Central Dogma of Biology
DNA→RNA→Protein
Introduction
• DNA info is in the form of specific sequences
of bases along the DNA strands.
• The DNA leads to specific traits by dictating
the synthesis of proteins.
• Proteins are the links between genotype and
phenotype.
– For example, Mendel’s dwarf pea plants lack a
functioning copy of the gene that specifies the
synthesis of a key protein, gibberellins.
– Gibberellins stimulate the normal elongation of
stems.
One Gene - One Polypeptide
• George Beadle and Edward Tatum were to
establish the link between genes and
enzymes in their exploration of the
metabolism of a bread mold, Neurospora
crassa.
• Their results provided strong evidence for the
one gene - one enzyme hypothesis.
• Later research refined the one gene - one enzyme
hypothesis.
• First, it became clear that not all proteins are
enzymes and yet their synthesis depends on
specific genes.
– This tweaked the hypothesis to one gene - one
protein.
• Later research demonstrated that many proteins
are composed of several polypeptides, each of
which has its own gene.
• Therefore, Beadle and Tatum’s idea has been
restated as the one gene - one polypeptide
hypothesis.
DNA vs RNA
• Components of DNA
– Sugar (deoxyribose)
– Base (A,G,C,T)
– Phosphate group
• Components of RNA
– Sugar (ribose)
– Base (A,G,C,Uracil)
• RNA does not contain thymine
– Phosphate group
DNA vs RNA continued
• Structural Characteristics of DNA
– Double stranded
– Base-pairing rules apply (A:T & G:C)
• Structural Characteristics of RNA
– Primarily single stranded
– Limited base-pairing (G:C & A:U)
Types of RNA
• Messenger RNA (mRNA)
– Complementary to info in DNA strand
– Variable in length
– Contains specific structural info for the
sequence of amino acids
– Processed before using
Types of RNA continued
• Transfer RNA (tRNA)
– Multiple varieties, each specific for a specific
amino acid
– Relatively small, with a consistent 3-d shape
– Specificity for each amino acid is
accomplished by a triplet base-pairing
relationship between codon on mRNA and
anti-codon on tRNA
Transcription and translation are the two
main processes linking gene to protein:
an overview
• Genes provide the instructions for making
specific proteins.
• The bridge between DNA and protein synthesis
is RNA.
• RNA is chemically similar to DNA, except that it
contains ribose as its sugar and substitutes the
nitrogenous base uracil for thymine.
– An RNA molecules almost always consists of a single
strand.
DNA→RNA→Protein
• DNA is TRANSCRIBED to messenger
RNA (mRNA)
• mRNA carries the message to tranfer RNA
(tRNA)
• tRNA is TRANSLATED to an amino acid
chain, which makes up proteins
• In DNA or RNA, the four nucleotide
monomers act like the letters of the
alphabet to communicate information.
• The specific sequence of hundreds or
thousands of nucleotides in each gene
carries the information for the primary
structure of a protein (the linear order of the
20 possible amino acids)
• To get from DNA, written in one chemical
language, to protein, written in another,
requires two major stages, transcription
and translation.
• During transcription, a DNA strand provides a
template for the synthesis of a complementary
RNA strand.
– This process is used to synthesize any type of RNA
from a DNA template.
– Transcription is from the 3’→5’ strand (template
strand)
• Transcription of a gene produces a messenger
RNA (mRNA) molecule.
– mRNA carries the message from the nucleus to the
ribosomes
• During translation, the information contained in
the order of nucleotides in mRNA is used to
determine the amino acid sequence of a
polypeptide.
– Translation occurs at ribosomes.
• The basic mechanics of transcription and
translation are similar in eukaryotes and
prokaryotes.
• Because bacteria lack nuclei, transcription
and translation are coupled.
• Ribosomes attach to the leading end of a
mRNA molecule while transcription is still in
progress.
Fig. 17.2a
• In a eukaryotic cell, almost all transcription occurs
in the nucleus and translation occurs mainly at
ribosomes in the cytoplasm.
• In addition, before the
primary transcript
can leave the nucleus
it is modified in
various ways during
RNA processing
before the finished
mRNA is exported
to the cytoplasm.
– Introns are removed
Fig. 17.2b
RNA Packaging
• Introns are removed (only exons contain
genetic info)
• Addition of a 5’ cap on mRNA (for
orientation purposes)
• Addition of a 3’ tail on mRNA (allows it to
last longer)
• To summarize, genes program protein
synthesis via genetic messenger RNA.
• The molecular chain of command in a cell is
DNA → RNA → protein.
This is referred to as the Central Dogma of
Biology