FREE ESSAY ON NUCLEOTIDES

NUCLEOTIDES

I. nucleotides: sub units "? DNA
A. Phosphate group
B. 5-carbon sugar molecule (deoxyribose)
C. nitrogen base
1. Adenine
2. Guanine
3. Thymine
4. Cytosine
Chargaff "? base pairing rule: 1 = 3 and 2 = 4
II. Maurice Wilkins and Rosalind Frankilin "? X-ray diffraction photos of DNA molecule
- DNA molecules: tightly coiled helix and composed of two or three chains of nucleotides
III. Chargaff builds a model of the double helix: ??a spiral staircase?? of 2 strands of
nucleotides twisting around a central axis.
A. alternating sugar and phosphate units
B. purine and pyrimidines paired up 
- A can only form hydrogen bonds with T
- C can only for ?? ?? with G.
"? 2 strands that are complementary to each other
IV. Copying of DNA:
A. Replication: process of synthesizing a new strand of DNA. 
B. Helices: open up the double helix by breaking the hydrogen bonds that link the
complementary bases.
C. Two strands separated: additional enzymes and proteins attach to the individual
strands and hold them apart "? twisting prevented
D. Replication fork : where double helix separates
1. DNA polymerases: enzymes that move along each strand "? adding nucleotides to the
exposed bases.
2. Proofreading
"? one old and one new DNA strand
8.3 The Structure of a Gene
- genes hold info specifying how to build particular proteins
- affects the phenotype of an individual because of the activity of the protein that it
specifies
- DNA encoded info that specifies particular proteins; each gene is made of a specific
sequence of nucleotides
I. Genes in Eukaryotes
A. interruped by necleotides that have no coding info.
1. Introns:: noncoding sequences/ intervening
- must be removed
2. Exons :nucleotide segments that code for amino acids
-stitched together once all introns are removed
II. Multigene families: clusters of almost identical sequences; cells exist in multiple
copies
A. Some are ??silent?? genes: don??t make protein
B. Some are only active during embryonic or fetal development
C. Some encode the two chains of