The Flow of Genetic Information
•  Cells Inherit Molecules, Energy, and Genetic Information
•  Genetic Information Is Encoded in DNA
•  DNA Has the Structure of a Double Helix
•  DNA Is Replicated Before Cell Division
•  Genetic Information Flows from DNA to RNA to Protein
•  Cells Express Their Genetic Information Selectively
•  Chromosome Structure Changes During the Cell Cycle
•  Chromosome Number Changes During the Life Cycle
Cells Are the Smallest Units of Life
As Cells Divide They Pass on Matter, Energy,
and … ?
As Cells Divide They Pass on Matter, Energy,
and … Genetic Information
The Flow of Genetic Information
•  Cells Inherit Molecules, Energy, and Genetic Information
•  Genetic Information Is Encoded in DNA
•  DNA Has the Structure of a Double Helix
•  DNA Is Replicated Before Cell Division
•  Genetic Information Flows from DNA to RNA to Protein
•  Cells Express Their Genetic Information Selectively
•  Chromosome Structure Changes During the Cell Cycle
•  Chromosome Number Changes During the Life Cycle
Human
Sperm
Sperm,
carrying a
complete set of
genetic
information,
are essentially
mobile nuclei.
Finn Dorset ewe
Blackface ewe
Mammary gland
epithelial cells
Egg in
metaphase II
Genetic Traits
Are Transfered
with Nuclei
G2
M
Cell
culture
S
G1
Enucleated egg
Starved cells (G0)
Cell fusion
Renucleated egg
Gestation in Blackface
foster ewe
Finn Dorset lamb
Dolly, the cloned sheep, showed the
racial and molecular characteristics of
the nuclear donor cell (Finn Dorsett),
not the characteristics of her foster
mother (Blackface).
Chromosomes in Cell Nucleus
The most
conspicuous
elements in a
cell nucleus
are the
chromosomes,
which stain
deeply (blue
in this photo)
for DNA.
Griffith Experiment
An experiment done in 1928 by
Frederick Griffith led to the
identification of DNA as the
genetic material. He showed that
live Pneumococcus bacteria could
change both their morphological
appearance and their
pathogenicity by taking up a
“transforming principle” from
dead bacteria.
Campbell & Reece (2002)
The Flow of Genetic Information
•  Cells Inherit Molecules, Energy, and Genetic Information
•  Genetic Information Is Encoded in DNA
•  DNA Has the Structure of a Double Helix
•  DNA Is Replicated Before Cell Division
•  Genetic Information Flows from DNA to RNA to Protein
•  Cells Express Their Genetic Information Selectively
•  Chromosome Structure Changes During the Cell Cycle
•  Chromosome Number Changes During the Life Cycle
Deoxyribonucleic Acid (DNA)
DNA is a polymer of nucleotides, each consisting of a phosphate group, a
sugar (deoxyribose), and one of four nitrogenous bases (A, C, G, or T).
The Four Nitrogenous Bases of
DNA
T and C have single-ring structures, A and G have
double-ring structures. One of the nitrogen atoms of
the ring is bonded to the deoxyribose ring in DNA.
X-Ray Diffraction Pattern of DNA
Rosalind Franklin obtained the
critical X-ray diffraction pattern
of DNA, from which the double
helix model was deduced.
Double Helix Model of DNA
James Watson (left)
and Francis Crick
proposed the double
helix model of DNA.
The model is critical
to the ability of
DNA to serve as
carrier of genetic
information.
The Twisted Rope Ladder Model of DNA
The ropes on the
sides represent the
sugar-phosphate
backbones of two
DNA polymer
strands. The rungs
of the ladder are
formed by
matched pairs of
nitrogenous bases. The Flow of Genetic Information
•  Cells Inherit Molecules, Energy, and Genetic Information
•  Genetic Information Is Encoded in DNA
•  DNA Has the Structure of a Double Helix
•  DNA Is Replicated Before Cell Division
•  Genetic Information Flows from DNA to RNA to Protein
•  Cells Express Their Genetic Information Selectively
•  Chromosome Structure Changes During the Cell Cycle
•  Chromosome Number Changes During the Life Cycle
DNA Replication Is Semi-Conservative
DNA replication
generates two
daughter
molecules, each
of which
consists of one
parental DNA
strand and one
newly
synthesized
complementary
strand.
