Transcript Why do Cells Divide?

Why do Cells
Divide?
Cell Size Limits
Animal & Plant Cells
DNA Replication
1. Replication (DNA DNA)
DNA Replication
Replication = DNA copies itself exactly
(Occurs within the nucleus)
 Any mistake in copying = mutation
 DNA mutation = chromosomal mutation
A. Basic Facts of DNA
Replication
1. Complementary
base pairing
makes
replication possible
C-G
A-T
A. Basic Facts of DNA
Replication
2. One side of DNA
molecule is
a
template
for
making
the
other side
(strand)
A. DNA Replication-occurs during “S” phase
of interphase
1. Occurs in the nucleus prior to any cell
division
2. Enzyme is used to “unzip” or “unwind” the
DNA
a. Forms a bubble at the origin site
B. Process of DNA
Replication
1. Uncoil (topoisomerase & unzip DNA molecule
 Enzyme (-ase) breaks
weak Hydrogen Bonds
between bases
B. Process of DNA
Replication
2. Enzyme brings in complementary N-bases
DNA Polymerase
B. Process of DNA
Replication
3. Insert N-bases
4. Seal bonds with ligase
Replication Moovie
C-G
A-T
C. Semi-conservative
replication
1. Each new DNA
molecule
contains
one old strand
&
one new strand
DNA replicationreplication bubbles
DNA replicationleading/lagging strands
DNA replication
DNA Replication (cont.)
3. Another enzyme is used to build a
complementary strand of DNA from
the template piece of original DNA
a. Nitrogenous bases pair up
1. A – T
2. C - G
4. As a result, you create two identical
strands of DNA
Let’s Practice
 Replicate the following strand of DNA
using the correct nitrogenous bases:
ATCGGCTATTAGGCATATCCGACGGTC
TAGCCGATAATCCGTATAGGCTGCCAG
Why do cells divide?
A. If they get too big
 Surface area to volume ratio, etc.
 Diffusion is fast over short distances but slow
over long distances
 So, b/c of slow rate of diffusion, cells can’t be
giants b/c they would starve to death or be
poisoned from the build up of wastes!!
 When you increase the size of the cell, the
vol. increases faster than the surface area of
the cell membrane!!
Why do cells divide?
B. Growth of organism
 In order for an organism to grow, it needs more cells
C. Repair
 New cells replace old/damaged ones
D. Reproduction
 ASEXUAL- offspring is identical to parent
 SEXUAL- offspring is a combo of both parents
Cell Division
 Mitosis
 Occurs in
somatic cells
(body cells)
 Daughter cells
are exactly like
parent
 Diploid
 2n
 In humans n=23
Meiosis
– Produces gametes
(germ cells/sex cells)
– Daughter cells have
half the chromosomes as parent
“Reduction division”
– Haploid
1n
Why?
Mitosis- divides somatic cells
to get tissues and organs
built; makes individual grow
into an adult
DNA VOCAB
1. Chromatin when a cell is not dividing it’s DNA is a tangled mass of thin
threads.
2. Chromosome When the DNA is ready to divide it condenses into individual
chromosomes
 In humans- there are 23 HOMOLOGOUS PAIRS of chromosomes
 The two chromosomes in a pair are SIMILAR but not exactly alike
 One is from mom and one from dad
3. Sister Chromatid When the DNA replicates (to make one copy for each cell) the
two copies join together by a centromere and each copy is called
a SISTER CHROMATID
 These sisters are EXACTLY ALIKE!
Chromatin to Chromosomes
TYPICAL HOMOLOGOUS
CHROMOSOME PAIR
Before & After DNA
replication (“S” phase)
The Cell Cycle
(Somatic Cells)
A. INTERPHASE: growth and
preparation
1. G1
2. S
3. G2
B. MITOSIS: Nucleus divides
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
C. CYTOKINESIS: Division of
cytoplasm
Reminder from cell theory
 All cells come from pre-existing
cells!!
-Cell Division follows this b/c it’s the
process where new cells are made
from 1 cell
 Cell Division makes 2 new daughter
cells that are identical to the parent
cell
Cell Cycle- The life of a cell
Cell Cycle- Interphase
Interphase (Nucleus enclosed in
nuclear membrane, nucleolus
present, chromosomes appear as
chromatin (in other words,
chromosomes are not yet visible))

a) G1 phase- intense cellular
activity and growth; organelles
double

b) S phase- DNA synthesischromosomes replicate
(copy)
Chromatin to Chromosomes
Interphase continued
 c) G2 phase- spindle fibers are
assembled/ centrioles
replicate
- cell prepares for cell division
Interphase Diagram
MITOSIS PHASES
Prophase (1st stage)
Prophase- 1st stage of actual cell division
 a.) Chromosomes condense and shorten,
become distinguishable
b.) Chromatids attached near center by
centromere
 c.) Centrioles separate, migrate to the poles
 d.) Spindle fibers form between centrioles,
asters form
 e.) Nuclear membrane disintegrates,
nucleolus disappears (in order to
separate contents of nucleus, it must
disappear)
Prophase
Early prophase
Late prophase
Metaphase(shortest)
nd
2
stage
 a.) chromosomes line up in pairs
at equator (center of cell)
 b.) centromere of each
chromosome is attached to a
separate spindle fiber
Metaphase diagram
Metaphase
Anaphase- 3rd stage
 a.) Centromere of each chromosome
separates
 b.) Separation of the chromatids in each
pair
 c.) Spindle fibers appear to shorten,
pulling the chromatids apart at the
centromere (now called
chromosomes)
 d.) migration of the chromosomes ends
with the arrival at the poles and the
formation of clusters
Anaphase diagram
Anaphase
Telophase – 4th stage
 a.) Cleavage furrow forms b/w 2 cells
and spindle fibers disappear
 b.) Nuclear membrane forms around
each set of chromosomes (forms 2
new nuclei)
 c.) Chromosomes uncoil to form
chromatin net (getting ready for
interphase)
 d.) Nucleoli reorganized
Telophase Diagram
Telophase
Cytokinesis
 Cytokinesis- division of the
cytoplasm
 In animal cells- cell membrane
pinches together, furrow forms along
the equator.
 -In plant cells- cell plate forms in the
middle of the dividing cell