Download unit iv - dna & cell division

UNIT IV - DNA & CELL DIVISION
Big Campbell –
Ch 12, 13, 16
Baby Campbell –
Ch 8, 10.1-10.5
I. INTRODUCTION TO DNA
• Genome
o All of an organism’s DNA
o Provides working instructions for cell
through ______________________
o Must be copied prior to cell division
• Chromosome
o Single molecule of DNA wrapped in
protein. Proteins maintain
chromosome structure & control DNA
activity
 Somatic cells
 Gametes
• Chromatin
o Term used to describe fine strands of
uncoiled DNA
II. A CLOSER LOOK AT DNA
• Discovery of DNA
o Early 1900s – Scientists determined genes determined inherited
characteristics. Also realized chromosomes were composed of DNA &
protein.
o Griffith (1928) – Studied 2 strains of bacteria. Determined that
pathogenicity could be transferred when living non-pathogens were
exposed to remains of dead pathogens.
o Avery (1944) – Identified “transforming substance” as DNA
o Hershey & Chase (1952) – Used radioactively-viruses that infect bacteria known as bacteriophages. Virus is made up of DNA & protein – Hershey &
Chase proved it was the DNA component that was injected into host cell
and used to make new virus particles.
o Rosalind Franklin (late 1950s) – Produced x-ray crystallography image of
DNA; “borrowed” by Watson & Crick
II. A CLOSER LOOK AT DNA, cont
• Watson & Crick
o 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
II. A CLOSER LOOK AT DNA, cont
• Monomers of DNA
o Nucleotides
o Composed of
 Pyrimidines


 Purines


II. A CLOSER LOOK AT DNA, cont
• Structure of DNA
 Double helix
 Strand of nucleotides held
together by covalent bonds
 Nitrogen bases held
together with hydrogen
bonds


 2 nucleotide strands are
antiparallel
 Each strand has a 3’ end
(terminus) and a 5’end;
named for carbon on
deoxyribose
II. A CLOSER LOOK AT DNA, cont
III. DNA REPLICATION
• DNA Replication
o Prior to cell division, DNA must be replicated
o Known as semiconservative model of replication
 Meselson-Stahl Experiment
III. 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
III. DNA REPLICATION, cont.
• The 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
III. 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
III. 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’; therefore, only one side of the DNA (3’ – 5’)
molecule can be replicated as a continuous strand. Known as the
leading strand.
III. 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
III. DNA REPLICATION, cont.
An Overview of Replication
III. 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
IV. ASEXUAL REPRODUCTION
•Cell Theory
Virchow
•Cell Cycle
Single-celled
Organisms
Multicellular
Organisms
V. PROKARYOTIC CELL DIVISION
• Known as 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
VI. EUKARYOTIC CELL CYCLE
• Can be divided into:


VI. EUKARYOTIC 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 – Period of time following cell
division in which cell is growing to
normal size. Protein production,
metabolism high.
• S – DNA replication. Known as “point
of no return”. Chromosome now
consists of 2 sister chromatids.
• G2 – Preparation for division;
replication of centrioles in animal
cells
VI. EUKARYOTIC CELL CYCLE, cont
• Mitosis
– Nuclear division
– Requires all the
cells energy,
resources
– Last step is
cytokinesis –
splitting of the
cell
VI. EUKARYOTIC CELL CYCLE, cont
VI. EUKARYOTIC CELL CYCLE, cont
VI. EUKARYOTIC CELL CYCLE, cont
• Cytokinesis in Animal & Plant Cells
VI. EUKARYOTIC CELL CYCLE, cont
VII. CONTROL OF THE CELL CYCLE, cont
• Internal Signals
o Three major checkpoints in
cell cycle
o Regulated by enzymes
known as cyclin-dependent
kinases or Cdks; activated
when bound to proteins
known as cyclins
o Cdk concentration fairly
constant; cyclin
concentration is critical
factor
o Most important checkpoint
is the G1 checkpoint. If
there is no signal, cell exits
cell cycle and switches to G0
VII. CONTROL OF THE CELL CYCLE
• External Signals
o Growth Factors
 Proteins released by certain cells
that stimulates other cells to
divide.
 Cells stop dividing when growth
factor is depleted.
 Examples include erythropoetin,
interleukin
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
VII. 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
o Named after type of cells affected
 Carcinoma – originate in linings & coverings; for example, skin or
lining of digestive tract
 Sarcoma – originate in support tissues; for example, bone and
muscle
 Lymphoma/Leukemia – originate in blood-forming tissues; for
example, bone marrow, spleen, lymph nodes
VII. 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
VIII. 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.
Autosomes – Human somatic cells contain 44 autosomes
Sex chromosomes – 2 per somatic cell
XX =
XY =
VIII. 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
• Meiosis
o Occurs in ovaries, testes
o DNA replicated once, cell
divides twice
o Produces 4 cells with ½ the
original chromosome
number
VIII. MEIOSIS, cont
VIII. MEIOSIS, cont
VIII. MEIOSIS, cont
• Nondisjunction – Failure of chromosomes to separate properly
in meiosis
IX. MEIOSIS vs MITOSIS
X. GENETIC VARIATION
X. 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