Central Dogma of Molecular
BiologyDNA RNA Protein
Chapter 17
The DNA sequence inherited by an organism leads to specific traits by
dictating the synthesis of proteins
▪ Proteins are the links between genotype and phenotypeGene expression is the process by which DNA directs protein
synthesis. Includes two stages:
Transcription is the synthesis of RNA under the direction of
DNA.
Translation is the synthesis of proteins under the direction of
RNA
Neurospora crasa (bread
mold)
Haploid species of fungi –
means it is homozygous for
any mutationCells placed on complete
medium.
Cells subjected to X-rays.
Surviving cells form colonies.
Cells placed in minimal
medium. Nutritional mutants
identifiedNutritional mutants placed in vials
with a variety of media.Vials observed for growthNext, investigated the collection of mutants that required
arginine to grow
Isolated colonies (mutants) that had different nutritional needs
Neurospora crasa (bread
mold) Results
The results of the experiments provided support for the one
gene – one enzyme hypothesis
▪ The hypothesis stated that the function of a gene is to dictate
production of a specific enzyme
▪ Not all proteins are enzymes, so researchers later revised
the hypothesis: one gene – one protein
▪ Therefore, Beadle and Tatum’s hypothesis is now restated
as the one gene – one polypeptide hypothesis
▪ Many proteins are composed of more
than one polypeptides - each of
which has its own gene (eg. alpha-
globin and beta-globin are folded
together to make hemoglobin)
Ribonucleic acid (RNA)
RNA is different from
DNA because it
– uses the sugar
ribose (instead of
deoxyribose in
DNA) and
– RNA has the
nitrogenous base
uracil (U) instead of
thymine
Genetic information written in codons is
translated into amino acid sequences
The flow of information from gene
to protein is based on a triplet
code: a series of nonoverlapping,
three-nucleotide words
▪ The words of a gene are
transcribed into complementary
nonoverlapping three-nucleotide
words of mRNA
▪ These words are then translated
into a chain of amino acids,
forming a polypeptideThe genetic code dictates how codons are
translated into amino acids
Messenger RNA (mRNA)
encodes amino acid sequences
– In prokaryotes, translation of RNA into protein occurs in the
same place that mRNA is made.
– In eukaryotes, mRNA must exit the nucleus via nuclear pores
to enter the cytoplasm before being translated.
The genetic code is universal
one species
can read the DNA code from a second species
* Testament to the unity of life
* Allows transgenics – the introduction of foreign
DNA into the genome of an organism
Eukaryotic gene structure
Promoter – stretch of DNA to which RNA polymerase and transcription
factors bind
Exons – stretches of DNA that code for RNA and proteins
Introns – stretches of DNA that separate exons and which don’t code for
RNA or protein
Terminator – stretch of DNA that marks the end of the gene
Transcription produces genetic messages in the
form of RNA
Transcription of a gene occurs in three main steps:
1. initiation, involving the attachment of RNA
polymerase to the promoter and the start of RNA
synthesis,
2. elongation, as the newly formed RNA strand
grows, and
3. termination, when RNA polymerase reaches the
terminator DNA and the polymerase molecule
detaches from the newly made RNA strand and
the gene.
Transcription factors – proteins that regulate
gene expression by binding to the promoters of
specific genes and recruiting RNA polymerase to
transcribe the gene
Play a critical role in
deciding when and where
a gene is expressed in the
organism
The gene for Mendel’s purple flower color encodes a
bHLH transcription factor that regulates the
expression of the genes that make the anthocyanin
purple pigment
Eukaryotic mRNA processing
Eukaryotic mRNA undergoes
processing before leaving the
nucleus as mRNA
– Pre-mRNA is a copy of the
gene sequence including all
the exons and introns
– RNA splicing process
removes introns and joins
exons from the pre-mRNA to
produce a continuous coding
sequence, the mature mRNA,
that is exported to the
cytoplasm to be translated
Eukaryotic mRNA may be spliced in more
than one way
Alternative RNA splicing
– splicing can remove some exons as well as introns
– produces different mRNAs from the same transcript
– results in the production of more than one polypeptide from the same
gene, and
– is very common in humans
Drosophila Down syndrome cell adhesion molecule
(Dscam) gene: can generate 38,016 isoforms by the
alternative splicing of 95 variable exons
Functions in neural tissue to control self non-self
recognition
RNA processing
enzymes in the eukaryotic nucleus modify
pre-mRNA before export to cytoplasm
– The 5′ end receives a modified nucleotide 5′ cap
– The 3′ end gets a poly-A tail
▪ These modifications share several functions
▪ They facilitate the export of mRNA to the cytoplasm
▪ They protect mRNA from hydrolytic enzymes – alters longevity of
mRNA in the cell
▪ They help ribosomes attach to the 5′ endEarly estimates of human gene
numbers was based on
sequencing mRNA molecules
cDNAs arising
from splice
variants were
often counted as
separate genes
Transfer RNA molecules serve as interpreters
during translation
reading the sequence of
codons in the mRNA into a sequence of
amino acids Transfer RNA – is a
strand of RNA that
forms a structure that
– picks up a
specific amino
acid and
– using a special
triplet of bases,
called an
anticodon,
inserts the amino
acid at the
appropriate
position in the
coding sequence
Ribosomes build polypeptides
Translation occurs on the
surface of the ribosome.
– Ribosomes coordinate the
functioning of mRNA and
tRNA and, ultimately, the
synthesis of polypeptides.
– Ribosomes have two
subunits: small and large.
– Each subunit is composed of
ribosomal RNAs and
proteins.
– Ribosomal subunits come
together during translation.
– Ribosomes have binding
sites for mRNA and tRNAs
An initiation codon marks the start of an
mRNA message
Translation can be divided into the same three phases as
transcription:
1. initiation,
2. elongation, and
3. terminationInitiation brings together
– mRNA,
– a tRNA bearing the first amino acid, and
– the two subunits of a ribosome.
Elongation
Termination
