Download Unit 08 - Lessons 1-3

8.2 Structure of DNA
From Gene to Protein
• deoxyribonucleic acid - (DNA) - the ultimate source of all
information in a cell
• This information is used by the cell to produce the protein
molecules which are responsible for virtually all the major
functions of the organism.
8.2 Structure of DNA
DNA is composed of four types of nucleotides.
• DNA is made up of a long, double chain of nucleotides.
• Each nucleotide has three parts.
– a phosphate group
– a deoxyribose sugar
– a nitrogen-containing base
phosphate group
deoxyribose (sugar)
nitrogen-containing
base
8.2 Structure of DNA
• The nitrogen containing bases are the only difference in
the four nucleotides.
- Purines (adenine and guanine) have a double ring
- Pyrimidines (cytosine and thymine) have a single ring
8.2 Structure of DNA
Watson and Crick determined the three-dimensional
structure of DNA by building models.
• DNA is a double helix
• made up of a sugar-phosphate
backbone on the outside with
nitrogenous bases on the inside
8.2 Structure of DNA
• Watson and Crick’s discovery built on the work of Rosalind
Franklin and Erwin Chargaff.
– Franklin’s x-ray images suggested that DNA was a
double helix of even width.
– Chargaff’s rules stated that A=T and C=G.
8.2 Structure of DNA
Nucleotides always pair in the same way.
• The base-pairing rules show
how nucleotides always pair
up in DNA.
– A pairs with T
– C pairs with G
• Because a pyrimidine
(single ring) pairs with a
purine (double ring), the
helix has a uniform width.
G
C
A T
8.2 Structure of DNA
• The backbone is connected by covalent bonds between the
phosphate and the sugar.
• The nitrogenous bases are connected by hydrogen bonds.
hydrogen bond
covalent bond
8.2 Structure of DNA
• ribonucleic acid – (RNA) differs from DNA in three major
ways.
– RNA has ribose as the 5-C sugar
– RNA has uracil instead of thymine
– RNA is a single-stranded structure
8.3 DNA Replication
Replication copies the genetic information.
• A single strand of DNA serves as a template for a new
strand.
• The rules of base pairing direct
replication.
• DNA is replicated during the
S (synthesis) stage of the
cell cycle.
• Each body cell gets a
complete set of
identical DNA.
8.3 DNA Replication
Proteins carry out the process of replication.
• DNA serves only as a template.
• Enzymes and other proteins do the actual work of
replication.
– Enzymes unzip the double helix.
– Free-floating nucleotides form hydrogen bonds
with the template strand.
nucleotide
The DNA molecule unzips
in both directions.
8.3 DNA Replication
– DNA polymerase enzymes bond the nucleotides
together to form the double helix.
– Polymerase enzymes form covalent bonds between
nucleotides in the new strand.
new strand
nucleotide
DNA polymerase
8.3 DNA Replication
• Two new molecules of DNA are formed, each with an
original strand and a newly formed strand.
• DNA replication is semiconservative.
original strand
Two molecules of DNA
new strand
8.3 DNA Replication
Replication is fast and accurate.
• DNA replication starts at many points in eukaryotic
chromosomes.
There are many origins of replication in eukaryotic chromosomes.
• DNA polymerases can find and correct errors.
8.4 Transcription
RNA carries DNA’s instructions outside of the nucleus.
• Central Dogma of
Molecular Biology states that information
flows in one direction from DNA to RNA to
proteins
• The central dogma
includes three processes.
– Replication
– Transcription
– Translation
replication
transcription
translation
8.4 Transcription
• DNA stores information using the four letter code of its
nitrogenous bases (A, T, C, G)
• Depending on the arrangement of these letters a certain sequence of
DNA, called a gene, can code for a specific type of protein.
• Proteins are polymers of amino acids
8.4 Transcription
• transcription - copies DNA to make a strand of mRNA
• Transcription is catalyzed by RNA polymerase.
– RNA polymerase and other proteins form a
transcription complex
– The transcription complex recognizes the start of
a gene and unwinds a segment of it.
start site
transcription complex
nucleotides
8.4 Transcription
– RNA nucleotides pair with one strand of the DNA
– RNA polymerase bonds the nucleotides together to form
a strand of mRNA
– The DNA helix winds again as the gene is transcribed
DNA
RNA polymerase
moves along the DNA
8.4 Transcription
– The RNA strand detaches from the DNA once the gene
is transcribed.
– It can now move out of the nucleus to a ribosome (on the
RER or floating on the cytoplasm) to be translated.
RNA
8.4 Transcription
• Transcription makes three types of RNA.
– messenger RNA (mRNA) – carries a copy of the message
from the DNA out of the nucleus to a ribosome where it will
be translated into a protein
*contains codons – sequence of 3 nucleotides that code
for a specific amino acid
– ribosomal RNA (rRNA) - forms part of ribosomes where
proteins are made
– transfer RNA (tRNA) – contains a sequence of 3
nucleotides (called an anticodon) that brings the amino
acid called for by the mRNA to a ribosome
8.5 Translation
Amino acids are coded by mRNA base sequences.
• translation - converts mRNA messages into a long chain of
amino acids (polypeptide)
• codon - (contained on the mRNA) - sequence of three
nucleotides that codes for an amino acid
codon for
methionine (Met)
codon for
leucine (Leu)
8.5 Translation
– one start codon - (AUG = methionine) – tells ribosome to start
translation
– three stop codons – tells the ribosome to stop translation
– The same amino acid may be coded for by more than one codon
The genetic code matches each RNA codon with its amino acid or function.
8.5 Translation
• A change in the order in which codons are read changes
the resulting protein.
• Universal Genetic Code - regardless of the organism,
codons code for the same amino acid
8.5 Translation
Amino acids are linked to become a protein.
• An anticodon is a set of three nucleotides that is
complementary to an mRNA codon.
• An anticodon is carried by a tRNA.
8.5 Translation
• Ribosomes consist of two subunits.
– The large subunit has three binding sites for tRNA.
– The small subunit binds to mRNA.
8.5 Translation
• For translation to begin, tRNA binds to a start codon and
signals the ribosome to assemble.
– A complementary tRNA molecule binds to the exposed
codon of the mRNA, bringing its amino acid close to the
first amino acid.
8.5 Translation
– The ribosome helps form a peptide bond between the
neighboring amino acids. The tRNA then releases the
amino acid once the peptide bond is formed.
– The ribosome pulls the mRNA strand the length of one
codon.
8.5 Translation
– The now empty tRNA molecule exits the ribosome.
– The next complementary tRNA molecule binds to the next
exposed mRNA codon, bringing another amino acid. Once
the peptide bond is formed, the tRNA releases the amino
acid and exits the ribosome. This continues until a stop
codon is reached.
8.5 Translation
– Once the stop codon is reached, the ribosome releases the
polypeptide and disassembles.
– The polypeptide eventually folds into a protein and
performs whatever function it does in the cell (enzymes,
hemoglobin, insulin etc.)