Basic Molecular Biology
Information
Central Dogma of Molecular
Biology
• Concerns the flow of information in the cell.
Nucleic Acids
• Only 2 types: DNA and RNA
• linear chains of nucleotides
• DNA: 2 chains running antiparallel twisted together into
a double helix
• RNA: usually 1 chain of
nucleotides, with secondary
structure caused by base
pairing between nucleotides
on the same strand.
Nucleotides
•
Each nucleotide has 3 parts: sugar, phosphate,
base.
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–
–
–
Sugar is ribose (RNA) or deoxyribose (DNA)
Bases are attached to the 1’ carbon of the sugar
Base is purine or pyrimidine.
Purines: 2 carbon-nitrogen rings, adenine or
guanine
– Pyrimidines: 1 carbon-nitrogen ring, cytosine,
thymine (DNA only), uracil (RNA only)
•
In the backbone, nucleotides are bonded
together between the phosphate on the 5'
carbon and the -OH on the 3' carbon.
– Thus each nucleic acid has a free 5' phosphate on
one end and a free 3' -OH on the other.
– Used to write the polarity of the molecule: each
nucleotide chain has a 5’ end and a 3’ end.
•
DNA has -H on 2' carbon of the sugar; RNA has OH.
Base Pairing
• A bonds with T (or U); G bonds with C.
Held together by hydrogen bonds
– A-T has 2 hydrogen bonds; G-C has 3.
This makes G-C stronger and more
stable at high temperatures.
• In DNA, 2 antiparallel chains are held
together by this pairing.
– Implies that the amount of A = amount
of T, and G = C in DNA.
• But: GC content varies between
species. Human DNA has 39.4% GC
content (implying 60.6% A + T).
• RNA is usually single stranded and
held in a folded conformation by base
pairing within the RNA molecule e.g.
tRNA.
Replication
• Main enzyme: DNA polymerase. Several other enzymes also
involved (see below)
• Replication is semiconservative:
– DNA helix is opened up and unwound by a helicase
– Each old strand gets a new strand built on it.
– DNA polymerase can only add bases to the 3’ –OH group on a preexisting nucleic acid that is base-paired with the template strand it is
copying. This means that DNA synthesis starts with the enzyme
primase synthesizing a short RNA primer. DNA polymerase then adds
bases to this primer.
– DNA polymerase can only add new bases to 3' end, so one strand is
synthesized continuously (leading strand) and the other is built up of
short fragments: discontinuous synthesis on the lagging strand.
– The short (100-1000 bp ) DNA fragments, called Okazaki fragments, are
built in the opposite direction of fork movement and then ligated
together (by DNA ligase).
– The whole process starts at several points on each chromosome and
goes in both directions. Takes 8 hr to complete.
Replication
Transcription
• Transcription is making an RNA copy of a short region of DNA.
• Only part of the DNA is transcribed. A transcribed region is called a
transcription unit, which is approximately equivalent to “gene”.
– most transcription units code for proteins
– some code for functional RNAs that never get translated into proteins
(RNA genes).
• When transcription starts, the DNA double helix is unwound and
only one strand is used as a template for the RNA.
– the template DNA strand is called the antisense strand, and the other
DNA strand, not used in transcription is called the sense strand. This is
because the sense strand has the same base sequence as the RNA
transcript. Gene sequences are generally written as the sense strand.
– Genes are oriented from 5' to 3' based on transcription direction (even
though the template DNA is read 3' to 5'). Thus, 5' end of a gene is
where transcription starts. Upstream and downstream also relate to this
direction.
Transcription Process
•
The primary enzyme used for
transcription is RNA polymerase
– There are 3 types of RNA
polymerase: pol2 does the protein
coding genes, while pol1 and pol3
do ribosomal RNA and other
structural RNA genes.
•
RNA polymerase binds to a
promoter sequence just upstream
from the transcription starting
point, with the help of several
proteins called transcription
factors.
– some transcription factors are
used for all transcriptions, but
others are very specific for cell
type, hormonal stimulus,
developmental time, etc.
•
RNA polymerase then moves in a
3’ direction, adding new RNA
nucleotides to the growing RNA
molecule.
Gene Regulation
•
•
Much of the control of gene expression occurs at the point of transcription.
Transcription regulation is based on interactions between transcription
factors (proteins) and DNA sequences near the gene .
– transcription factors are trans-acting: they diffuse freely through the cell and
affect any DNA sequence they can bind to.
– in contrast, DNA sequences near the gene are cis-acting: they can only affect
transcription of the gene they are next to. (and not, for example, the same gene
on the other homologous chromosome).
•
Types of cis-acting sequence:
– promoters: several short regions within 100 bp of transcription start, especially
the TATA box, which are all similar to TATAAA.
– enhancers: can be up to several kilobases from the gene, either upstream or
downstream, and in either orientation. Increase transcription level.
– silencers: similar to enhancers, but opposite effect.
•
Genes are also affected by the region of chromosome they are in: some
areas are highly condensed and unable to be transcribed (depending on cell
type).
RNA Processing
•
The RNA molecule that results for RNA polymerase
transcribing a gene is called a primary transcript. It
is an exact copy of the DNA. Before it can be
translated into protein, it must be processed, then
transported to the cytoplasm. RNA processing has
3 steps:
1.
2.
3.
