Download dna & cell division

UNIT V - DNA & CELL DIVISION
Big Campbell – Ch 12, 13, 16
Baby Campbell – Ch 8, 10.1-10.5
Hillis – Chp 7, 9
I. ASEXUAL REPRODUCTION
• Purpose
 Unicellular Organisms
Single-celled organisms like
bacteria and yeast use asexual
reproduction to make
new organisms – reproduce .
I. ASEXUAL REPRODUCTION
• Multicellular Organisms
Growth & Development –grow through cell division, rather than
unlimited growth because if a cell gets too large, it cannot meet its
own needs and maintain homeostasis.
Renewal & Repair –used to replace cells that die from normal wear
& tear; Ex: skin cells.
II. PROKARYOTIC ASEXUAL REPRODUCTION
• Binary Fission
o Asexual reproduction
o Much shorter than euk cell
cycle
 Single chromosome
replicates
 Each copy begins moving to
opposite ends of cell
 Cell elongates
 When bacterium is 2X
original size, cell membrane
pinches inward
 Cell wall deposited
 2 identical cells produces
Binary Fission, cont.
III. EUKARYOTIC CELL DIVISION – THE CELL CYCLE
• Can be divided into:
 Interphase
 M –Phase or Mitotic Phase
III. CELL CYCLE, cont
• Interphase
 Portion of cell cycle in which cell is
carrying out normal activities.
 Approx 90% of normal cell cycle is
spent in interphase.
 DNA found in chromatin form
 3 sub-phases
 G1 -Cell grows, carries out normal cell
activities. Organelles are replaced if needed.
 S or Synthesis - Replication of DNA; occurs
in nucleus, also known as “point of no return”
 G2 - Preparation for mitosis. Centrioles and
all organelles are replicated.
III. CELL CYCLE, cont
• Mitosis
 Nuclear division
 Requires all the cells energy, resources
 Last step is cytokinesis – splitting of the cell
III. CELL CYCLE, cont
Cell at the END of interphase
a. _DNA_ has been replicated – still
in chromatin form.
b. _Centrioles_ have been replicated in
_animal_ cells only. This replication
results in 2 pairs of centrioles,
composed of _microtubules__.
c. The _nuclear envelope_ and
_nucleolus are still present.
Interphase
PROPHASE
1.
2.
3.
4.
_Nucleolus_ and _nuclear envelope
disassemble.
Replicated DNA thickens & condenses 
Results in _chromosomes_ made up of 2 sister
chromatids
held together by the _centromere_.
_Centrioles_ begin moving to opposite sides of
the cell.
Microtubules_extend to form spindle fibers from
centriole to centriole. Some of these fibers,
known as kinetochore microtubules, begin to
attach at the kinetochore of each chromatid.
(located at the centromere). Other fibers, known
as non-kinetochore microtubules span the cell
to aid in support and structure.
Prophase
Late Prophase (Prometaphase)
III. CELL CYCLE, cont
METAPHASE
1. _Spindle fiber network_ network is
fully formed with _centrioles_ at
opposite ends.
2. Each _sister chromatid_ is attached
by their kinetochore (at the
centromere) to a spindle fiber from
each pole.
3. Sister chromatids align in _equator
(middle)__ of cell.
Metaphase
Anaphase
1. _Centromeres__ split.
2. Sister _chromatids_ are pulled
apart by _kinetochore
microtubules_ to opposite ends of
the cell.
3. Genetic material is now known as
daughter _chromosomes__.
4. Cell is elongating; preparing for two
new nuclei.
Anaphase
Telophase
1. Complete set of _chromosomes_ at
each pole of the cell.
2. Spindle fibers _ disassemble.
3. New _nuclear envelope_ forms around
nucleus.
4. chromosomes_ uncoil  _chromatin_.
