Biology II –
Chapter 9: DNA: The Molecule of Heredity & Chapter 10: Gene Expression and Regulation
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Every cell must store and transmit the information necessary for the manufacturing of proteins.
Proteins form the structural units of cells and control all chemical processes within the cell.
Both DNA and RNA are polymers – formed from the monomers called nucleotides.
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DNA stores the information for protein synthesis.
RNA transmits the information for the building of proteins.
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DNA makes RNA  RNA makes proteins.
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DNA (Deoxyribonucleic Acid)
o Molecule that carries the blueprint for all forms of life on Earth
 Directs the life of each cell in an organism
 Enables organisms (or the cells of the organism) to transmit information
accurately from one generation to the next
o The discovery of DNA is one of the greatest achievements of the 20th century
 Beginning in the late 1920’s, hundreds of scientists and decades of research
have contributed to this achievement, even though only a handful of Nobel Prize
winners are credited with the discovery.
o Genes – subunits that carry heritable information and determine many of the heritable
differences among individuals within a species
 Consist of DNA
 Located on structures called chromosomes.
 Chromosomes consist of DNA and protein.
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DNA Composition
o Sugar called deoxyribose
o phosphate group
o 4 Nitrogen bases:
 Adenine – A
 Guanine – G
 Purines – a double ring of carbon and nitrogen atoms
 Thymine – T
 Cytosine – C
 Pyrimidines – a single ring of carbon and nitrogen atoms
o The DNA of any given species contains equal amounts of adenine and thymine, as well
as equal amounts of guanine and cytosine
o Consists of two long rope-like strands – called a double helix – each is a chain of
nucleotide monomers
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The Double Helix: the three-dimensional structure of DNA
o Late 1940’s – Rosalind Franklin recorded the structure of the DNA helix using x-ray
diffraction to measure the DNA pattern and dimensions – research led to her early
death due to radiation poisoning
o 1953 - James Watson and Frances Crick were the discoverers of the double helix
structure.
o Two strands bonded together that twist around a central axis to form the spiral structure
known as the double helix.
o Pairs of nitrogen bases bonded to a sugar-phosphate backbone
 Has a right-handed twist – each turn consisting of ten base pairs
 Think of a twisted ladder:
 Sides – alternating sugar-phosphate units
 Rungs – the bonded pairs of nitrogen bases
o Uniform in length – one base with a double ringed purine; the
other with a single ringed pyrimidine
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The pairs are held together by hydrogen bonds – form between purines and
pyrimidines.
 Adenine == Thymine
o Form 2 hydrogen bonds
 Cytosine == Guanine
o Form 3 hydrogen bonds
o The sequential arrangement of bases on one strand therefore matches its exact
complement of base arrangement on the other strand – therefore, the strands are said to
be complementary base pairs or strands
 Not random!! – the sequence of nitrogen bases determines the structure and
function of an organism
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DNA Replication
o The complementary strands allow for exact self-replication (duplication)
o This process of duplicating a DNA molecule is called replication.
o During the process of replication, the strand unwinds – each strand providing a
template to build a new complementary strand – therefore, producing two new DNA
strands that are exact replicas of the original.
o The Process of Replication:
 Begins when the enzyme called DNA helicase attaches and breaks the hydrogen
bonds that hold the bases together.
 Next, each strand now “picks up” a new complementary base that is freefloating in the nucleus and hydrogen bonds form with the help of another
enzyme called DNA polymerase.
 The final result is two new DNA molecules – each consisting of one “old”
strand of DNA and one “new” strand of DNA
o This process does not begin at one end of the DNA molecule and end at the other
 Occurs at many points on the molecule – all happening at the same time
 Reduces the copying time dramatically
o The cell where replication occurs has a built-in “proofreader” – allows for very little
error in copying – approximately one error per billion nucleotides occurs
o DNA also has the ability to repair itself – assisted by a group of 20 or more enzymes
that recognize, remove, and replace damaged nucleotides
 Factors that can damage DNA in Humans:
 Body heat
 Chemicals
 Radiation
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RNA (Ribonucleic Acid)
o The primary function of RNA is to synthesize proteins.
