Cell Size Limitations
 Most cells are 2-200 µm (micrometer) in diameter


1) Diffusion limits cell size
 If larger, cell can not diffuse substances in & out efficiently
2) DNA limits cell size
 Only so much protein can be made at one time from DNA
 3) Surface Area to Volume Ratio limits size
 If cell size increases, cell membrane can not keep up with needs

Cell Division
 “All cells are made from preexisting cells through cell division” (Cell Theory)
 Cell division results in 2 new cells, identical to the original
 Helps organisms grow larger, develop, change, heal, etc

DNA
 DNA:
Deoxyribonucleic
Acid
 In nucleus of eukaryotes
 Contains genetic info
 Composed of nucleotides
(monomers)

Chromosomes
 When cells divide, chromosomes develop from DNA
 Chromosome: DNA & protein in a coiled, rod-shaped form
 Histones: proteins that help DNA form chromosomes
 Together, DNA & a histone form a unit called a
Nucleosome

Nucleosome Structure

Chromosome Structure
 Chromosomes have
2 identical parts called sister chromatids
Chromatid →
 Connected by a centromere
Centromere
Chromatid
↓

The Cell Cycle
 Cell Cycle: the repeating set of events that make up the life of a cell
 Interphase: time between divisions
(G
1
, S, G
2 phases)
 M phase: when division occurs
(Mitosis)

Phases of Cell Cycle
 Interphase: most of cell’s lifetime


1) G
1
: cell growth
2) S: DNA replication (copied)
 3) G
2
: centrioles form & cell prepares to divide
 Mitosis: division of nucleus
 1) Prophase
 2) Metaphase
 3) Anaphase
 4) Telophase
 5)Cytokinesis

Pre-Mitosis Cell

Phases of Mitosis
 1) Prophase
 Chromosomes form
 Nuclear membrane breaks down
 Centrioles and spindle fibers appear
 Prophase
The arrow is pointing to the dark region, which is condensing chromatin

Phases of Mitosis
 2) Metaphase
 Spindle fibers move chromosomes to the center of the cell
 Fibers attach to each sister chromatid at the centromere
 Metaphase
Ch = chromosome,
S = spindle

Phases of Mitosis
 3) Anaphase
 Sister chromatids separate and move to opposite sides of cell
 once apart, are considered individual chromosomes
 Anaphase
Cell in mid anaphase

Phases of Mitosis
 4) Telophase
 Spindle fibers dissolve and nuclear membrane reforms
 Chromosomes unwind back into chromatin
 Telophase
Cell in early telophase. The arrow points to the formation of a cell plate.

Phases of Mitosis
 5) Cytokinesis
 division of the cytoplasm

Cleavage Furrow: area that pinches in to form 2 separate cells in animals

Cell Plate: forms cell walls around each new cell
The arrow is pointing to the cell plate, which divides the cell into two new cells.

Live Mitosis!!
 Mitosis
 http://www.phschool.com/science/biology_pla ce/biocoach/mitosisisg/process.html

Control of the Cell Cycle
 Cell cycle is controlled by genes (DNA)
 If genes become damaged, cells undergo mitosis uncontrollably!!
 Sudden growth & reproduction of cells creates tumors
 Benign Tumor: non-harmful; stays in place
 Malignant Tumor: harmful b/c spreads throughout body

Causes of Cancer
 1) Genetics: inherit from parents
 2) Chemicals: cigarette tar, food preservatives, pollution
 3) UV Radiation: overexposure to sunlight
 4) Viruses: herpes, HIV

Skin Cancer
 Most common form
 Caused by overexposure to UV rays
 Types:
 1) Basal Cell: most common form; easy to treat
 2) Squamous: affects upper layers of skin
 3) Malignant: most deadly and fastspreading; found in moles

Chapter 8

Meiosis
 “Sexual reproduction”
 Produces Gametes : human reproductive cells (sperm/egg)
 Cells begin with 2 sets of chromosomes
 Must divide twice to become 4 single cells
 Occurs in 2 stages:
 1) Meiosis I
 2) Meiosis II

Types of Chromosomes
 2 Types:
 Sex Chromosomes: determine sex of organism (male/female)

Male: XY

Female: XX

Humans have 1 pair
 Autosomes: all others

Humans have 22 pairs

Homologous Chromosomes
 Organisms produced by sexual reproduction
(2 parents) have
2 copies of each autosome
 Homologous Chromosomes: carry genes for same traits
 1 from each parent
 same size & shape

Karyotypes
 Karyotype: a picture of an individual’s chromosomes
 May be used to identify genetic disorders
 Humans:

22 autosome pairs

1 sex chromosome pair
Homologous
Chromosomes
↓
Sex Chromosomes
↑ ↑

Phases of Meiosis I
 1) Prophase I Prophase I
 Same as Mitosis, except …
 Homologous chromosomes pair up to form a tetrad
 Chromatids twist around each other & exchange info
 Crossing-over: creates new mixture of genetic material

Maternal & Paternal DNA exchanged

Crossing Over
Tetrad →

Phases of Meiosis I
 2) Metaphase I
 Tetrads line up randomly along center of cell
 Metaphase I

Phases of Meiosis I
 3) Anaphase I
 Each homologous chromosome moves to either side of cell
 Independent Assortment: random separation of homologous chromosomes to opposite sides of cell

Creates genetic variation

Anaphase I

Phases of Meiosis I
 4) Telophase I & Cytokinesis I

Same as Mitosis, except…
 New cells contain 1 chromosome from each homologous pair

Different genetic material than original

Telophase I & Cytokinesis I

Phases of Meiosis II
 1) Prophase II
 Chromosomes form and spindles attach
 Prophase II

Phases of Meiosis II
 2) Metaphase II
 Chromosomes move to center of cell
 Metaphase II

Phases of Meiosis II
 3) Anaphase II
 Chromatids separate and move to either side of cell
 Anaphase II

Phases of Meiosis II
 4) Telophase II & Cytokinesis II
 Nuclear membrane reforms
 4 new cells form (gametes)
 Telophase II & Cytokinesis II

Results of Meiosis
 After meiosis has occurred, 4 new cells are created
 All genetically different
 Will become 4 new sperm or 1 new egg

Eukaryotic Cell Division
 Mitosis: results in new cells with identical genetic material as original
 Used for growth, repair, etc.
 Meiosis: cuts chromosome # in half (to 23)
 2 different cells must later join to form 1 whole cell

“sexual reproduction”

Diploid & Haploid Cells
 Diploid Cells: cells with 2 sets of chromosomes - Mitosis


Describes all human cells except sperm & egg
Abbreviated “2n”
 i.e.: Human 2n = 46
 Haploid Cells: cells with 1 set of chromosomes - Meiosis


Describes human sperm & egg cells
Abbreviated “1n”
 i.e. Human 1n = 23

Binary Fission
 Binary Fission: division of a prokaryotic cell into 2 identical offspring cells
 Steps:
 1) Cells makes copy of DNA

2) Cell grows to 2x’s normal size
 3) Cell wall forms, splitting into 2 cells, each with own copy of DNA

Binary Fission (con’t)

Chromosome #’s
 Different species have different #’s of chromosomes inside their nucleus
 Humans have 46
 Lettuce only has 18
 But, dogs have 78!!
 (more chromosomes ≠ more complex!!)