•
Splicing out of introns, which are non-protein coding
regions in the middle of protein-coding genes. . Most
genes are interrupted by introns: up to 99% of the
gene in some cases. Exons are the regions of genes
that code for protein. Primary transcript contains
introns, but spliceosomes (RNA/protein hybrids) splice
out the introns. There are signals on the RNA for this,
but it can vary between tissues (alternative splicing).
5' cap: a 7-methyl guanine linked 5’ to 5’ with the first
nucleotide of the RNA.
3' poly A tail: several hundred adenosines added to 3’
end. Signal for poly A marks end of gene, but
transcription continues past this without having a
definite end point. All except histone genes have poly
A. Stability of mRNA is probable reason for it.
After processing, the RNA is called messenger
RNA, and it gets transported to the cytoplasm.
Translation
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In cytoplasm, messenger RNA molecules
are translated into polypeptides by
ribosomes (RNA/protein hybrids).
Starts at first AUG in the messenger RNA,
goes to first stop codon. (So, only one
polypeptide per messenger RNA.) Open
reading frame (ORF): region between a start
codon and a stop codon.
Protein is synthesized from N terminus to Cterminus, based on free NH2 and COOH
groups on the polypeptide. These
correspond to the ribosome moving down
the messenger RNA from 5’ end to 3’ end.
There are 5' and 3' untranslated regions
(UTRs) on the RNA.
Most mRNA molecules are translated
multiple times.
transfer RNA: short RNA molecules with
several modified bases that act as adapters
between codons on the mRNA and the
amino acids.
Genetic Code
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•
•
•
Three bases of DNA or RNA code
for 1 amino acid = codon.
Since there are 4 bases, there are
43 = 64 codons. 61 of these code
for amino acids, while the last 3
are stop codons that end the
translation process.
Most amino acids have more than
1 possible codon: code is
degenerate. Most variation is in
third position of codon.
Nearly all organisms use the same
code, with minor variations mostly
in mitochondria and chloroplasts.
– human mitochondria use a slightly
altered genetic code
•
All translations start with ATG,
methionine.
Proteins
• Proteins: one or more polypeptides plus
(optionally) co-factors
– polypeptides: linear chain of amino acids,
coded for by genes
– co-factors: can be single atoms: metal ions
commonly, or small organic molecules: heme,
vitamins like riboflavin or biotin or thiamin
Amino Acids
• Amino acid structure: amino group, acid group,
side chain
• amino covalently bonded to acid: peptide bond
(= amide)
• types of side chain: acidic, basic, uncharged
polar, non-polar (some non-polar are very small
and not very hydrophobic)
• also: cysteine-cysteine bridges, proline-induced
bends
Amino Acid Structures
Protein Structure
• Classification of protein
structure:
– primary: amino acid
sequence;
– secondary : alpha helix,
beta sheet, random coil
(i.e. something more
complex);
– tertiary: overall shape of
polypeptide;
– quaternary: how all
subunits and co-factors fit
together.
Forces holding proteins together
• primary structure = amino acids covalently bonded together
• hydrogen bonds
– based on electronegativity differences: C or H vs. O or N
– H... H-O, both within the molecule and with water.
– Involved in the 2 main secondary structural characteristics: alpha helix
and beta pleated sheet (and beta turn), which involve interactions
between C=O and NH2 groups of the peptide.
• ionic bonds between charged groups (acidic, basic, polypeptide
ends, co-factors)
• hydrophobic forces. groups that can't form H-bonds with water tend
to cluster together--they are excluded from the water, like oil
droplets. Form interiors of proteins and membrane-spanning
regions.
• Van der Waals forces: very mild attraction of all atoms with strong
repulsion if they get too close.
Protein folding
• Mostly spontaneous to most stable
configuration.
• Some proteins are assisted by chaperone
proteins, which also assist in recovery from heat
shock by causing re-folding to proper
configuration. Thus, chaperone proteins are
also often called heat shock proteins.
• However, predicting protein structure from the
primary sequence is (so far) an unsolved and
very difficult problem in biochemistry.
Post-translational modification
• Addition of various groups:
– Glycosylation: adding sugars. occurs in smooth ER. Mostly for proteins
that are secreted or on outside of plasma membrane or inside of
lysosomes. Large blocks of sugars added. Proteins called
glycoproteins.
• N-glycosylation : Occurs on asparigine (Asn).
• O-glycosylation: On serine (Ser) or threonine (Thr)
– Phosphorylation: adding phosphates. An important way to active
various enzymes , especially for turning genes on and off. On serine,
threonine, or tyrosine.
– Adding lipids: so proteins get anchored to membrane. Various names
depending on which lipid is added. For example, myristoyation,
prenylation, palmitoylation, etc. Proteins called lipoproteins.
– Others as well.
• Cleavage. Often the N-terminal Met is removed. Other regions can
also be removed: middle region of insulin, removal of signal
peptides.
Localization
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How do proteins get to the proper
location in the cell?
Polypeptides often contain signal
sequences that cause protein to
end up in proper organelle, or be
secreted, or become embedded
in the membrane. Often a leader
sequence (or signal sequence) at
N terminus that is then removed.
Best known is for secretion into
ER, into membrane, and
extracellular: About 20 mostly
hydrophobic amino acids at the
N-terminus of the polypeptide. A
Signal Recognition Particle
(RNA/protein hybrid) recognizes
this during translation and guides
ribosomes to the rough ER where
translation finishes.
Also signals for nucleus,
lysosome, mitochondria. Some
are internal to protein and not
removed.