Download unit iv - dna & cell division

UNIT IV - DNA & CELL DIVISION
I. INTRODUCTION TO DNA
• ________________ – All of an
organism’s DNA; must be copied
prior to cell division
• __________________ – Single
molecule of DNA wrapped in
protein. Proteins maintain
chromosome structure & control
DNA activity
 Somatic cells - ___________
_________________________
 Gametes - ___________
_________________________
• ________________ – 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
• _______________ & _______________
o Realized DNA was a __________ composed of _____
nucleotide strands
o Franklin suggested backbone of DNA was composed of
alternating ______________________________ molecules
o Watson & Crick determined interior of DNA was made up of
paired ___________________
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 _____________,
_____________, & _________
o _______________
o Single-ring N-base
o Ex: Cytosine & Thymine
 _______________
 Double-ring structures
 Ex: Adenine & Guanine
II. A CLOSER LOOK AT DNA, cont
• Structure of DNA
 Double helix
 Strand of nucleotides held together
by ________________ bonds
 Nitrogen bases held together with
________________ bonds
 Pairs
 _____________________
 _____________________
 2 nucleotide strands are
____________________
 Each strand has a ______ end
(terminus) and a _______ 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 _______________________ model of replication
 Meselson-Stahl Experiment
III. DNA REPLICATION, cont.
• ________________ Two identical DNA
molecules; copied
prior to cell division
 ________________ –
Site where sister
chromatids are most
closely attached
III. DNA REPLICATION, cont.
• Steps Involved in Replication
 ___________________________ unwinds the DNA double helix
 Replication begins at specific points on the DNA molecule known
as ____________________________________________. The
Y-shaped region where new strands of DNA are elongating are
called ________________________________
III. DNA REPLICATION, cont.
 As DNA is “unzipped”, _______________________ binding proteins
hold the DNA open
 A ______________________________ untwists the nucleotide strand.
 DNA polymerase can only add nucleotides to an already-existing strand
so an ___________________ is synthesized to get replication going
III. DNA REPLICATION, cont.
 ______________________________ add complementary
nucleotides to each side of the DNA molecule.
 Can only add nucleotides to the _______ end of the growing strand,
so the daughter DNA is synthesized _________________
 So…..only one side of the DNA (3’ – 5’) molecule can be replicated
as a continuous strand which is known as the
_______________________________.
III. DNA REPLICATION, cont.
• Synthesis of ____________________
 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 ________________ fragments.
 ____________________ binds fragments together to form a
continuous strand of nucleotides.
• Proofreading & Repair
 DNA Polymerase proofread nucleotides as they are added
III. DNA REPLICATION, cont.
III. DNA REPLICATION, cont.
An Overview of Replication
III. DNA REPLICATION, cont.
• ___________________
 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