DNA STRUCTURE AND FUNCTION
I. BASICS OF DNA
A. Chromosomes-organelles located in a cell’s nucleus.
1. These structures are composed of
2. Chromosomes contain a cell’s genetic information.
3. Genes-sequences of DNA found on chromosomes. These are responsible for giving
individuals their characteristics/traits. Genes are also involved in the production of
proteins and enzymes in cells.
4. Chromosomes are only visible during cell division.
a. Chromatin-this material contains the DNA of a non-dividing cell. These
condense to form chromosomes prior to cell division.
b. Karyotype-collection of chromosomes in an individual.
5. In most cells, chromosomes occur in pairs known as Homologous Chromosomes.
a. Each member of the pair is known as a sister chromatid. These chromatids are
held together by a centromere.
6. Replication-
a. This process ensures that a newly produced cell will have a copy of the
parental cell DNA.
B. Chromosome Number
1. Different species of plants and animals contain different numbers of chromosomes in
their cells. How many pairs of chromosomes do humans contain?
2. One thing to remember is that all healthy members of a species have the exact
number of chromosomes that is required for that species. Fewer or extra chromosomes
causes genetic disorders.
3. Humans have a total of 23 pairs of chromosomes.
4. Terms That Refer to Chromosome Number
a. Diploid1) Diploid cells contain homologous chromosomes. These are
abbreviated 2N.
2) Most living cells are diploid.
b. Haploid1) These are abbreviated
2) Gametes are the only cells that are haploid. What are gametes?
a) Gametes fuse to produce a structure known as a zygote.
C. Autosomes-nonsex chromosomes. There are 22 pairs of these. These produce nonsex related
traits (hair color, eye color, etc).
D. Sex Chromosomes-differ in males and females. These determine the sex of an individual.
1. Females-have 2X sex chromosomes (XX).
a. Most of a females sex related traits are produced by the first X. However, she
can pass the second X to her offspring.
2. Males-have an X and a Y sex chromosome (XY).
a. The Y chromosome produces few traits in males. Usually, the Y leads to the
production of testosterone.
b. Who determines the sex of a child?
II. HISTORICAL STUDIES ON DNA:
A. Johann Meischer-1868-
B. Fred Griffith-1928-
C. Oswald Avery-1930’s-
D. Alfred Hershey and Martha Chase-1952
E. Watson and Crick-1953
F. Rosalind Franklin-1940’s and 1950’s-
III. THE STRUCTURE OF DNA
A. DNA is composed of many nucleotides that are covalently bonded together.
B. Each nucleotide of DNA contains:
C. Erwin Chargaff-1940’s-discovered that different species of plants and animals have different
amounts of DNA. He also observed that the proportions of the nitrogen bases varied from
species to species.
1. Chargaff also observed several points that today are known as Chargaff’s Rules:
a. The amount of adenine in a single DNA molecule is always equal to the
amount of thymine in that same DNA molecule.
b. The amount of guanine in a single DNA molecules is always equal to the
amount of cytosine in that same DNA molecule.
c. The amount of each base varies from species to species.
2. Chargaff had no idea why the above rules were true.
3. Watson and Crick were able to use these rules in their discovery of the structure of
the DNA molecule.
D. Watson and Crick discovered that DNA has a Double Helix Shape.
1. This refers to the fact that DNA is composed of 2 strands of bonded nucleotides that
are covalently bonded together.
2. In this Double Helix structure, the nitrogen bases face inward.
a. These nitrogen bases always form weak hydrogen bonds with each other.
These bonds actually hold the DNA double helix together.
b. The nitrogen bases always bond as follows:
c. These bonding patterns perfectly fit Chargaff’s Rules.
IV. DNA REPLICATION-the production of 2 new DNA strands from an existing DNA strand.
A. Steps in DNA Replication
1. DNA polymerase-an enzymes; opens the double helix of DNA to form a Y-shaped
structure known as a replication fork.
2. Once the DNA is open, the DNA polymerase begins to form a new DNA strand along
1 of the old DNA strands. This is accomplished as the enzyme attaches the
appropriate nucleotides together. Specifically, how do the bases bond?
a. This process is often referred to as Complementary Base Pairing.
3. RNA primers-are used to add bases to the second strand of DNA. This process is
similar to the above process. It functions by placing short primers on this DNA strand.
a. DNA Polymerase recognizes the primers and adds the correct bases to build
a new segment of DNA.
4. Once DNA Polymerase and the RNA Primers have passed, the new DNA strands
wind together. DNA Ligase, and enzyme, helps to seal any gaps in the forming DNA.
