DNA: The Molecule of Heredity
Ch. 12 Notes: Thursday - Tuesday
Test (Ch. 12-13): Wednesday, 10/13/2010
DNA Introduction
• DNA is the genetic material of life
• DNA structure is a double helix
– Discovered by James Watson and Francis Crick
DNA Structure
• Made of repeated subunits/monomers
• Nucleotide – 3 part subunit of DNA molecule
• 3 subunits
– sugar (deoxyribose)
– phosphate group (phosphorous and 4 oxygens)
– nitrogen base
•
•
•
•
adenine (A)
thymine (T)
guanine (G)
cytosine (C)
DNA is a Long Polymer
– Chain of subunits shaped in a helix
– Looks like a twisted ladder
– The sides of the ladder are the sugar-phosphate
bonded groups and the rungs are the nitrogen
bases.
– See picture on board / draw in notes
Using the Code
• A nucleotide sequence provides information
to assemble amino acids.
• In turn, amino acids assemble to make
proteins.
• SO, GENES ARE DNA CODES FOR
MAKING PROTEINS!!!
RNA
• RNA carries out the instructions of DNA
in the cell.
• Differs from DNA in 3 ways:
– RNA is single stranded; not double like DNA
– RNA uses ribose sugar; not deoxyribose
– RNA has uracil (U) instead of thymine, still
acts the same by binding to adenine.
Types of RNA
• mRNA – messenger RNA brings codes
from nucleus to cytoplasm.
• rRNA – ribosomal RNA links onto
mRNA to assemble amino acids.
• tRNA – transfer RNA transports amino
acid to ribosome to assemble protein.
Transcription
• The process of making RNA from DNA.
• In the nucleus, sections of nucleotides from
DNA replicate the gene – code sections of RNA
to be sent out for protein production.
• Ex: DNA code TGATTC will make RNA code
ACUAAG.
• Codon – a 3 nucleotide section that codes for
amino acids.
Codons
• Every different amino acid – has its own
signature codon. (Example, methionine is coded
by the codon AUG.)
• Find the corresponding amino acid using the
genetic code in your book. (p. 303)
– AAA
– GCC
– UUC
• Find all the possible codons for isoleucine
Translation
• The process of decoding mRNA codons
into the amino acid sequence of proteins.
• Example: GGC links to tRNA carrying
the sequence CCG to produce an amino
acid.
• What tRNA would TTG link to? What
amino acid would be coded for?
Mistakes in Transcription and
Translation
• Point mutation – a single mistake in DNA
transcription to RNA where one nitrogen
base is replaced by another. Single
change.
• Frameshift mutation - when one base gets
inserted or deleted throwing off the entire
sequence. Extensive change.
• (SEE EXAMPLES)
Chromosomal Mutations
• Mutagen – environmental condition that causes
mutation. Radiation, heat, chemicals . . .
• Mutation - a change in the normal genetic code.
• Deletion - a part of a chromosome is removed.
• Insertion - extra material is added, leading to
nonfunctional genes.
• Inversion - codes flip-flop - out of order.
• Translocation - part of a chromosome is cut out
and bonds to a different chromosome.
Objectives: Friday, 11/4/05
• Notes: after lecture, you should be able to
– describe the function of DNA polymerase,
helicase, and RNA polymerase
– explain the central theme of molecular biology
• Lab: after lab activity, you should be able to
– understand base pairing rules
– analyze molecular basis of heredity / DNA
Central Dogma (Theme) of
Molecular Biology
DNA
RNA
Proteins
Replication
• Double helix must be unwound. This
requires the helicase enzyme.
• The unwound strands separate and act as
templates for the formation of 2 new
strands.
• The addition of new nucleotides is
determined by base-pairing rules. This
addition (polymerization) requires the DNA
polymerase enzyme.
RNA vs. DNA
• there are 3 major differences between the 2
forms of nucleic acids
– Composition of sugar backbone (reflects name)
• RNA - ribonucleic acid (ribose)
• DNA - deoxyribonucleic acid (deoxyribose)
– number of strands: RNA = 1 and DNA = 2
– RNA does not have thymine as a nitrogen base,
instead uracil (U) is substituted in its place
Transcription: mRNA Production
• Short segment of DNA (gene) is transcribed
into mRNA for movement out of the
nucleus to the ribosome
• DNA is used as a template to make a
complementary piece of “messenger
RNA”. This requires the aid of the RNA
polymerase enzyme.
Translation: Protein Synthesis
• mRNA transcript arrives at the ribosome to
act as the genetic code for protein assembly
• proteins are composed of a unique sequence
of amino acids.
• mRNA carries the code for the order and
type of amino acids to be included in the
protein. The “genetic code” is the unique
order of triplet codons located on the
mRNA strand.
What does a “Gene” really do?
• It is simply a blueprint in the DNA that tells
how to build a protein.
• “WHO” we are genetically is really just a
collection of “WHAT” proteins are
expressed in our phenotype.
Transcription / Translation Lab
• Obtain the following from Mr. Smith:
–
–
–
–
1 per student
an mRNA transcript (little slip of paper)
a genetic “dictionary” - Genetic Code
Lab Worksheets - 3 pages
• Work on your own, but check your work
with those around you. Remember, you
each possess your own code, so you won’t
get the same sequences as everyone else.