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