Download Molecular Basis of Heredity

3.01: Molecular Basis of Heredity
3.01: DNA STRUCTURE
DNA
• DNA is the called the “genetic blueprint”
because it contains the instructions needed
for your cells to carry out all of the functions
to sustain life. DNA stands for
deoxyribonucleic acid.
• Information encoded in your cells’ DNA is
organized into units called genes. A gene is
a segment of DNA that codes for a protein or
RNA molecule. By the 1950s, scientists
knew that genes were made of DNA, but
they didn’t know what this molecule looked
like.
DNA
• James Watson and Francis
Crick, two researchers at
Cambridge University,
discovered the structure of
DNA, which clarified how DNA
served as the genetic
material.
• The DNA molecule is a double
helix – two strands twisted
around each other like a
winding staircase or a twisted
ladder.
DNA
• Each strand is made of
linked nucleotides.
Remember—nucleotides
are the subunits that make
up nucleic acids like DNA
and RNA.
• Each nucleotide is made of three parts: a
phosphate group, a five-carbon sugar called
deoxyribose, and a nitrogen-containing base.
The sugar and the phosphate group are the
same for each nucleotide in a DNA molecule,
but the nitrogen base may be one of four
kinds.
DNA
Guanine
Cytosine
Thymine
Adenine
• The sugar-phosphate backbones are like
the side rails of a ladder, while the paired
nitrogen bases are like the rungs (steps) of
the ladder.
Discovering DNA’s Structure
• In 1949, Erwin Chargraff observed that for
each organism’s DNA that he studied, the
amount of adenine always equaled the
amount of thymine (A = T) and the
amount of guanine always equaled the
amount of cytosine (G = C).
Discovering DNA’s Structure
• In 1952, Maurice Wilkins and Rosalind
Franklin developed high-quality X-ray
photographs of DNA and showed that it
must be a tightly coiled helix composed of
chains of nucleotides.
• In 1953, Watson and Crick put together
Chargraff’s findings and stolen X-ray data
from Franklin and Wilson to come up with
the three dimensional double helix model.
Base Pairing in DNA
• Adenine always pairs with Thymine and
Guanine always pairs with Cytosine
(A – T)
(G – C)
• These base-pairing rules are supported
by Chargraff’s observations.
Base Pairing in DNA
• The nucleotides are paired together with
Hydrogen bonds, resulting in two strands
that contain complementary base pairs.
This means, the sequence of nitrogen
bases on one strand determines the
sequence of nitrogen bases on the other
strand.
Base Pairing in DNA
• In other words, one strand acts as a template
for figuring out the complementary strand.
Ex:
T C G A A C T
A G C T T G A
DNA Replication
• The process of making a copy of the DNA
is called DNA replication. Remember, this
occurs during the S phase of the cell
cycle, before the cell divides.
• Step 1: The DNA double helix unwinds
using an enzyme called DNA helicase.
Proteins hold each strand apart from each
other, forming a Y shape called a
replication fork.
DNA Replication
• Step 2: At the replication fork, enzymes
called DNA polymerase move along each
of the DNA strands, adding
complementary bases to the exposed
nitrogen bases according to base-pairing
rules.
DNA Replication
• Step 3: DNA polymerases add nucleotides to a
growing double helix until all the DNA has been
copied and the polymerases are signaled to
detach. Result: two new DNA molecules, each
composed of one original strand and one copied
strand. The nucleotide sequences of the two DNA
molecules are identical.
• This process is called semi-conservative because
each new DNA helix is made of half original DNA
molecule and half new DNA molecule.
DNA Replication
DNA Replication
• During DNA replication, errors can occur
in which the wrong nucleotide is added to
a new strand. Changes to the DNA are
called mutations.
• DNA polymerases proofread the new
strand as they build it to avoid these
errors. This results in an error rate of only
one error per 1 billion nucleotides.
3.01: TRANSCRIPTION AND
TRANSLATION
Transcription and Translation
• Traits, like eye color and hair color, are
determined by proteins that are built
according to instructions coded in DNA.
