Genes and
Chromosomes
Central Dogma 
Once information has got into a protein it can’t get out again.
transcription
RNA is synthesized
translation
Proteins are synthesized
Genes Has Evolved
responsible for phenotype
one gene ->one enzyme
genes are defined as segments of DNA that
code for peptides and RNA
DNA
Very Large Macromolecule
in a virion or cell
compact forms
In transcription
DNA acts as the molecular template for RNA synthesis
Three nucleotides code for one “codon.”
In translation
triplets of nucleotides in mRNAbind
to complementary triplets in tRNA
RNA
Many viruses
only RNA or DNA surrounded by protein coat
use the host’s genes
small and single-stranded
during the life cycle
change from circular to linear
Eukaryotic Chromosome
are in this state after replication
at metaphase during mitosis.
Mitochondria and Chloroplasts
Double-stranded circles
Human mitochondrial DNA (mtDNA)
Plant mtDNA
Chloroplast DNA (cpDNA)
Mitochondrial DNA
codes for
mitochondrial rRNAs
mitochondrial tRNAs
mitochondrial specific proteins
Most mitochondrial proteins (at least 95%)
are encoded by nuclear genes.
types of RNA
most of eukaryotic DNA is noncoding
less than 400 genes.
Is Packaged with Proteins
Viral genomic DNA
capsid proteins
Prokaryotic DNA
nucleoid
Eukaryotic DNA
organized with proteins
into a complex called chromatin
Composition of the Human Genome
small fraction (1.5%) of the total genome
encodes for proteins
noncoding sequences
not entirely clear
participate in the regulation of
gene expression (promoters, termination signals, etc.)
encodes for small regulatory RNA with poorly
understood functions
may be junk (pieces of unwanted genes,
remnants of viral infections)
Introns
are transcribed but
not translated
Exons
are expressed sequences (translated into
amino acid sequence).
do not encode polypeptide sequence
are removed after transcription
exon mRNA sequences are spliced together after transcription
Bacterial Genomes
Introns
do not interrupt protein-coding sequences
interrupt mainly tRNA sequences
Many bacterial introns encode catalytic RNA sequences
have the ability to insert and reverse transcribe themselves into the genomic DNA.
Transposons
can move around within the genome of a single cell
The ends of transposons
terminal repeats.
Eukaryotes Contain Highly Repetitive
DNA or Simple Sequence Repeats (SSRs)
Repeated millions of times
“satellite” DNA
Associated with centromeres and telomeres
Centromere Sequences
held together
during mitosis
equal distribution of chromosome sets to daughter cells
AT-rich repeated
Telomere Sequences
Cap the End
keep DNA ends
from unraveling
contain multiple repeats with general sequence
(TxGy)n
(AxCy)n
added by enzyme telomerase (protein+nucleotides)
Associated with Cellular Aging
telomeres are shortened after each round of replication
losing the ability to divide
DNA Supercoiling
Nonsupercoiled DNA is called relaxed
Many circular DNAs are supercoiled.
great influence on transcription and
replication of DNA.
highly regulated
Linking Number (Lk)
Twist (Tw)
Writhe (Wr)
twists or turns of the helix
coils
Cruciforms
palindromes
Topoisomers Are DNAs That Differ
Only in Linking Number
Same # bp, same sequence but different degree of supercoiling
Conversion between topoisomers requires a DNA
strand break.
Topoisomerases
Change Lk
DNA unwinding and rewinding during transcription and replication.
Type I
cut in one DNA strand
changes Lk by 1
Type II
cut in both DNA strands
change Lk in steps of 2
Topo I and III
relax DNA
increase Lk
use single-stranded breaks
Topo II
introduces negative supercoils
decreases Lk
uses ATP and double-stranded breaks
Type IIA and Type IIB
can relax both positive and negative supercoils
Are Targets for Antibiotics
Coumarins (novobiocin, coumermycin A1)
inhibit bacterial Type II topoisomerases from binding ATP
Quinolones (nalidixic acid; ciproflaoxadin, Cipro)
resealing the DNA strand breaks
Used as Chemotherapy Agents
Eukaryotic Type I topoisomerase inhibitors
trap the enzyme-DNA complex in its cleaved state
Eukaryotic Type II topoisomerase inhibitors
Forms Plectonemic or
Toroid/Solenoid Structures
Plectonemic
in plasmids
Toroid/solenoid
in chromatin
Packing into Chromatin
fibers of protein
DNA
a small amount of RNA.
associates tightly with proteins called histones
small proteins with lots of basic (Lys, Arg) residues
DNA and protein are packed into discrete units
called nucleosomes