5. End Result of DNA Replication:
B. DNA replication is described as being a semiconservative process. This means that each new
DNA strand is composed of a parental (old) strand and a newly formed strand.
V. As we know, genes play a role in providing traits to individuals. Recall that they are also involved in
the production of proteins. This is the area that we are going to focus on in this chapter.
A. In 1940, George Beadle and Edward Tatum proposed the “One Gene, One Enzyme
Theory” which stated that individual genes are responsible for producing specific proteins
in living organisms.
VI. RIBONUCLEIC ACID (RNA)
A. This nucleic acid, like DNA plays a role in protein synthesis.
B. RNA, like DNA, is composed of nucleotides that are covalently bonded together. However,
RNA occurs as a single strand, not a double helix.
1. All RNA nucleotides contain the following components:
a. Ribose
b. A phosphate group
c. One of four possible nitrogen bases:
C. Types of RNA:
1. Ribosomal RNA (rRNA)-is involved in the formation of ribosomes. What are
ribosomes?
2. Messenger RNA (mRNA)-occurs as a long, slender strand.
a. mRNA is produced from DNA in a cell’s nucleus.
b. During protein production, mRNA carries information from the chromosome
to the ribosome. mRNA specifically tells the ribosome which protein to
produce.
c. Codons-
3. Transfer RNA (tRNA)-is located in the cytoplasm of cells.
a. tRNA functions by
b. Remember that proteins are composed of amino acids that are covalently
bonded together.
c. Anticodon
1) mRNA codons are complementary to anticodons. How do they
match?
D. RNA vs. DNA
VII. PROTEIN SYNTHESISA. 2 major events occur during protein synthesis. These events are:
B. Transcription-
1. This occurs at chromosomes in the nuclei of cells.
2. Steps/Events in Transcription
a. RNA Polymerase-what type of molecule is this?
1) This process is initiated by a special promotor molecule.
2) RNA polymerase attaches to and opens the double helix of DNA.
3) Next, the enzyme begins adding bases to a growing RNA strand.
4) The RNA polymerase will move over the length of DNA producing
mRNA as it travels. This is known as elongation.
5) How do the bases match in this process?
6) RNA polymerase will continue moving along DNA producing DNA
until it reaches a special stop sequence where termination occurs.
7) The DNA double helix reforms after the enzyme has passed by.
b. Next, the mRNA will release from the DNA. It then is released into the
cytoplasm of the cell. From here, the mRNA will float to a ribosome where it
will attach and aid in the production of proteins.
3. When transcription is complete, mRNA contains a series of protein coding regions
known as exons. There are also introns on the mRNA which are not involved in
coding for proteins. Introns allow cells to choose which exons will go into a specific
mRNA molecule through a process known as Alternative mRNA Splicing.
C. Translation-
1. This process occurs at the ribosome.
2. Ribosomes are composed of 2 subunits: a large subunit and a small subunit.
3. Steps/Events in Translation
a. mRNA aligns itself between the large and small subunits of the ribosome.
b. The codons of mRNA face upwards into the ribosome. mRNA contains a
special start codon (AUG) to begin translation.
c. A tRNA molecule, with its amino acid, enters the ribosome.
1) What is the anticodon for this first tRNA molecule?
2) This portion of Translation is known as Initiation.
d. Next, a second tRNA with its amino acid enters the ribosome. It attaches as
above. This begins the elongation phase.
e. Once the two tRNA molecules are in place, their amino acids will form a
covalent bond with each other. What is the bond referred to as?
f. Next, the first tRNA releases its amino acid and exits the ribosome.
g. The ribosome will move to the next mRNA codon.
h. A third tRNA molecule with its amino acid enters the ribosome.
i. What happens next?
j. This process continues until the ribosome reaches a special stop codon on the
mRNA molecule. This initiates termination.
4. What does translation produce?
VIII. MUTATIONS-any change in a cell’s DNA.
A. Mutations can be helpful or harmful.
B. Causes of Mutation
1. Ionizing radiation-including gamma and X-rays.
a. This produces free radicals in which portions of DNA lose electrons.
b. This can lead to the formation of cancer.
2. Ultraviolet Radiation-where does this come from?
a. This may produce dimers. Dimers occur when the bases of DNA do not pair
and bond properly. Dimers can also block DNA replication.
3. Chemical Mutagens
a. Many chemicals can lead to the formation of mutations.
b. What chemicals can act as mutagens?
c. Nicotine-causes mutation in the DNA of epithelial cells. Where are these cells
found in the human body?
1) Nicotine damages the gene that regulates cell division.
2) When this gene is damaged, the lung cells begin to divide in an
uncontrollable fashion.