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Chapter 11: DNA and Genes
11.1 – DNA: The Molecule of Heredity
1. DNA stands for
deoxyribonucleic acid
2. DNA contains all the genetic
information that determines an
organism's traits
3. DNA controls ALL cell activities
by its production of proteins
DNA Makes Proteins
1. All living things contain proteins. Proteins
are essential to all of life.
2. ENZYMES ARE PROTEINS!!
3. Enzymes control all the chemical reactions
in an organism and are extremely important
molecules in biology.
4. DNA contains all of the information
needed to build all of the proteins for our
bodies.
The Structure of DNA
1.DNA is made up of repeating
subunits called nucleotides.
2.Nucleotides have 3 parts:
a)A simple sugar
b)A phosphate group
c) A nitrogen base
The Structure of DNA
1. A nitrogen base is a carbon ring structure that
contains one or more atoms of nitrogen.
2. There are 4 possible nitrogen bases in DNA
a) Adenine (A)
b) Guanine (G)
c) Cytosine (C)
d) Thymine (T)
3. Thus, there are 4 possible nucleotides in DNA
DNA Nucleotides
1. The amount of
Thymine is always
equal to the amount
of Adenine
2. The amount of
Cytosine is always
equal to the amount
of Guanine.
Who Discovered the Structure of DNA?
1. In 1953 James Watson and
Francis Crick discovered that the
structure of DNA is made of two
chains of nucleotides joined
together by the nitrogen bases in
the form of a double helix
Double Helix DNA
1. Adenine
always bonds
with Thymine
2. Cytosine
always bonds
with Guanine
3. A-T
4. C-G
How does DNA carry information?
1. DNA carries information in the form of
nucleotide sequences.
2. For example:
a) The sequence ATATGCTCTA carries
different information from the sequence
CGGTATTAAC.
3. Different combinations of the same 4
nucleotides creates a near infinite number of
unique genes.
Replication of DNA
1. Recall that before a cell can divide
via mitosis it must first copy its
DNA during interphase
2. The DNA in a cell is copied in a
process known as DNA replication
Steps of DNA Replication
1. Double stranded DNA is cleaved into two
separate strands by the enzyme DNA
helicase. DNA helicase “unzips” the DNA
by breaking the bonds between the
nitrogen base pairs.
2. Next, free floating nucleotides are matched
to their complementary nucleotides
through base pairing.
3. Another enzyme called DNA polymerase
bonds these new nucleotides into a chain.
4. When the whole process is complete, there
will be 2 copies of the DNA.
11.2 – From DNA to Protein
1.Proteins are made up of long
strands of amino acids.
2.The sequence of nucleotides
in a gene contains the
information for assembling
the amino acids that will
eventually become a protein.
RNA
1. RNA is similar to DNA in that it is a nucleic acid.
2. RNA differs from DNA in 3 ways:
a) RNA is single stranded (it looks like half a
zipper) and DNA is double stranded.
b) The sugar in RNA is ribose and in DNA it is
deoxyribose.
c) RNA contains 4 base pairs like DNA but RNA
has a base pair called Uracil (U) instead of
Thymine (T). Uracil still bonds with Adenine (A)
just like Thymine (T) does.
What is the role of RNA in the cell?
1. Making proteins is like making a car in a
factory.
2. In order to make proteins, like making a car, you
need 3 main things:
a) Instructions on how to make the protein
b) The parts necessary to make the protein
c) Workers to assemble the parts into a protein.
What is the role of RNA in the cell?
1. DNA provides the instructions for building the
protein.
2. Proteins are made up of amino acids. Amino
acids are the parts needed to make the protein
3. There are 3 main types of RNA that act as
workers to interpret the instructions and
assemble the amino acids into a complete protein
What is the role of RNA in the cell?
1. The 3 types of RNA are:
a) Messenger RNA (mRNA) – Brings the
information from the DNA in the nucleus to
the cytoplasm where protein synthesis occurs.
b) Ribosomal RNA (rRNA) – rRNA makes up
ribosomes. Ribosomes are the organelle
responsible for assembling amino acids into
proteins.
c) Transfer RNA (tRNA) – transports specific
amino acids to the ribosome for assembly.
Protein Synthesis Overview
1.Protein synthesis occurs in
two steps:
a) Transcription
b) Translation
Transcription
1. Transcription is the process of making
mRNA from a DNA template.
2. Transcription is similar to DNA
replication except that a single
stranded mRNA molecule is formed
instead of a double stranded DNA
molecule.
Transcription
1. mRNA needs to be transcribed because
DNA can’t leave the nucleus, but protein
synthesis takes place in the cytoplasm.
2. mRNA carries the DNA’s message to the
cytoplasm and tells the ribosome which
proteins to make.
The Genetic Code
1. There are 20 different amino acids (the building
blocks of proteins) but there are only 4 different
nucleotides in RNA.
2. It takes 3 nucleotides to code for an amino acid.
a) For example: the nucleotide sequence UUU
will code for the amino acid phenylalanine to
be placed in the protein.
The Genetic Code
1. Each set of 3 nucleotides in mRNA that
codes for an amino acid is called a codon.
2. How many different codons are possible
with 4 different possible nucleotides?
a) Answer: 43 = 64 different possible
codons.
3. There are 64 possible codons but only 20
possible amino acids.
The Genetic Code
1. More than one codon can code for the same
amino acid.
2. However, for any one codon, there can only
be one amino acid.
3. Certain codons do not code for amino acids;
they are instructor codons.
a) For example: UAA is a stop codon and
tells protein production to stop at this
point.
Amino Acid Codon Chart
Translation: From mRNA to Protein
1. The process of converting the information in a
sequence of nitrogen bases in mRNA into a
sequence of amino acids that make up a protein
is called translation.
2. Translation takes place at the ribosome in the
cytoplasm.
3. In eukaryotic organisms, the mRNA is made in
the nucleus and then moves to the cytoplasm
where translation takes place.
Translation (continued)
1. When the mRNA arrives in the cytoplasm,
Ribosomes attach to it like clothespins to a
cloths line.
2. For proteins to be built, amino acids must be
brought to the ribosomes.
3. tRNA is the molecule for responsible for
bringing the amino acids to the ribosomes.
Ribosome
Translation (continued)
1. Each tRNA molecule has three nucleotides
that are complementary to the three
nucleotides on the mRNA strand.
2. The nucleotides on the tRNA are called an
anticodon because they complement a
codon on the mRNA.
3. Each tRNA carries only the amino acid
specified by the anticodon.
Translation (continued)
1. As the amino acids are bonded together, the
ribosome slides down the mRNA and moves
to the next codon.
2. Amino acids are added to the chain until a
stop codon is reached.
3. As all of the amino acids are built into a
chain, a polypeptide is formed.
4. This polypeptide will eventually be a protein
with a specific job in the body.
Polypeptide