Transcript Chapter 11 - BickfordBiology

Chapter 11
DNA: The Molecule of
Heredity
What is DNA?
• Deoxyribonucleic Acid – sugar is
deoxyribose
• Carries complete instructions for
manufacturing proteins
• Genetic information that determines an
organisms traits
• DNA produces proteins
• Proteins are enzymes
• Enzymes control chemical reactions in
body
Structure of DNA
• Very long polymer made of repeating
nucleotides
– Nucleotides = simple sugar, a phosphate
group, and a nitrogen base
• Four possible nitrogen bases
– Adenine (A)
– Guanine (G)
– Thymine (T)
– Cytosine (C)
• Phosphate group of one nucleotide
binds with deoxyribose sugar of
adjacent nucleotide
– These form the backbone of the chain
• Nitrogen bases stick out like rungs of a
ladder up the chain
• # of Adenine is always equal to the # of
Guanine
• # of Thymine is always equal to the #
of Cytosine
Watson & Crick
• In 1953 proposed DNA made of 2
chains of nucleotides joined together
via hydrogen bonds at the nitrogen
bases
Double Helix
• Nucleotides match up with
their complementary base
pairs
• Adenine opposite Thymine
• Guanine opposite Cytosine
• Looks like rungs of a ladder
• Ladder twists forming a
spring-like form called a
double helix
Importance of Nucleotide Sequencing
• Sequence of nucleotides forms a
unique code for each organism
• Used to determine whether two
people are related
• The closer the relationship between
two organisms, the greater the
similarities in the order of DNA
nucleotides
Replication of DNA
• Before Mitosis/Meiosis cells make a
copy of their DNA
• An enzyme breaks the hydrogen
bonds between nitrogen bases and
helix unzips forming two strands
• Free nucleotides pair with their
complementary base pairs
• Results in formation of two new DNA
molecules
11.2
From DNA to Protein
• Genes to Proteins
– There are an estimated 80,000 genes in
a human cell
– Proteins are polymers of amino acids
– The sequence of nucleotides determines
the protein
– Proteins form complex 3D shapes that
become key cell structures and
regulators of cell function
Actin & Tubulin form the cytoskeleton
Transcription
• In the nucleus, enzymes make an RNA
copy of a DNA strand
• Role of RNA?
– Protein synthesis
– RNA takes instructions from DNA on
how to assemble the proteins
– There are Three types of RNA
– What is RNA?
RNA – Ribonucleic Acid
• Structure differs from DNA in three ways
– RNA is a single strand
– Sugar in RNA is ribose
– RNA replaces Thymine with Uracil and base
pairs with Adenine
mRNA – messenger RNA
• Brings
information
from DNA to
cytoplasm
rRNA – ribosomal RNA
• Clamps onto
mRNA and
assembles
amino acids
in correct
order
tRNA – Transfer RNA
• Transports amino acids to the ribosome to be
assembled into proteins
• Composed of approx. 80 nucleotides
• Each tRNA molecule is specific for one amino
acid
• Shaped like a T with an amino acid on one
end and three nucleotides on the other end,
the nucleotides are the complementary base
pairs to mRNA – they are referred to as an
Anticodon
The Genetic Code
• A code is used to convert the language
of mRNA into proteins
• There are 20 different amino acids;
however, mRNA contains only 4 types
of bases (A-U-G-C)
– A group of three nucleotides codes for one
amino acid  CODON
• The order of nitrogen bases in mRNA
determines type and order or amino
acids in a protein
• There are 64 possible combinations;
therefore, there are 64 different mRNA
codons
• The codon below codes for Alanine
• Some codons don’t code for amino
acids, they give instructions
– UAA – is the stop codon, it tells where
the protein stops
• One codon gives instructions as
well as code for an amino acid
– AUG – is the start codon, as well as
the Methionine codon, it tells where
the protein starts
• Amino acids can have more than one
codon; however, an individual codon
only codes for one amino acid
– Leucine has 6 codons
– UUG codes for Leucine only
• All organisms use the same genetic
code for amino acids and assembling
proteins
Translation
• Process of converting mRNA into proteins
• mRNA leaves nucleus and enters
cytoplasm; ribosomes attach to mRNA
• tRNA brings the first amino acid to the
mRNA
• Anticodon binds to mRNA
• Ribosome slides down the mRNA
chain to the next codon and a new
tRNA molecule brings the next amino
acid
• The new tRNA binds and the first
tRNA is released
• This process continues until a stop
codon is reached
• Translation ends
• Amino acid strand is released from
the ribosome, it twists and forms
complex 3-D structures and
becomes protein
11.3 Genetic Changes
Mutation: A change in DNA
• Mutations are changes in a DNA
sequence that also change the protein
it codes for
• Can affect reproductive cells and
altered gene will be part of offspring’s
genetic makeup
– Can produce new trait
– May result in a malfunction of a protein,
resulting in structural or functional
problems
– In rare cases, mutation can be positive,
make an organism faster or stronger
• In non-reproductive cells, a mutation
would not be passed to offspring, it
would only affect the individual
• Damage to a gene may impair cell
functions
– Cause it to lose its ability to work
– Cause it to divide uncontrollably
Point Mutations
• A change in a single base pair
– This changes the entire structure of the
protein
Frameshift Mutation
• A single base is added or deleted
– Results in a shift in translation causing
every codon after addition or deletion to
be out of position by one
Chromosomal Mutations
• Can occur in different ways
– Chromosome can break off during mitosis
or meiosis
– Chromosome can break and rejoin
incorrectly
• Attach backwards
• Join wrong chromosome
• Common in plants
Causes of Mutations
• Spontaneous mutations
– Mutations that just seem to happen
(mistakes during base pairing)
• Environmental
– Mutations that are caused by a mutagen
(an agent that causes a change in DNA)
– Radiation (X-Rays, UV Light)
– Chemicals (dioxins, asbestos,
formaldehyde)
Repairing DNA
• Organisms have enzymes that
proofread DNA and replace incorrect
nucleotides with the correct ones
•  the exposure to a mutagen the  the
chance that the mistake will not be
corrected
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