Proteins are not built directly from DNA,
though. Ribonucleic acid, RNA, is also
involved.
Transcription and Translation
• RNA differs from DNA in 3 important ways:
1.RNA is a single strand of nucleotides, instead
of two strands as in DNA.
2.RNA contains the sugar ribose, instead of
DNA’s sugar deoxyribose.
3.RNA has the nitrogen bases A, G, and C as
in DNA, but instead of T it has U for Uracil. U
is complementary to A, just like Thymine is in
DNA.
Transcription
• The instructions for making a protein are
transferred from a gene, which is a segment
of DNA, to an RNA molecule in a process
called transcription.
Transcription = DNA  RNA
• Like DNA replication, RNA transcription uses
DNA nucleotides as a template for making a
new molecule. The new molecule that gets
made, however, is RNA in transcription
instead of DNA as in replication.
Transcription
DNA  A C G T C A G T C A A T T C G
RNA  U G C A G U C A G U U A A G C
• In prokaryotes, transcription occurs in the
cytoplasm because prokaryotes have no nucleus.
• In eukaryotes transcription occurs in the nucleus
because that is where the DNA is kept
Translation
• After transcription is complete, cells use
the RNA that was made to put together
the amino acids that will make up a
protein. This process is called translation.
Translation = RNA  protein
Translation
• Amino acids that make up the protein are
held together by peptide bonds. The
protein is also called a polypeptide
because of these bonds.
• The entire process by which proteins are
made based on information coded in the
DNA is called gene expression or protein
synthesis.
Types of RNA
• Different types of RNA are made during
transcription depending on what gene is
being expressed:
 mRNA carries a message from the DNA to
make proteins
 tRNA carries amino acids to build proteins
 rRNA makes up the ribosome
Types of RNA
• When a cell needs a protein, it is
messenger RNA (mRNA) that is made.
mRNA carries the instructions for making
proteins from a gene in the nucleus and
delivers it to the site of translation in the
cytoplasm.
• This information then gets translated from
the language of RNA—nucleotides—to the
language of proteins—amino acids
Transcription and Translation
• The RNA instructions are written as a
series of three-nucleotide sequences or
“words” called codons. Each codon in the
mRNA strand stands for an amino acid or
signifies a start or stop signal for
translation.
Transcription and Translation
• Ex: See the
genetic code
on the
screen.
Whenever we
use this table,
we plug
mRNA into
the chart to
find the
amino acids
coded by the
DNA.
Transcription and Translation
• Start with DNA:
T A C A T G T G T
• Translate to RNA: A U G U A C A C A,
then split into 3-letter codons
• Transcribe into proteins: (use the table)
• Methionine, Tyrosine, and Threonine
• Translation takes place in the cytoplasm.
Here, transfer RNA molecules and
ribosomes work together to make proteins.
Transcription and Translation
• Transfer RNA (tRNA) carries a specific
amino acid on one end. Each tRNA
molecule has a three-nucleotide anticodon
that is complementary to an mRNA codon.
Write the anticodons for the mRNA codons
you translated above:
• tRNA anticodons: U A C—A U G—U G U
Transcription and Translation
• Ribosomes are made of ribosomal RNA
(rRNA) and proteins. Remember—
ribosomes are the site of protein synthesis
Steps of Translation
1. Transcription, in the nucleus, creates an mRNA
molecule.
2. mRNA leaves the nucleus and enters the
cytoplasm.
3. A ribosome attaches to mRNA at a “start” codon, a
tRNA molecule containing the anticodon to
mRNA’s codon attaches to the ribosome as well.
This signals the beginning of protein synthesis.
4. The ribosome slides down the mRNA molecule
while tRNA molecules, carrying amino acids, join
up to the mRNA in the ribosome just long enough
to drop off their amino acids, which join together to
form a protein chain (a polypeptide).
Steps of Translation
• This continues until a “stop” codon is
reached. Then the newly made protein is
released into the cell.
