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Chapter 12
Remember! Chargaff’s rules
• The relative amounts of adenine and
thymine are the same in DNA
• The relative amounts of cytosine and
guanine are the same.
• Named after Erwin Chargaff
Review of the double-helix
• A twisted ladder with two long chains of
alternating phosphates and sugars. The
nitrogenous bases act as the “rungs” joining
the two strands.
How long is the DNA molecule?
Male vs Female
• MALE
• Usually the Y
• FEMALE
• Usually the X
chromosome.
• Y is usually smaller
• Male genotype = XY
chromosome.
• Larger than the Y
• Female genotype XX
Except Birds
Male = XX
Female = XY
Chromosome structure:
DNA replication
• Replication must
occur before a
cell divides.
• Each new cell
needs a copy of
the information
in order to grow.
DNA replication. Why needed?
• Before DNA strand
can be replicated or
copied it must be
“unzipped”
• DNA polymerase
(enzyme that unzips)
• Starts at many
different points. Why?
Completing the replication
• After the DNA
molecule comes
apart, bases of
free nucleotides
in the nucleus
join their
complimentary
bases.
RNA
• Ribonucleic Acid
• It is a DISPOSABLE copy of DNA
• Its job? To make PROTEINS!!!
• Very similar to DNA w/ 3 Exceptions:
1) Ribose is the 5-carbon sugar
2) Uracil replaces thymine
3) Single-stranded
Types of RNA
• A. mRNA (messenger)
• Copies genetic code of
DNA by matching
bases.
• Occurs in the nucleus.
• Carries copies of
information
TRANSCRIPTION
• The process of turning DNA into mRNA
• Requires the enzyme RNA polymerase.
1. RNA polymerase binds to the DNA and
unwinds it.
2. Separates the DNA strands
3. Uses one strand as a template to make
mRNA
• Where does this take place? NUCLEUS!
Transcription continued:
Confusing genetic terms:
• Amino acids = building blocks of a
polypetide
• Polypeptide = building blocks of a protein.
• Protein = a bunch of polypeptides
Reading the genetic code
• The genetic code is responsible for
building all the proteins in the body
using 20 different amino acids.
• Extra credit: How many 3 letter words
can you make from the letters A,T,G
and C?
• Answer: 64
Codons
• A three letter “word” that specifies
an amino acid.
Genetic code:
tRNA (transfer)
• approx. 80 nucleotides
•
•
•
•
in length.
Cross-like shape
At one end an amino
acid is attached
At the other end there
is an anticodon
Acts like a truck
Polypeptide assembly
• Translation = reading
or “translating” the
RNA code to form a
chain of amino acids.
• Known as protein
synthesis
• Occurs in the
cytoplasm. (p.304)
Mutations
• The source of variation in a genetic
sequence.
• Can be either gene or chromosomal
mutations.
• Point mutations = a change in a single
nucleotide in a sequence of DNA.
Frameshift Mutation
• Inserting an extra nucleotide which, in turn,
shifts the entire sequence one way or the
other.
Chromosomal mutations
• Involves a change in the number or
structure of the chromosomes.
• Deletion : when a piece of a chromosome
breaks off and is lost.
• Duplication : when a segment of a
chromosome is repeated
• Inversion : when a segment of a
chromosome is reversed.
More chromosomal mutations
• Translocation :
when part of a
chromosome
breaks off and is
attached to a nonhomologous
chromosome.
Advanced Honors: Control of gene
expression
• Genes are often like light switches
that can be turned off and on.
• Operon = occur in prokaryotes.
(bacteria) different genes that work
together to activate gene functions
Eukaryotic gene expression
• Controlled by
complex
sequences of
DNA.
• Example:
“TATA box”
Factors:
• Overall gene control is more difficult
for eukaryotes because functional
genes may be on different
chromosomes.
• Environmental such as chemicals and
temperature.
Hox and Oncogenes
• Hox genes
• Genes that
actively control
embryonic
development.
• Oncogenes
• Genes known to
cause cancer.
• Usually these are
switched “off”, but
can be switched
“on” by a number
of factors.