5. nucleolus_ reforms  _ribosomes_ are
produced _protein_ synthesis
resumes  cell _metabolic activity_
resumes
Telophase
III. CELL CYCLE, cont
• Cytokinesis – IN ANIMAL CELLS
Cytokinesis in Plant Cells
Telophase and Cytokinesis
III. CELL CYCLE, cont
IV. CONTROL OF THE CELL CYCLE
• Internal Signals
o Three major checkpoints in cell cycle
 G1
 G2
M
o Regulated by enzymes known as cyclindependent kinases or Cdks
 Activated when bound to proteins
known as cyclins
 Kinase concentrations fairly
constant; cyclin concentrations vary
IV. CONTROL OF THE CELL CYCLE, cont
• External Signals
o Growth Factors
 Proteins released by certain cells that stimulate other cells to
divide.
 Cells stop dividing when growth factor is depleted.
 Examples include erythropoetin, interleukin, pdgf
IV. CONTROL OF THE CELL CYCLE, cont
• External Signals
o Density-dependent Inhibition
Results from crowded conditions
When one cell touches another, cell division stops
o Anchorage Dependence
Most cells must be in contact with solid surface to divide
IV. CONTROL OF THE CELL CYCLE, cont
• Cell Cycle Out of Control = CANCER
o Cancer cells do not respond to normal cell cycle controls
 Apoptosis – Programmed cell death
o Uncontrolled growth
o Deprive normal cells of nutrients
IV. CONTROL OF THE CELL CYCLE, cont
o Tumor – Mass of abnormal cells
 Benign – Mass remains at original site
 Malignant – Mass spreads to other parts of the body
 Metastasis – Separation of cancer cells from tumor; travel
through circulatory system
V. MEIOSIS
• Somatic Cells
o Body cells
o Human somatic cells contain 46 chromosomes, 23 from mom, 23
from dad
o 2n or diploid
o Matched pairs of chromosomes called homologous pairs. Each
chromosome making up a homologous pair is known as a
homologue. Both carry genes for same traits. The location of a gene
on a chromosome is known as a locus.
44 Autosomes
2 Sex chromosomes
XX =
XY =
V. MEIOSIS, cont
• Gametes
o Egg and sperm cells
o Haploid or n
o Contain 23
chromosomes
o In fertilization,
haploid (n) sperm
fuses with haploid
(n) egg → diploid
(2n) zygote
V. MEIOSIS, cont
• Description of Meiosis
o
o
o
o
o
o
Special type of cell division that occurs to produce gametes
Occurs in ovaries, testes only
Involved specialized cells
DNA replicated once, cell divides twice
Produces 4 cells with ½ the original chromosome number
In humans,


V. MEIOSIS, cont
V. MEIOSIS, cont
V. MEIOSIS, cont
• Nondisjunction – Failure of chromosomes to separate properly
in meiosis
VI. GENETIC VARIATION
VI. GENETIC VARIATION, cont
• Crossing Over
o Further increases genetic
variability
o Occurs during prophase I
when tetrads are forming
o Piece of one sister
chromatid breaks off &
exchanges places with
piece of sister chromatid
of homologue
o Known as chiasma
o Occurs very frequently
VII. A COMPARISON OF MEIOSIS & MITOSIS
VIII. DNA – THE MOLECULE OF INHERITANCE
• Chromosome
o Single molecule of DNA
wrapped in histone proteins.
Proteins maintain
chromosome structure &
control DNA activity
o Gene
VIII. DNA, cont
• Genome
o All of an organism’s DNA
o Provides working instructions for
cell through
______________________
o Must be copied prior to cell
division
IX. DISCOVERY OF DNA
• Early 1900s – Scientists determined genes determined inherited
characteristics. Also realized chromosomes were composed of DNA &
protein.
• Griffith (1928) – Studied 2 strains of bacteria. Determined pathogenicity
could be transferred when living non-pathogens were exposed to
remains of dead pathogens.
• Avery (1944) – Identified “transforming substance” as DNA
IX. DISCOVERY OF DNA, cont
o Hershey & Chase
(1952)
 Used bacteriophage
with labeled
phosphorus, sulfur
 Tested bacterial cells,
supernatant following
exposure
 Proved it was the DNA
component that was
injected into host cell
and used to make new
virus particles.