DNA/RNA REVIEW
DNA/RNA Review
• DNA and RNA are both nucleic acids. This
means they are composed of subunits called
nucleotides, which are made of a sugar, a
phosphate, and a nitrogen base.
• The structure of DNA is a double helix, which
was discovered by Watson and Crick
• One strand of DNA acts as a template for the
other strand, following base pairing rules: A
bonds with T, G bonds with C
DNA/RNA Review
• DNA replication means making an exact
copy of the DNA.
• The structure of RNA is a single strand of
nucleotides, not a double helix.
• RNA has the sugar Ribose, while DNA
has the sugar deoxyribose.
• RNA has A, G, and C nitrogen bases, but
has U instead of T.
DNA/RNA Review
• Transcription is making mRNA from DNA,
which occurs in the nucleus.
• Translation is making proteins from
mRNA, which occurs at the ribosome.
• Transcription and translation together
make up the process called protein
synthesis (making proteins).
DNA/RNA Review
• Practice the following processes:
• DNA replication—using base pairing
rules, make the complementary strand of
DNA from the original:
• T A C C G G C T A A T A
• A T G G C C G A T T A T
DNA/RNA Review
• Transcription—using the new DNA strand
you just created as a template, tell what
the mRNA would be :
• U A C-C G G-C U A-A U A
• Translation— split your mRNA into codons
and use p. 303 in your book to write out
the amino acids that would result from this
strand of mRNA:
• Tyrosine, Arginine, Leucine, Isoleucine
GENE REGULATION AND
MUTATIONS
Gene Regulation
• For the most part, all of the cells in an
organism’s body have the same DNA.
However, cells are specialized for specific
tasks and parts of the body. How can
they be specialized but have the exact
same DNA, the exact same instructions?
Gene Regulation
• Cell differentiation is the way that cells
become different from each other as they
go through mitosis. At first, all cells are
the same and are not specialized. These
are called stem cells. As they grow and
divide they become differentiated and
specialized into heart cells, brain cells,
liver cells, etc.
Gene Regulation
• The differentiation of cells, despite the fact
that all cells have the same DNA, occurs due
to gene regulation—organisms’ cells can
regulate which genes are expressed and
which are not, depending on the cell’s
needs.
• As different cells respond to the environment
they produce different types and amounts of
proteins by “turning on” some genes and
“turning off” other genes. This protein
production is what specializes different cells
for different jobs.
Mutations
• A mutation is a change in the nucleotidebase sequence of a gene or DNA
molecule.
• When the cell replicates its DNA, the
enzyme DNA Polymerase is in charge of
proofreading the new DNA strand for
errors. If it makes an error and doesn’t
correct it, a mutation occurs.
Mutations
• Mutations can disrupt the functions of
proteins. Look at the example to see how
they can change proteins:
• DNA  TAC – GGC – GAG – TAG – CCT
• mRNA AUG – CCG – CUC – AUC – GGA
• Amino acids: Methionine, Proline, Leucine,
Isoleucine, Glycine
• These amino acids in this order make up a
specific protein that is needed by the body. If
one DNA nucleotide is changed, look what
happens to the protein:
Mutations
• DNA  TAC – GTC – GAG – TAG –
CCT
• mRNA  AUG – CAG – CUC – AUC GGA
• Amino acids: Methionine,Glutamine,
Leucine, Isoleucine, Glycine
• Now a different protein has been made, or
perhaps an amino acid sequence that is
unstable and will do nothing.
Mutations
• A different type of mutation can occur called
a frameshift mutation. What would happen if
the cell accidentally removed one of the
nucleotides in the original DNA sequence?
• DNA  TAC – GGC – GAG – TAG – CCT

TAC – GGC – AGT – AGC – CT…
• mRNA AUG – CCG – UCA – UCG – GA
• Amino acids: Methionine, Proline, Serine,
Serine,
__________
Mutations
• In these ways, protein synthesis can be
disrupted by even a small mutation in the
DNA sequence.