Download Biology DNA and Protein Syn

Biology:
STRUCTURE OF DNA
DNA and Genes
• How do genes work? How do they determine
the characteristics of organisms?
• To truly understand genetics, biologists first had
to learn the chemical composition of a gene.
• It took the work of many scientists over several
years to identify DNA as the genetic material,
and to discover its structure.
DNA Code is Universal
Griffith and Transformation
• 1928- Frederick Griffith was studying the
mechanism by which bacteria cause disease.
• He used two strains of the same type of
bacteria: one that caused pneumonia
(pathogenic), and one that did not.
• He discovered that even when the pathogenic
bacteria were killed by heating them, they could
pass their disease-causing ability to the
harmless strain.
• Griffith called this transformation, since one
strain of bacteria(harmless) had been changed
into another (disease-causing) strain.
Avery and DNA
• 1944- Avery and a group scientists repeated
Griffith’s experiments, making an extract from
the heat-killed bacteria.
• They used enzymes to remove carbohydrates,
proteins, lipids, and other molecules (but not
DNA). Transformation still occurred.
• When enzymes were used to break down DNA,
transformation did not happen.
• They concluded that DNA is the nucleic acid
that stores and transmits genetic
information.
Hershey and
Chase Experiment
• Alfred Hershey and Martha Chase studied
bacteriophages, viruses that infect bacteria by
injecting its DNA into the bacterial cell.
• They wanted to determine if it was the protein
coat, or DNA that entered the infected cell.
• They used radioactive phosphorus (32P) to mark
the DNA, and radioactive sulfur (35S) to mark the
protein.
• When the virus infected the bacterial cell, 32P
was found.
• Hershey and Chase concluded that the bacteria
was infected with DNA, the genetic material.
Structure of DNA
•
DNA is made up of units called
nucleotides.
• Each nucleotide is made up of three
basic parts:
1) A 5-carbon sugar, deoxyribose
2) A phosphate group
3) Nitrogenous base: adenine and guanine
are purines, cytosine and thymine are
pyrimidines
Structure of DNA, cont.
• Chargaff discovered that in a DNA sample,
the percentage of cytosine and guanine is
equal, and the percentage of thymine and
adenine is equal.
• Chargaff’s rules: [C]=[G] and [A]=[T]
• The backbone of a DNA chain is formed
by sugar and phosphate groups of each
nucleotide.
X-ray Evidence
• In the early 1950s,
Rosalind Franklin studied
DNA using X-ray
diffraction.
• The patterns in her
pictures showed that the
strands in DNA formed a
coil shape (helix)
• Her studies indicated that
there were two strands,
and that the nucleotides
were toward the center of
the molecule.
X-ray Diffraction of DNA
Watson and Crick
• James Watson and Francis
Crick were working on the
structure of DNA in the 1950s.
• Using information from
Chargaff, Franklin, and other
scientists, they put together a
3-D model of DNA.
• Watson and Crick’s model was
a double helix, with hydrogen
bonds between the nitrogen
bases holding the strands
together.
• They won the Nobel Prize for
their work.
DNA Replication
• During DNA
replication, the double
helix is separated into
two individual strands.
• The rules of base
pairing are applied,
and the result is two
identical new DNA
strands: each with
one side from the
original.
• Each strand of the double helix serves
as a template, or model, for the new
strand.
• The new strands are called
“complementary” strands.
• DNA replication is carried out by a
series of enzymes. One enzyme
“unzips” the original DNA strand.
• DNA polymerase brings nucleotides
together to produce the new strand and
proofreads it to make sure it is an exact
copy.
RNA and Protein Synthesis
•
The structure of RNA has several
differences from DNA:
1. The sugar is RNA is ribose. DNA has
deoxyribose.
2. RNA is single-stranded. DNA is a
double helix
3. RNA contains uracil instead of
thymine. Uracil pairs with adenine in
RNA.
Types of RNA
• There are three types of RNA, and
all are involved in protein
synthesis:
1. Messenger RNA (mRNA)- carries
messages from DNA for
assembling amino acids into
proteins
Types of RNA, cont.
2. Ribosomal RNA (rRNA)- Proteins and
rRNA make up ribosomes. Ribosomes
are sites of protein synthesis.
3. Transfer RNA (tRNA)- transfers each
amino acid to the ribosome as
specified by codes in the mRNA. These
amino acids are assembled into
proteins.
Transcription
• RNA molecules are produced by
copying part of the nucleotide
sequence of DNA into a complementary
sequence in RNA.
• Transcription requires an enzyme
called RNA polymerase that is similar
to DNA polymerase.
Transcription
Adenine (DNA and RNA)
Cystosine (DNA and RNA)
Guanine(DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
RNA polymerase
DNA
RNA
• During transcription, RNA polymerase
binds to DNA and separates the DNA
strands.
• RNA polymerase then uses one strand
of DNA as a template to assemble
nucleotides into a strand of RNA.
• The new RNA strand is edited before
they become functional. Parts removed
are called introns, and the remaining
parts are exons.
The Genetic Code
• Proteins are made by joining amino
acids into long chains called
polypeptides.
• Each polypeptide contains
combinations of the 20 amino acids.
• The “language” of the instructions in
mRNA is called the Genetic Code.
• RNA contains four bases: adenine,
uracil, cytosine, and guanine.
• The code for these bases is read in
groups of three called a codon.
• Each codon specify for a specific
amino acid to be added to the
polypeptide.
• Example: UCGCAC is read UCG-CAC,
and represents the amino acids serine
and histidine.
The Genetic
Code
Translation
• The order of the amino acids in a protein
are coded for in the mRNA.
• This “translation” of the mRNA takes place
on the ribosomes.
• mRNA is transcribed in the nucleus and
released into the cytoplasm.
• There the mRNA attaches to a ribosome
and translation begins.
Translation
Nucleus
Messenger RNA
Messenger RNA is transcribed in the nucleus.
Phenylalanine
tRNA
The mRNA then enters the cytoplasm and
attaches to a ribosome. Translation begins at
AUG, the start codon. Each transfer RNA has
an anticodon whose bases are complementary
to a codon on the mRNA strand. The ribosome
positions the start codon to attract its
anticodon, which is part of the tRNA that binds
methionine. The ribosome also binds the next
codon and its anticodon.
Ribosome
Go to
Section:
mRNA
Transfer RNA
Methionine
mRNA
Lysine
Start codon
Importance of Proteins
• Most genes contains instructions for
making proteins.
• Proteins are essential for all living
things: many are enzymes that
catalyze and regulate chemical
processes.
• Proteins determine many
characteristics of living things, and
control their growth and
development.
• Proteins are key to everything that
living cells do!