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Chapter 8 Notes
I. DNA (deoxyribonucleic acid); Strands that contain the genetic information that is
found in our chromosomes. The shape is a double helix.
A. Composition; Nucleotides that are bonded together. A nucleotide is made of a
phosphate group that is bonded to a deoxyribose sugar and a nitrogenous base.
1. Nitrogenous bases; Purines will bond to pyrimidines. They
are held together with hydrogen bonds.
a. Purines; adenine and guanine
b. Pyrimidines; thymine and cytosine
* thymine and adenine always bond together, cytosine and guanine
always bond together.
B. DNA replication; Replication is directional, meaning it has to occur in a very
specific direction.
1. An enzyme, DNA helicase splits the DNA strand at the
hydrogen bonds, separating the base pairs, and unzipping the molecule.
2. Another enzyme, DNA polymerase attaches free nucleotides to their
complementary base pair by hydrogen bonding.
3. Because this is directional, the pairing of bases must occur in a 5’ to 3’
direction.
4. When replication is complete, there are 2 identical molecules of DNA.
Each new molecule is made of a parent strand and a complementary daughter
strand.
II. RNA (ribonucleic acid); Single stranded
A. Composition; like DNA, RNA is composed of nucleotides. There are two
major differences in DNA and RNA
1. ribose sugar instead of deoxyribose
2. uracil instead of thymine
B. Types;
1. r-RNA; Ribosomal RNA and protein make up the beadlike
structures known as ribosomes.
2. m-RNA; Messenger RNA takes instructions from DNA in the nucleus to the
ribosomes in the cytoplasm. Instructions are in the form of three
nitrogenous bases called a codon.
* Codon determines which amino acid will be attached at the ribosome.
3. t-RNA; Transfer RNA carries amino acids to the ribosome so that they can
form a chain.
III. Protein Synthesis; Making proteins
A. Transcription; Occurs in the nucleus. DNA is split, as if replication is about to
occur. A complementary strand of m-RNA codons is formed using the DNA
strand as a template.
B. Translation;
1. m-RNA leaves the nucleus and attaches to a ribosome.
2. The start codon (AUG) attracts a complementary anticodon of t-RNA
that carries the amino acid methionine. This starts the protein synthesis.
3. A new t-RNA molecule carrying an amino acid attaches to the ribosome
and m-RNA strand next to the previous molecule.
4. The amino acids on the t-RNA molecules join by peptide bonds, and the
t-RNA molecule is released.
5. A chain of amino acids is formed until a stop codon is reached on the
m-RNA strand.
IV. Mutations; May be caused by errors in replication, transcription, cell division, or by
external factors. Mutations may have negative effects, or beneficial effects.
A. Cell types
1. Body cells; mutations that occur in body cells might cause problems for the
individual that has the mutation.
2. Reproductive cells; if mutations that occur in reproductive cells the altered
gene may become part of the genetic makeup of the offspring.
B. Mutation types;
1. Changes in single base pairs
a. Frame shift: A single base is added or deleted from the DNA sequence.
When the m-RNA is transcribed the entire strand will be out of position by
one base.
b. Point mutation; A change in a single base pair in DNA.
2. Chromosomal mutation; Alterations in the structure of chromosomes
a. Deletion; part of a chromosome is left out.
b. Insertion; part of a chromatid breaks off and attaches to its sister
chromatid. This causes a duplication of genes on the same chromosome.
c. Inversion; Part of a chromosome breaks off and reattaches backwards.
d. Translocation; Part of one chromosome breaks off and is added to a
different chromosome.
V. Causes of mutations;
A. Mutagens; Any agent that can cause a change in DNA.
1. Radiation; x-rays, cosmic rays, ultraviolet light, and nuclear radiation.
2. Chemical; some causes are dioxins, benzene, asbestos, and formaldehyde.
3. High temperatures
B. Spontaneous; mistakes in base pairing, mitosis, or meiosis.