THE STRUCTURE OF DNA Chapter 12… section 12.1 & 12.2.
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Transcript of THE STRUCTURE OF DNA Chapter 12… section 12.1 & 12.2.
THE STRUCTURE
OF DNA
Chapter 12… section 12.1 & 12.2
What do we know about DNA?
What is the importance of DNA?
Chromosomes contain many genes made of DNA.
A gene is a segment of DNA that codes for a particular trait.
GENES (and DNA)DO 3 CRITICAL THINGS ….
1. carry information from one generation to the next
2. determine the inheritable characteristics of an organism
3. can be easily copied
Deoxyribonucleic Acid (DNA)
DNA is a long thin molecule in a human cell with over 6 billion nucleotides.
DNA- is a long molecule made up of units called nucleotides.- Determines the production of proteins.
A nucleotide has 3 parts:1. Deoxyribose Sugar2. Phosphate Group3. Nitrogenous Base
There are 4 kinds ofnitrogenous bases in DNA…..
1. Adenine (A) 2. Guanine (G)3. Cytosine (C) 4. Thymine (T)
Adenine and Guanine are PURINESand have a “double” ring structure
Thymine and Cytosine are PYRIMIDINES and have a “single” ring structure
DNA looks like a twisted ladder….the sides (BACKBONE)
are composed of alternating…
deoxyribose sugars and
phosphate groups(Covalently bonded together)
the steps (RUNGS) are composed of ….
complementary pairs of nitrogenous bases
“untwisted” “twisted”
The nucleotides can be joined in any order….
this means that, ANY SEQUENCE of N bases is possible…
thereby allowing for great diversity in living things – even among members of the same species
Watson and Crick – determined the structure of DNA to be a DOUBLE HELIX in which two strands were wound around each other
DNA is like a spiral
staircase
HYDROGEN BONDS…bond between a positive H atom of one molecule and a negative atom of another molecule; intermolecular force
weak bond no electrons involved
Form between the N bases in DNA
provide enough force to hold the 2 strands of DNA together
Hydrogen Bonds form between certain base pairs
ADENINE (purine) and THYMINE (pyrimidine) – 2 H bonds
GUANINE (purine) and CYTOSINE (pyrimidine) – 3 H bonds
because the N bases can only pair in a certain way this is called…..
COMPLEMENTARY BASE PAIRING
a purine will always bond with a pyrimidine
HYDROGEN BONDS
Chargaff’s Rule (because of complementary base pairing) for every adenine there is exactly 1 thymine
for every cytosine there is exactly 1 guanine
Covalent Bonds – occur when 2 atoms share electrons; intramolecular force
strong bonds that are not easily broken found between…
• the sugars and phosphates • the sugars and nitrogen bases
maintain the….• backbone (sides) of the DNA molecule• the integrity of the DNA code (sequence of N bases)
DNA Structure- 10 base pairs make up one full twist of DNA
What is the complement?
If the sequence of N bases on 1 strand of DNA is….
A T C G G C T T A A T A T C GWhat is the sequence on the other strand?
REMEMBER…DNA is a double stranded molecule
T A G C C G A A T T A T A G C
During most of the cell cycle chromosomes are not visible, instead the DNA is seen in a form called chromatin.
During cell division, the chromatin condenses and coils around proteins (called histones) to form chromosomes.
Semiconservative DNA REPLICATION…. (hyperlink)making an exact copy of DNAoccurs before cell division (mitosis + meiosis) - interphase highly accurateMUTATIONS may occur ….
changes in the (sequence of nucleotides) DNA
Watson and Crick realized that each strand of a DNA molecule has all the
information needed to RECONSTRUCT THE OTHER STRAND
DNA Replication a simplistic view….
DNA Replication – 3 Steps
1. UNWINDING … DNA Helicase (enzyme) separates the DNA
molecule at Hydrogen bonds2. BASE PAIRING… DNA Polymerase (enzyme) adds new
nucleotides forming new strands of DNA3. JOINING… DNA ligase (enzyme) connects new
“daughter” strands together
The sites where separation and replication occur are called replication forks
DNA replication produces ….
2 new complementary strands(of DNA) Following (Chargaff’s) rules of base
pairing
Each “parental” (old) strand of DNA serves as a template (pattern) for the making of (2) “daughter” (new) strands
When DNA replication is complete…. 2 molecules of DNA are made
Each molecule is composed of… 1 old strand and 1 new strand
Prokaryotic Cells…. have no nucleus have no membrane bound organelles… mitochondria, golgi bodies, vacuoles,… DNA is in the cytoplasm have a single, circular chromosome DNA replication begins at a single point
and often proceeds in 2 directions until the entire DNA is replicated
Eukaryotic Cells… have a nucleus have membrane bound organelles… mitochondria, golgi bodies, vacuoles,… DNA is in the nucleus have more DNA…. DNA replication occurs at hundreds of places
and produces segments called OKAZAKI FRAGMENTS
replication proceeds in both directions until the DNA is completely copied
DNA is usually in a form called CHROMATIN
But during cell division, the chromatin Condenses and coils around proteins (called histones)to form CHROMOSOMES (of DNA and protein)
chromosome
chromatin