o RNA is different from DNA in 3 ways:
 Consists of only one strand of nucleotides
 Ribose is RNA’s five-carbon sugar
 Instead of Thymine – RNA has Uracil as a nitrogen base
o 3 Structural Forms/Types
 Messenger RNA – (mRNA) – an uncoiled strand – transmits information from
DNA for use during protein synthesis
 Serves as a template, or pattern, for the assembling of amino acids
during protein synthesis
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Transfer RNA – (tRNA) – cloverleaf or hairpin form – allows some
complementary bases to pair
 Exists in 20 or more varieties – each able to bond to one specific type of
amino acid
Attached
amino acid
anticodon
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Ribosomal RNA – (rRNA) – globular form – the major component of the
ribosomes
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Transcription
o Definition of transcription: to make a written copy
o The process where RNA is produced from DNA is called transcription
 Occurs in the nucleus.
o During transcription:
o All three types of RNA are transcribed in the same manner.
o Each then moves from the nucleus to the cytoplasm – where it is involved in protein
synthesis.
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Translation
o Definition of translation: to convert words of one language into another
o The process of protein synthesis from RNA is called translation.
 Ribosomes carry out the process of translation
o The number of different proteins that make up an organism depends on the organism’s
complexity.
o Proteins are made up of amino acids – these must be in a specific sequence for the
protein to function properly.
 All structural and functional characteristics of a protein are determined by its
amino acid sequence.
o Codons
 The region of DNA that directs the formation of a protein (or polypeptide) is
called a gene.
 Gene – a short segment of DNA that contains coding for a polypeptide
or protein.
 Since most proteins are made up of more than one polypeptide, several
genes may contribute to the production of a particular protein.
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Genetic code – the system that contains information needed by cell for proper
functioning
 This is built into the arrangement of the sequence of DNA
o Therefore, DNA is ultimately responsible for the correct
sequencing of amino acids.
 Virtually every organism uses the same genetic code.
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This code is found in a group of three sequential bases of mRNA called a
codon.
 Codon – a group of three sequential nitrogen bases of an mRNA
molecule
Each codon recognizes a specific amino acid using tRNA.
There are 64 possible codons – each amino acid may have several codons
 The codons differ from each other by the base in the 3rd position
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Some codons do not encode amino acids
 Instead, they are “start” signals that make a ribosome begin to read an
mRNA molecule or are “stop” signals that cause a ribosome to stop
reading mRNA.
AUG is the universal “start” code.
o During translation:
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Mutations
o Mistakes can occur in any process – a mistake in the gene sequence is called a
mutation.
o A single faulty mRNA molecule or single defective protein may not affect a cell very
much
o A single faulty gene may be more serious because all of the proteins synthesized
according to the base sequence will be defective
o Mutations that occur in gamete cells can be passed on to future generations if not lethal.
o 4 Types of Mutation:
 Point Mutation – or nucleotide substitution – when individual nucleotides in
DNA sequence are changed or the wrong pair of nucleotides are matched
 Substitution can result in at least four different outcomes:
o Unchanged protein
o New protein is equivalent to the original protein
o Protein function may be changed
o Protein function may be destroyed
 Insertion Mutation – when one or more new nucleotides are inserted into a
gene
 Almost always results in a nonfunctioning protein
 Deletion Mutation – when one or more nucleotides are removed (or deleted)
from a gene
 Almost always results in a nonfunctioning protein
 Neutral Mutation – a change that does not directly affect or change the
function of the encoded protein
o Mutations are essential for evolution – enable the random changes in DNA sequence
that provides possible genetic advantages for survival within the current environment.
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Gene Regulation
o Individual cells express certain genes that are appropriate for the function of that cell
type.
 Individual genes – sex hormones, blood/albumin
 Regions of chromosomes – some regions don’t contain genes, while others are
found in condensed areas of DNA where it must uncoiled to reach the necessary
gene
 Entire chromosomes – sex chromosomes (XX or XY)
o Gene expression can change over time – depending on the organism’s present needs.
o An organism’s environment also helps determine gene expression.