IX. DISCOVERY OF DNA, cont
• Rosalind Franklin (late 1950s) – Produced x-ray
crystallography image of DNA; “borrowed” by Watson & Crick
Watson & Crick
o Realized DNA was a helix
composed of 2 nucleotide
strands
oFranklin suggested
backbone of DNA was
composed of alternating
sugar-phosphate molecules
o Watson & Crick determined
interior of DNA was made
up of paired N-bases
o Eventually deduced bases
always paired a specific
way
Chargaff – Chemically proved
the same base-pairing rules that
Watson & Crick proved
structurally
X. A CLOSER LOOK AT DNA
• Monomers of DNA
oNucleotides
o Composed of
- 5C Sugar
(deoxyribose)
- phosphate group
- nitrogen base
 Pyrimidines


Thymine
Cytosine
 Purines


Adenine
Guanine
X. A CLOSER LOOK AT DNA, cont
• Structure of DNA
 Each strand of
nucleotides held
together with
 Double helix
 2 nucleotide strands
are antiparallel
 Each strand has a 3’
end (terminus) and a
5’end; named for
carbon on
deoxyribose
X. A CLOSER LOOK AT DNA, cont
• Base Pairing
XI. DNA REPLICATION
• DNA Replication
o Prior to cell division,
DNA must be
replicated
o Occurs during _S_
of __interphase_ of
mitosis, meiosis
o Known as
semiconservative
model of replication
 Meselson-Stahl
Experiment
XI. DNA REPLICATION, cont.
• Chromatids
 Two identical DNA
molecules
 Result of replication
 Term is only used when
identical DNAs are
physically attached;
described as one
chromosome made up of
two sister chromatids
 Centromere – Site where
sister chromatids are
most closely attached
XI. DNA REPLICATION, cont.
• Steps of Replication:
 DNA helicase unwinds the DNA double helix
 Replication begins at specific points on the DNA molecule known as origins
of replication. The Y-shaped region where new strands of DNA are
elongating are called replication forks
XI. DNA REPLICATION, cont.
 As DNA is “unzipped”, single-strand binding proteins hold the DNA
open
 A topoisomerase relieves tension creating by unwinding of DNA by
making cuts, untwisting, & rejoining the nucleotide strand.
 DNA polymerase can only add nucleotides to an already-existing
strand so an RNA primer is synthesized to get replication going
XI. DNA REPLICATION, cont.
 DNA polymerases add complementary nucleotides to each side of
the DNA molecule.
 DNA polymerase can only add nucleotides to the 3’ end of the
growing strand, so the daughter DNA is synthesized 5’ – 3’, which
means parental DNA is “read” __ 3’ – 5’__.
 This means only one side of the DNA (3’ – 5’) molecule can be
replicated as a continuous strand. Known as the leading strand.
XI. DNA REPLICATION, cont.
• Synthesis of lagging strand
 To synthesize the other new strand of DNA, DNA polymerase must
work away from the replication fork. Leads to synthesis of short
pieces of DNA known as Okazaki fragments.
 DNA ligase binds fragments together to form a continuous strand of
nucleotides.
• Proofreading & Repair
 DNA Polymerase proofreads nucleotides as they are added
XI. DNA REPLICATION, cont.
An Overview of Replication
XI. DNA REPLICATION, cont.
• Telomeres
 5’ ends of daughter strands cannot be
completed because DNA polymerase
can only add nucleotides to the 3’ end
 Results in shorter and shorter DNA
molecules with jagged ends
 To protect genetic integrity, ends of
chromosomes do not contain genes –
instead there are nucleotide sequences
known as telomeres
 Contain nucleotide repeat sequences
 Telomeres shorten each time cell
divides - limits the number of times a
cell can divide; thought to protect
organism from cancer
 Telomerase – Enzyme produced by stem
cells, cancer cells that restores
telomere length