Transcript nitrogenous base - Brookwood High School

DNA / RNA Notes
l. DNA Structure
 A. Chromosomes are made up of DNA, or
deoxyribonucleic acid. DNA is the master copy, or
blueprint, of an organism’s information code.
 B. DNA is a type of
nucleic acid, which are made up of subunits called
nucleotides.
C. A nucleotide consists of 3 things:
A sugar- deoxyribose
A phosphate group
A nitrogenous base. In DNA, there are 4 bases:
adenine, guanine, cytosine, & thymine.
 D. DNA looks like a twisted
ladder. This structure, called
a double helix, was proposed
by Watson & Crick in 1953.
E. The “poles” of the ladder are made up of alternating
sugars and phosphates. The 2 poles are held together
by nitrogenous bases (the “steps” or “rungs” of the
ladder).
Poles of sugars &
phosphates
Nitrogen
bases
1. The nitrogenous bases are held together by weak
hydrogen bonds.
2. The bases pair very specifically:
• - Adenine only pairs with Thymine
• - Guanine only pairs with Cytosine
A–T
G-C
 F. The way the nucleotides are arranged on the
double helix forms a genetic code. The cell can
“decode” this code in order to make proteins.
Proteins determine the structure and function of
organisms.
ll. DNA replication or copying
 A. Every time a cell divides, it must
first make a copy of its
chromosomes, or DNA. This
process of copying is called DNA
replication.
B. How does it happen?
1. An enzyme breaks the hydrogen bonds that hold
the 2 strands together, unzipping the DNA.
2. This leaves 2 single strands of DNA with
unmatched/ unpaired bases.
 3. Free nucleotides in the surroundings pair to the
exposed bases.
 4. Each new strand is a complement of one of the
original, or parent, strands.
 5. The result is the formation of 2 DNA molecules,
each identical to the original.
lll. RNA and Protein Synthesis
How does a cell use the info in DNA to make proteins?
A. Parts of the DNA sequence are copied into RNA, or
ribonucleic acid. The RNA molecules carry out the
process of making proteins.
 B. RNA structure vs DNA structure
 1. RNA is single stranded
 2. The sugar in RNA is ribose
 3. RNA contains uracil instead of thymine
C. Transcription: Making RNA from DNA
1. Messenger RNA (or mRNA) is a “copy” of DNA
that can travel from the nucleus to the ribosome,
where proteins will be made.
2. We will talk about bases in 3’s, what is known as
a triplet codon.
lV. Translation
 A. Proteins determine the structure and function of
organisms. They are made of amino acids. Only 20 amino
acids occur naturally, but these 20 can be arranged to
produce an infinite number of proteins.
 B. Translation is the process of converting the sequence of
bases in mRNA into a sequence of amino acids, making a
protein.
C. Steps:
1. mRNA attaches to a ribosome.
2. tRNA (or transfer RNA) brings amino acids to the
ribosome so that they can be assembled into
proteins. How does the tRNA know where to bring
the amino acid?
 3. At the bottom of the tRNA are 3 nitrogen bases
known as the anticodon. The tRNA looks for the
complementary codon and attaches the amino
acid at that spot.
 4. The process continues and a chain of amino
acids is formed. The amino acids bond to form a
protein.
V. Mutations
 A. Sometimes mistakes occur in the process of
copying genetic material. Any mistake or change in
the DNA sequence is known as a mutation.
 B. Point mutations- a change in a single base pair in
DNA. This change can lead to a change in amino
acids, and therefore, a change in a protein.
 C. Frameshift Mutation: A single base is added or
deleted, changing every amino acid after the change.
AUG – ACC – AUU – ACG – GAU
Met Thr Ile Thr Asp
AUG – ACC – ACG – GAU
Met Thr Thr Asp
 D. Chromosomal Mutation: ex. Parts broken off.
Gametes may end up with extra copies of some
genes or a lack of some genes.
E. Disjunction errors: Failure of homologous
chromosomes to separate properly in meiosis is called
nondisjunction. Two kinds of gametes can result:
1. Extra chromosome 21: Down’s syndrome
2. Lack of chromosomes: zygotes usually die
G. Causes of mutations
1. Random/ Spontaneous
2. Environmental- x rays, UV, certain chemicals,
etc.