The genetic code is written in the DNA
DNA is a nucleic acid-a type of organic MACROmolecule (meaning ‘big’)-of which the smaller building
block is a “nucleotide”
a. The DNA is shaped like a twisted ladder or zipper, the sides of which are made of alternating sugars
and phosphate molecules-the sugar-phosphate backbone. Each individual piece of a rung + the side is
the building block/monomer o DNA-the nucleotide. The 2 strands of the double helix/ladder
structure are antiparallel –run in opposite direction from each other like “head to toe and toe to
head” or “A to Z and Z to A”
Nucleotide-consists of a 5-Carbon sugar, phosphate group and a base
b. The rungs of the ladder of made of nucleotide bases and the DNA itself is a double-stranded chain of
these nucleotides.
1. Adenine (A)
2. Thymine (T)
3. Guanine (G)
4. Cytosine (C)
c. In each rung are 2 bases that are paired according to the following rule (always for DNA):
1) Adenine-Thymine
2) Guanine-Cytosine
**Each of these bases is said to be complementary to its mate
d. The instructions for making proteins-i.e. the genetic code-is encoded in the order of these bases along
the strand of DNA.
Reflect for a minute: What in heavens does BOGGLE have to do with any of this?
In this case, the order does matter-the different sequences of nucleotide bases code/instruct for
different amino acids and proteins. The order of bases indicates ultimately which protein.
e. Human DNA contains 3 billion of these bases strung together in the chromosomesinteresting fact: only about 1% of the total sequence of these bases varies between people.
Central Dogma
The Central Dogma of Biology posits that DNA is copied and converted into RNA language, a language
that is then read and translated to produce a protein in the cell. There is a direct, linear flow from DNA
in the nucleus to the production of a protein in the cell.
Genes & Chromosomes
Background information on DNA
1. segments of chromosomes are called genes-lengths of DNA are called chromosomes
a. each gene codes for a different type or protein
b. as such, many genes have been identified as causing specific diseases or conditionsex. A gene on chromosome #15 causes Tay Sachs disease
c. There are 46 chromosomes in almost every human cells – 23 pairs (sex chromosomes:
XX vs XY)
** chromosomes are usually single strands of DNA but when the cell copies the DNA, the chromosomes
are doubled and look like X’s “doubled chromosomes”
Karyotype is an organized chart based upon photographs of the doubled chromosome form of
DNA from white blood cells and can be used to look for abnormalities.
For ex, this chart has a normal pattern-2 per chromosome. If there were a 3rd pait at #21, that individual
would have Down Syndrome (Trisomy 21-“tri” = 3)
Replication
DNA replication is a semi-conservative process. One strand serves as the template for the second
strand. DNA replication is initiated at a region on a chromosome called an origin of replication.
a. When the DNA is copied/replicated, the 2 strands of DNA separate from one another
b. In the nucleus, there are subunits of DNA-DNA nucleotides- floating around freely.
c. Free DNA nucleotides connect with the exposed bases of the “unzipped DNA” according to the base
pairing rules (A-T and G-C)
d. As the DNA nucleotides connect to form the rungs of 2 new DNA ladders, the sugars and phosphates
of the free nucleotides connect to form the sides of the DNA ladders
e. This process continues until the entire length of each chromosome has been copied; the process
takes about 6 hours
f. the 2 identical copies of each chromosome are connected by a centromere and = a double
chromosome. Each new strand/copy/double helix contains one original parent strand and one newly
synthesized strand (semiconservative)
Remember that DNA exists in the nucleus as a condensed, compact structure. To prepare DNA for
replication, a series of proteins aid in the unwinding and separation of the double-stranded DNA
molecule. These proteins are required because DNA must be single-stranded before replication can
proceed…and I will do you the favor of not having you memorize them all 