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DNA Structure and Replication
• Why is DNA important in your life?
– DNA contains information that is critical to the
structure and function of your body’s cells.
– When new cells are made in your body, they must contain
DNA and there needs to be a full set of DNA
transferred accurately to these new cells
– The instructions encoded in DNA play a major role in
determining how your body operates
– A child’s life depends on the accurate transmission of
genetic information from his or her parents
– DNA is the molecular basis of reproduction.
– DNA is required for the building, maintenance, and
regulation of all living organisms’ cells.
• How are copies of genetic information made?
– The process is called replication
• In humans, as in all eukaryotes, the genetic material consists of
long DNA molecules.
– Packaged tightly in chromosomes (DNA molecule
and many protein molecules)
– One of the middle sized human chromosomes
contains about 145 million nucleotides (nitrogen
bases)
– In each of your cells (except gametes and red
blood cells), you have 23 pairs of chromosomes,
for a total of 46 chromosomes.
• Nucleotides are the building blocks (subunits of
DNA)
• Nucleic acid is a long chain (polymer), or strand
made up of nucleotides
• DNA Nucleotides
– made up of a nitrogen base, a deoxyribose
sugar, and a triphosphate group
– will have one of four different nitrogen bases:
Adenine (A), Cytosine ©, guanine (G), and
thymine (T).
• The DNA sequence is the order of the nitrogen
bases along one strand of DNA. This sequence
conveys information to certain parts of the cell
How does this structure allow a DNA molecule
to be copied to make another identical
molecule?
• 1. Pairing between nitrogen bases on opposite
strands is always the same.
– This bonding pattern is called complementary
base pairing.
– Adenine, a large base , bonds with thymine, a
small base.
– Guanine, a large base, bonds with Cytosine, a
small base.
• 2. Specific Enzymes are required for replication
to take place
– Begins with enzymes that separate the two DNA strands
– Next, enzymes read the sequence of nucleotides on one
strand,
– Finally, enzymes create a new complementary strand by
adding one nucleotide at a time to the new strand (DNA
Polymerase).
• Remember: A always bonds with T, and G always
bonds with C. This means that the sequence of
bases in the old strand determines the sequence
of bases in the new strand. Each newly added
base must complement the base in the old strand.
Result:
There are two molecules of DNA where
previously there had been one. Each
molecule contains one old strand and
one new complementary strand. DNA
molecule served as a pattern for a new
copy of the genetic information that it
encodes.
Gene Mutations
deletion, insertion, substitution
pgs 582-583
• Mutations are changes in the DNA sequence
that affect genetic information.
• A. Gene Mutations result from changes in a
single gene.
1. Point mutations affect one nucleotide in the
DNA sequence.
Some substitute one nucleotide for another
which generally changes one amino in a protein.
2. Frame shift mutations occur when a point
mutation involves the insertion or deletion of a
nucleotide(nucleotides). Every amino acid in
the protein is effected because the codons are
now different. The protein is useless.
12-3 RNA and Protein Synthesis
• Genes are coded DNA instructions that control
the production of proteins within the cell.
• The Structure of RNA
– Consists of a long chain of nucleotides
– RNA nucleotides contain the sugar ribose in
place of deoxyribose and the nitrogen base
uracil in place of thymine.
• The ability to copy a single DNA sequence into
RNA makes it possible for a single gene to
produce hundreds or even thousands of RNA
molecules.
Types of RNA
• In the majority of cells RNA molecules are
involved in protein synthesis.
• mRNA or messenger RNA serve as messengers
from DNA to the rest of the cell.
• rRNA or Ribosomal RNA molecules along with
several dozen proteins form the ribosomes.
• tRNA or Transfer RNA molecules transfer amino
acids to the ribosomes as it is specified by coded
messages in the mRNA
• Transcription Model
– Put together a DNA molecule
– mRNA is transcribed similarly to the
first steps of DNA replication.
– How can a single strand of RNA be
produced from DNA?
– Is it necessary to use all the DNA?
– Where does the mRNA go and what
happens to the DNA?
Transcription
• An enzyme RNA polymerase attaches to the DNA at regions
of the DNA known as promoters.
– Promoters are specific nitrogen base sequences in DNA
which signal the binding of RNA polymerase.
– Similar DNA nitrogen base sequences serve as signals to
stop RNA transcription.
• RNA polymerase separates the DNA strands and uses one
strand of DNA as a template from which nucleotides are
assembled into a strand of RNA
• Laser Disk on transcription.
The Genetic Code
• Proteins are made by joining amino acids into
long chains called polypeptides or proteins.
• The properties of proteins are determined by
the order in which different amino acids are
joined together to produce polypeptides
• The language of mRNA instructions are in the
form of codons, three nucleotides that
specify a single amino acid that is to be added
to the polypeptide chain.
• There are 64 possible codons but some amino
acids can be specified by more that one codon
(Figure 12-17).
• Note: AUG (methionine) the start codon and
three stop codons.
RNA Editing
• Introns are intervening sequences of mRNA in the
nucleus.
• Exons or expressed sequences are spliced back
together to form the final mRNA.
• Some RNA molecules may be cut and spliced in
different ways in different tissues making it
possible for a single gene to produce several
different forms of RNA.
• Introns and exons may play a role in evolution
• Using your DNA nucleotides put together a strand
of DNA and then using the RNA nucleotides carry
out transcription. Show intron splicing and write
the nucleotide sequence of the mRNA on paper.
Translation
• During translation, the cell uses information from
messenger RNA to produce proteins.
• 1. mRNA must be transcribed from DNA in the
nucleus and released into the cytoplasm.
• 2. Translation begins when an mRNA molecules in
the cytoplasm attaches to a ribosome
– A start codon on the mRNA (AUG) attaches to
the anticodon UAC on a tRNA which is carrying
the amino acid methionine.
– As the ribosome (rRNA) continues to move to
the next codon of mRNA the anticodons of
another tRNA carrying another amino acid
bonds with the complimentary codons of
mRNA.
• 3.
• 4.
The ribosome forms a peptide (covalent) bond
between the first and second amino acids.
The ribosome also breaks the bond that had
held the first tRNA to its amino acid and
releases the tRNA molecule from the mRNA.
The ribosome moves to the next codon.
The ribosome eventually reaches a stop codon
on the mRNA molecule. The polypeptide
(protein) is released from the tRNA, the
mRNA molecule is released from the
ribosome and translation is complete.
The roles of RNA and DNA
• The DNA remains in the safety of the nucleus
• RNA molecules go to the protein-building sites in
the cytoplasm- the ribosome.
Genes and Proteins
• Many proteins are enzymes, which catalyze and
regulate chemical reactions .
• Genes for certain proteins can regulate the rate
and pattern of growth throughout an organism,
controlling its size and shape.
• Proteins are the keys to almost everything that
living cells do.
• Quiz tomorrow over section 3 chapter 12