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UNIT V - DNA & CELL DIVISION
Big Campbell – Ch 12, 13, 16Baby Campbell – Ch 8, 10.1-10.5Hillis – 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 Fissiono Asexual reproductiono Much shorter than euk cell
cycleSingle chromosome
replicatesEach copy begins moving to
opposite ends of cellCell elongatesWhen bacterium is 2X
original size, cell membrane pinches inward
Cell wall deposited2 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
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.
Cell at the END of interphase
PROPHASE1. _Nucleolus_ and _nuclear envelope
disassemble.2. Replicated DNA thickens & condenses Results
in _chromosomes_ made up of 2 sister chromatids
held together by the _centromere_. 3. _Centrioles_ begin moving to opposite sides of
the cell.
4. 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.
Late Prophase (Prometaphase)
III. CELL CYCLE, contMETAPHASE
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.
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.
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
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 cycleG1
G2
M
o Regulated by enzymes known as cyclin-dependent kinases or CdksActivated when bound to proteins
known as cyclinsKinase concentrations fairly
constant; cyclin concentrations vary
IV. CONTROL OF THE CELL CYCLE, cont• External Signals
o Growth FactorsProteins 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 conditionsWhen one cell touches another, cell division stops
o Anchorage DependenceMost 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 controlsApoptosis – Programmed cell death
o Uncontrolled growtho Deprive normal cells of nutrients
IV. CONTROL OF THE CELL CYCLE, conto Tumor – Mass of abnormal cells
Benign – Mass remains at original siteMalignant – Mass spreads to other parts of the bodyMetastasis – Separation of cancer cells from tumor; travel
through circulatory system
V. MEIOSIS• Somatic Cells
o Body cellso Human somatic cells contain 46 chromosomes, 23 from mom, 23
from dado 2n or diploido 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
• Gameteso Egg and sperm cellso Haploid or no Contain 23
chromosomeso In fertilization,
haploid (n) sperm fuses with haploid (n) egg → diploid (2n) zygote
V. MEIOSIS, cont• Description of Meiosis
o Special type of cell division that occurs to produce gametes
o Occurs in ovaries, testes onlyo Involved specialized cells o DNA replicated once, cell divides twiceo Produces 4 cells with ½ the original chromosome numbero 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
variabilityo Occurs during prophase I
when tetrads are formingo Piece of one sister
chromatid breaks off & exchanges places with piece of sister chromatid of homologue
o Known as chiasmao 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 DNAo 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, conto 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 & Cricko Realized DNA was a helix
composed of 2 nucleotide strands
o Franklin 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 DNAoNucleotidesoComposed 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 replicationMeselson-Stahl
Experiment
XI. DNA REPLICATION, cont.
• Chromatids Two identical DNA
moleculesResult of replicationTerm 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
