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All Cells Come From Cells
• Most of the cells on the outer layer of your
skin are dead
• Underneath the dead cells are living cells
providing life functions.
• The dead cells need to be replaced – the
regeneration of cells from other cells is
called mitosis
• Mitosis is necessary to both replace
damaged cells and to allow growth of an
• During mitosis, DNA is duplicated and the
cell splits. The resulting 2 cells are
genetically identical – this is called
asexual reproduction – no genetic
material (DNA) has been mixed
• Sexual reproduction involves the
combining of genetic material (DNA) – the
resulting cells or organisms are not
genetically identical to each other
Chromosome structure
• Our genetic material consists of chromatin, a
combination of DNA and protein molecules.
• When cells divide, the chromatin fibers condense,
becoming visible under a microscope as the structures
called chromosomes.
• Human body cells each have 46 chromosomes. Each
chromosome may contain many hundreds of genes.
The chromosome duplicates before mitosis begins
The centromere holds the two sister chromatids
When the two sister chromatids separate they are
genetically identical
Cell Cycle – all the processes that
result in new cells from old cells
• Interphase – Cells are functioning –– This is when the
cell is functioning – fulfilling life functions – Cells are
MOST OFTEN in interphase
• Prophase – DNA duplicates and uncoils as the cell
prepares for Mitosis
• Metaphase – chromosomes line up in the center of the
cell – this is part of the organizing part of mitosis
• Anaphase – The chromosomes are divided and moved
to opposite sides of the cell
• Telophase/Cytokinesis – the cell now splits into 2
identical cells
Prophase – DNA
becomes visible –
nuclear envelope
breaks up
Structures of mitosis
• During Prophase the DNA uncoils and
becomes visible
• Spindle fibers help to direct the
chromosomes as they line up during
• These spindles continue to direct
movement during anaphase as the
chromosomes are divided and pulled apart
Telophase and cytokinesis
• In animals, during telophase an
indentation appears and the two cells
• In plants, a cell plate forms that separates
the two cells
• Cytokinesis is the completion of the
creation of the 2 cells
Regulation of mitosis
• Hormones and
enzymes control the
processes involved in
the cell cycle
• When new cells are
needed, the cell is
large enough and
DNA is duplicated
mitosis can proceed.
• G1 and G2 are “checkpoints” where the
cells pause before continuing with mitosis.
• Cancer results when the cells do not
pause here, and just continue to grow.
• A tumor of growing cells is the result
Meiosis – reproduction of sex cells
• Sperm and egg are the sex cells that we
call gametes
• Meiosis produces 4 gametes that contain
½ of the total chromosomes of the parent
• DNA is also combined, restoring the full
DNA count
• Cells containing the entire chromosome
count are considered to be diploid – all of
our body cells are diploid
• Sex cells, containing only ½ the
chromosome count are described as
• When gametes combine during fertilization
a diploid zygote is formed
Fertilization restores full
chromosome count
•During fertilization, sperm
and egg combine DNA
This combining
of DNA is
• The stages of meiosis are very similar to
• However, there are 2 cycles
• The first cycle is the same as mitosis –
creating 2 cells – with a combination of
DNA derived from the male parent and
female parent
• These chromosomes are described as
homologous – They code for the same
genes, but different versions of the gene
Prophase II
• In the second cycle,
during prophase II the
DNA is NOT duplicated
• During prophase II of
meiosis, some genetic
material from the
chromosomes switches
from one homologous
chromosome to another
• Crossing over is genetic
recombination and
creates greater genetic