BIOLOGY 1111

CHAPTER 9

CELL DIVISION

the process of cell division is essential to the growth and development of all organisms

growth and tissue repair are common occurrences

abnormal cell division is commonly called cancer 

DNA REPLICATION

parent cells must  provide daughter cells with hereditary instructions and enough cytoplasmic machinery to start up with their own operation

cells divide DNA by:

Mitosis

occurs in somatic cells

Meiosis

occurs in germ cells

CHROMOSOMES

Chromosomes are the genetic material found inside the nucleus of the cell

Consist of condensed DNA and proteins

DNA controls protein synthesis

Controls the appearance of characters and metabolic activities of an organism

chromosomes are condensed DNA molecules

while in a threadlike form chromosomes undergo duplication
each duplicated strand is called a sister chromatid

duplicated chromosomes narrow to a small region called the centromere

Each species has a fixed number of chromosomes in the nucleus of each of its cells

Chromosomes always exist in pairs in the body cells

SOMATIC CELLS

somatic cells in each species of organism will have the same number of chromosomes specific to that species

ex: humans = 46                                                                    

            gorillas = 48                                                                                  

            pea plant = 14

DIPLOID

cells which have two of every chromosome are said to be diploid

ex: humans = 23 pair

ex: gorillas = 24 pair

diploid cell produces two identical diploid daughter cells

enables the chromosome number to remain constant generation  after generation

cells function properly only if they receive one of each of the 23 different chromosomes 

CELL CYCLE

Mitosis                          M

Gap phase 1                G₁

Synthesis phase         S

Gap phase 2                G₂

M    (mitosis)

nuclear division

cytoplasmic division

G₁    (gap 1)

interval before the onset of DNA replication

S    (synthesis)

replication of DNA

G₂     (gap 2)

interval between completion of DNA replication and the onset of mitosis

INTERPHASE

usually the longest phase of the cell cycle

consists of the G1,S, and G2 phases of the cell cycle

the cell increases in mass

chromosomes cannot be seen
 chromosomes are duplicated   

STAGES OF MITOSIS

Prophase
Metaphase
Anaphase
Telophase

PROPHASE

chromosomes start condensing

chromatids twist and fold on one another

spindle fibers begin to form
3) centrioles are duplicated and begin to migrate to opposite poles

centrioles later give rise to cilia and flagella

METAPHASE

nuclear membrane disappears

spindle fibers attach to chromosome centromeres

spindle fibers from opposite poles “pull” on the chromosomes orientating them at the equator

chromosomes become aligned at the cell equator

ANAPHASE

the two sister chromatids of each chromosome are separated

chromatids move to opposite poles

TELOPHASE

the daughter chromosomes arrive at opposite poles
chromosomes begin to unwind and decondense to a threadlike DNA molecule
nuclear membranes form, enclosing the DNA
each new daughter cell has the same number of chromosomes as the parent cell

 DIVISION OF THE CYTOPLASM

the cytoplasm divides during Telophase
cytokinesis- cytoplasmic division
plant cells form a cell plate  during cytokinesis

the cell plate is converted to a new cell wall dividing the daughter cells

animal cells “pinch in two” by a process called cleavage 

END