIBERS (made of microtubules) appear and begin to STRETCH across the cell
  • KINETOCHORE PROTEINS (attached to centromere) attach to spindle fibers
  • Nuclear membrane and nucleolus BREAK DOWN (DISAPPEAR)
  • METAPHASE: SHORTEST PHASE – Chromosomes are MOST VISIBLE
  • Sister chromatids line up at the center of the cell (MEET IN THE MIDDLE)
  • Microtubules connect the centromere (kinetochore) of each chromosome

to the two poles of the spindle

  • ANAPHASE:
  • CENTROMERE DIVIDES and liberate sister chromatids (now considered individual chromosomes
  • Microtubules shorten and pull chromosomes towards opposite poles (AWAY/APART from one another)
  • Animal cell begins to elongate
  • TELOPHASE: REVERSE OF PROPHASE
  • Chromosomes uncoil CHROMATIN
  • Nuclear membrane and nucleoli REFORM, REAPPEAR
  • Spindle breaks apart
  • 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 4haploid (N) nuclei from one DIPLOID (2N) cell.
  • Consists of TWO divisions - Meiosis I and MeiosisII(b/c nucleus can only divide into two cells at a time)
  • DNA replication occurs before Meiosis I
  • In Meiosis I, TETRADSare 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 ITETRADS LINE UP METAPHASE PLATE (HELD TOGETHER BY CHIASMA)

ANAPAHASE ITETRADS SPEARATE; HOMOLOGOUS PAIRS SEPARATE

TELOPHASE ISAME 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 IIANAPHASE IITELOPHASE 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  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  mature sperm

Secondary spermatocyteSpermatid  mature sperm Primary Spermatocyte

Secondary spermatocyteSpermatid  mature sperm

Spermatid  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