Cell Division

Cell division

increases number of somatic (body) cells
mitosis and cytokinesis

Mitosis

division of the nucleus
occurs in body cells
diploid cells divide to produce diploid cells
daughter cells are genetically identical to parent cells

cytokinesis

division of the cytoplasm
accompanies mitosis
newly formed cells receive a share of organelles made during interphase

Apoptosis

cell death; decreases the number of cells

Reasons for cell division

body growth, maintenance and repair, fighting infection, replacing worn/dead cells

Reasons for apoptosis

tail of tadpole disappears - frog
skin between human fingers and toes dies during development

Cell increase and decrease

both occur during normal development and growth

The Cell Cycle

an orderly sequence of events that occurs from the time a cell is first formed until it divides into two new cells

Interphase

G1 stage, S stage, and G2 stage
most of the cell cycle is spent in this phase

G1 stage

cell growth, cell doubles its organelles (cell structures), prepares for DNA replication

S stage

DNA replication occurs

G2 stage

cell makes proteins needed for cell division

G0 phase

nerve cells and muscle cells exit the cell cycle

M stage

follows interphase in the cell cycle
includes mitosis and cytokinesis

Control of the cell cycle (3 checkpoints)

1. During G1 - if DNA is damaged, apoptosis occurs
2. During G2 - will not proceed if DNA is damaged or not copied
3. During the M stage - if chromosomes are not properly aligned

Chromosome Structure

In a non-dividing cell - genetic material is in the form of chromatin (DNA and protein)
In a non-dividing cell - chromatin undergoes coiling to form chromosomes

Histones

package DNA so it can fit into the nucleus

Sister Chromatids

the two halves of a chromosome

Centromere

holds sister chromatids together

Number of chromosomes in humans

46 chromosomes

diploid number (2n)

2 chromosomes of each kind (1 from each parent)
in body cells

haploid number (n)

1 chromosome of each kind
in sex cells (egg and sperm)

late interphase

centrosomes (which contain pair of centrioles and an aster - which are short microtubules) duplicate
chromatin condenses into chromosomes

early prophase

chromosomes become visible
centrosomes move to opposite ends of the cell
nuclear membrane and nucleolus disappear

late prophase

spindle fibers form
chromosomes become attached to spindle fibers - centromere attaches to spindle fibers

metaphase

chromosomes line up at metaphase plate - equidistant from poles

anaphase

centromeres holding sister chromatids divide
sister chromatids separate, becoming daughter chromosomes, and move toward opposite ends of cell

telophase

spindle disappears
nuclear membrane and nucleolus reappear
chromosomes turn into chromatin

mitosis in plant cells

same phases as in animal cells
have centrosome and spindle, but no centrioles or asters

cleavage furrow

indentation of membrane where cell will divide

cytokinesis in animal cells

a cleavage furrow begins at the end of anaphase. A contractile ring slowly forms a constriction between the two daughter cells and completes the division

cytokinesis in plant cells

rigid cell wall surround plant cells cannot form a cleavage furrow. Instead a cell plate forms. New plant cell walls form and are later strengthened by cellulose fibers.

binary fission

the process of asexual reproduction in prokaryotes

cell division in prokaryotes

The two daughter cells are identical to the original parent cell, each with a single chromosome.
Following DNA replication, the two resulting chromosomes separate as the cell elongates
Cell divides without cell structures seen in plants & animals

Meiosis

Produces sex cells (gametes) - eggs & sperm
Reduces the chromosome number (2n -> n)
2 cell divisions and produces 4 haploid cells

Importance of Meiosis

Ensures that the next generation will have:
1. the diploid number of chromosomes
2. a combination of traits that differs from that of either parent

homologous chromosomes

chromosomes with the same genes
humans have 23 pairs

DNA replication

occurs prior to meiosis I

gametes

sex cells - eggs and sperm
what haploid daughter cells mature into

Fertilization

fusion of egg and sperm
restores the diploid number of chromosomes

Phases of meiosis

same four phases as mitosis (prophase, metaphase, anaphase, telophase) occur in both meiosis I and meiosis II

interkinesis

period of time between meiosis I and meiosis II
No replication of DNA occurs

synapsis

pairing of homologous chromosomes

Prophase I

nuclear membrane & nucleolus disappear
spindle forms
homologous chromosomes pair during synapsis

Metaphase I

homologous chromosomes line up at the metaphase plate

Anaphase I

homologous chromosomes separate

Telophase I

nuclear membrane and nucleolus reappear
cytokinesis occurs

Prophase II

Spindle reappears, nucleolus and nuclear membrane disappear, chromosomes attach to spindle

Metaphase II

chromosomes line up at the metaphase plate

Anaphase II

sister chromatids separate

Telophase II

spindle disappears, nuclear membrane and nucleolus reappear, cytokinesis divides the cells

Genetic Recombination

occurs in several different ways
reason why variation is endless in the human population

Genetic Recombination #1

crossing over - occurs during prophase I between non sister chromatids

Genetic Recombination #2

independent assortment of homologous chromosome - separate in a random manner

Genetic Recombination #3

combining of chromosomes of genetically different gametes during fertilization

Mitosis vs. Meiosis part 1

Mitosis:
-DNA replication occurs only once during interphase
-One cell division
-Two diploid daughter cells genetically identical to parent
Meiosis:
-DNA replication occurs only once during interphase
-Two cell divisions
-Four haploid daughter cells genet

Mitosis vs. Meiosis part 2

Mitosis:
-Daughter cells identical to each other
-Occurs in all somatic cells for growth and repair
Meiosis:
-Daughter cells are different from each other
-Occurs only in the reproductive organs for the production of gametes

The Human life cycle

requires both mitosis and meiosis

spermatogenesis

meiosis in males
produces four (4) haploid spermatids that mature into sperm with 23 chromosomes

oogenesis

meiosis in females
meiosis II will only be completed if sperm is present
Following meiosis II, there is one (1) haploid egg cell with 23 chromosomes and up to three (3) polar bodies

Polar bodies

serve as a dumping ground for extra chromosomes - will disintegrate

Sperm and egg cells

have 23 chromosomes

zygote

result of fertilization of the egg cell by a single sperm
has 46 chromosomes (diploid number)

Cell differentiation

occurs during development resulting in a variety of cell types