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ANSWER KEY
Section 2.3 From DNA to Proteins
As we have learned to date the nucleus of a cell contains chromosomes, which contain
DNA. DNA is a molecule that contains all the instructions to make, maintain, and repair
cells. This section will investigate how DNA is able to carry out all of these vital functions.
DNA Structure
It is useful to investigate how the DNA molecule is actually constructed. You may
have known that DNA looks like a twisted ladder, structure scientists refer to as a double
HELIX. All DNA is made from a basic unit called a NUCLEOTIDE, which consists of a
sugar, a nitrogen base, and a phosphate group. A pictorial of this is shown below.
Figure 1.
A nucleotide consists of a sugar, a nitrogen base, and a phosphate.
The sides of the DNA molecule are made from a SUGAR and phosphate joined
together, much like a backbone. The rungs of the ladder are made from a pair of
NITROGEN bases, and only four nitrogen bases are possible in DNA: adenine (A), guanine
(G), thymine (T), and _CYTOSINE_ (C). As you will notice from the picture below,
adenine always partners with _THYMONE_ and cytosine partners with GUANINE_.
Consequently, every rung of the DNA ladder is either made of adenine and thymine (A-T) or
cytosine and GUANINE (C-G); these two combinations are referred to as BASE PAIRS.
Figure 2.
A close-up of the DNA molecule. Notice the sugar-phosphate backbone and the nitrogen
bases that make up the rungs of the DNA ladder. Each base pair is either A-T or C-G.
ANSWER KEY
The Genetic Code
The bases in the DNA molecule can be thought of in much the same way as the
LETTERS in the alphabet. From the letters in the alphabet it is possible to make hundreds
of thousands of different combinations that are called _”WORDS”_. The four nitrogen
bases (A, T, C, G) do the same thing; they can combine in many ways to make a short three
letter combination. These short combinations of three nitrogen bases are called AMINO
ACIDS , and there are TWENTY (20) of them.
These different three letter combinations called amino acids, can combine together to
make larger molecules called PROTEINS. These proteins determine the characteristics of
an organism and the INSTRUCTIONS that all of the body’s cells must follow.
Figure 3. A section of the DNA molecule. Notice that every three
nitrogen bases are marked on the diagram; this three-letter sequence
represents an amino acid, the building block of proteins.
DNA to Genes
The DNA molecule in a chromosome is organized into _GENES_. A gene is a short section
of DNA that contains INSTRUCTIONS_ for a specific trait. In other words, a gene
represents a specific CODE_ for a characteristic, such as eye colour or hair colour. For each
gene, of course, there can be many VARIATIONS_. That is why no two people have the
exact same DNA……….except for IDENTICAL TWINS_! The entire set of an organism’s
DNA is called its GENOME_. The human genome is contained in __46__ chromosomes,
which is present in all of your body cells.
ANSWER KEY
Summary of Vocabulary and Concepts
The DNA molecule looks like a “twisted ladder”, a structure called a DOUBLE helix.
A nucleotide is the basic unit of the DNA molecule; sugar and PHOSPHATE make up
the SIDES of the ladder.
The NITROGEN bases make up the rungs of the ladder and there are 4 (FOUR) of them; A
always partners with T and C with G. The two nitrogen bases that make up each rung are
often called BASE PAIRS.
A section of three nitrogen bases represents a(n) AMINO ACID; there are 20 (TWENTY)
of them possible. It is these small molecules that can be assembled to form _PROTEINS_,
which determine the characteristics of an organism and the instructions each cell must
follow.
A short section of DNA with instructions to make a specific protein is called a _GENE_.
The sum of these for an organism is called its GENOME.