A: Because there are limits to Cell Growth

Two main reasons why cells need to divide rather than grow indefinitely

1. DNA Overload: Larger cells place more demand on its DNA DNA in small cells is able to meet the cells

NEEDS

Think of a city- large cities can’t be run by one small town hall- it has to be split up to be managed effectively.

2. Exchange of Materials Larger cells have more trouble moving NUTRIENTS

and waste across the cell membrane and through the cytoplasm The RATE OF MOVEMENT of food, water, nutrients, and

waste exchange depends on the surface area of the cell

May lead to the cell not getting food and water into the cell and waste out

Chromosomes Rod shaped structures made of DNA and proteins

Each chromosome is a single DNA molecule with proteins

In eukaryotes DNA wraps tightly around proteins called histones

Chromosome structure

Structure of a chromosome

Consists of 2 identical halves Each half is called a

chromatid When cells divide

each of the 2 new cells receive one chromatid

The center of each chromosome is called a centromere

Chromosome numbers Humans and animals have sex

chromosomes and autosomes Sex chromosomes determine the gender of

an organisms For humans X or y

Males XY Females XX

2 sex chromosomes All other chromosomes are called autosomes

44 autosomes

46 total chromosomes in humans

Homologous chromosomes

Each organism produced via sexual reproduction has 2 copies of autosomes One comes from each parent The two copies are called homologous chromosomes

or homologues Are the same size & shape and carry genes for the same

traits

Called- Binary fissionThe division of a prokaryotic cell into two different

offspring

3 general stages1. DNA is copied2. Cell begins to divide3. Two identical haploid cells are made

Most prokaryotes reproduce by binary fission, in which 2 identical cells are produced from one cell.

This process is asexual- meaning just one cell is involved

The cell cycle Interphase

Mitosis Prophase Metaphase Anaphase telophase

Interphase Most of the cell’s lifetime

is spent in this cycle

Busiest part of the cell cycle Making of ATP Repairing themselves Excreting wastes Making proteins Producing new

organelles Copying

chromosomes

Mitosis Division of the nucleus, which occurs during cell

division

Continuous process divided into 4 phases Prophase Metaphase Anaphase Telophase

PMAT

Mitosis- division in eukaryotes

Process in which a cells nucleus replicates and divides in preparation for division of the cell. Result: two cells

genetically identical a necessary condition for the normal functioning of

virtually all cells. Used for

repair and replacement of damaged or worn out cell asexual reproduction, or reproduction without eggs

and sperm.

Prophase1st Phase(PACK)

Chromatin condenses (PACKS) into chromosomes

Centrioles separate

Spindle begins to form- microtubule structure helps to separate chromosomes

Nuclear envelope disappears

Metaphase- 2nd Phase (MIDDLE)

Chromosomes line up across the center (middle) of the cell

Chromosomes attach to the spindle fiber at its centromere

Anaphase- 3rd Phase (APART)

Sister chromatids separate (move apart) into individual chromosomes

Spindle fibers pull the chromosomes to opposite sides of the cell

Telophase- Final Stage (TWO)

Chromosomes gather at opposite ends of the cell and lose their shape

Two new nuclear membranes form

Nucleolus reforms (darkens)

Cytokinesis Occurs with Telophase

Each daughter cell has an identical set of chromosomes

The cytoplasm pinches in half in animal cells- called a cleavage furrow

In plants, a plate forms midway between the two nuclei and helps form the cell wall.

In short… Chromosomes PACK in Prophase

They line up in the MIDDLE in Metaphase

Sister chromatids (each half of the chromosome) move APART in Anaphase

Two new cells are formed in Telophase when cytokinesis occurs.

Division ControlDivision Control- Cells have internal & external regulators - Cells have internal & external regulators

that control cell division that control cell division

- Cancer cells do not respond to the - Cancer cells do not respond to the regulators so they have uncontrolled cell regulators so they have uncontrolled cell

growth and form tumors growth and form tumors

* benign tumors* benign tumors* malignant tumors (metastasize)* malignant tumors (metastasize)