The Flow of Genetic Information
•  Cells Inherit Molecules, Energy, and Genetic Information
•  Genetic Information Is Encoded in DNA
•  DNA Has the Structure of a Double Helix
•  DNA Is Replicated Before Cell Division
•  Genetic Information Flows from DNA to RNA to Protein
•  Cells Express Their Genetic Information Selectively
•  Chromosome Structure Changes During the Cell Cycle
•  Chromosome Number Changes During the Life Cycle
Transcription and Translation
Cells express
genetic
information in
two steps:
1.  Transcription
into RNA in
the nucleus
2.  Translation
into protein in
the cytoplasm
RNA and DNA
Both are long
nucleotide polymers. RNA differs from
DNA in
•  being mostly singlestranded
•  having the
nitrogenous base uracil
instead of thymine
•  having ribose as a
sugar instead of
deoxyribose
Codons Are Informative
Units of Three Nucleotides
During translation,
mRNA is read in units
of 3 nucleotides called
codons, and for each
codon a specific amino
acid is incorporated
into the newly
synthesized protein. Campbell & Reece (2002)
Genetic Code
Polypeptides Fold into Functional Proteins
The immediate
result of
translation is a
linear
polypeptide,
which is later
cut and folded
into a threedimensional,
biologically
active protein.
The Flow of Genetic Information
•  Cells Inherit Molecules, Energy, and Genetic Information
•  Genetic Information Is Encoded in DNA
•  DNA Has the Structure of a Double Helix
•  DNA Is Replicated Before Cell Division
•  Genetic Information Flows from DNA to RNA to Protein
•  Cells Express Their Genetic Information Selectively
•  Chromosome Structure Changes During the Cell Cycle
•  Chromosome Number Changes During the Life Cycle
Cells Express Genes Selectively
Controlling the Initiation of Transcription
Key elements
are regulatory
proteins known
as transcription
factors. Binding
to specific DNA
sequences called
promoters and
enhancers, the
proteins form a
threedimensional
complex that
positions RNA
polymerase at
the transcription
initiation site.
The Flow of Genetic Information
•  Cells Inherit Molecules, Energy, and Genetic Information
•  Genetic Information Is Encoded in DNA
•  DNA Has the Structure of a Double Helix
•  DNA Is Replicated Before Cell Division
•  Genetic Information Flows from DNA to RNA to Protein
•  Cells Express Their Genetic Information Selectively
•  Chromosome Structure Changes During the Cell Cycle
•  Chromosome Number Changes During the Life Cycle
Cell
Cycle
For clarity, only
one of 2n
chromosomes
is shown
DNA Packaging
in Chromosomes
In eukaryotic cells, DNA is
packaged with histones and other
proteins. The resulting chromatin
fibers are coiled and supercoiled.
The chromatin exists in segments
called chromosomes, which form
a loose meshwork during
interphase. During mitosis,
chromosomes become condensed
even further and are then visible
as compact units. Chromosome Duplication and Separation
This illustration
focuses on a single
chromosome.
During S phase,
DNA replication
lags behind at a
region called the
centromere. Each
chromosome then
consists of two
sister chromatids,
which are separated
during mitosis.
The Flow of Genetic Information
•  Cells Inherit Molecules, Energy, and Genetic Information
•  Genetic Information Is Encoded in DNA
•  DNA Has the Structure of a Double Helix
•  DNA Is Replicated Before Cell Division
•  Genetic Information Flows from DNA to RNA to Protein
•  Cells Express Their Genetic Information Selectively
•  Chromosome Structure Changes During the Cell Cycle
•  Chromosome Number Changes During the Life Cycle
Homologous Pairs
of Chromosomes
In most somatic cells,
chromosomes occur in
homologous pairs, one
derived from the paternal and
the other from the maternal
parent. The human
chromosome set (karyotype)
includes 22 autosomal pairs
and one pair of sex
chromosomes.
Error: Lac/lac refer to genes
for lactase, not lactose.
Number of
Chromosomes
per Nucleus
Changes During
Life Cycle
The number of
chromosomes per cell
nucleus is reduced from
diploid (2n) to haploid
(1n) during gamete
formation. The diploid
number is restored at
fertilization.
Meiosis
Campbell & Reece (2002)
This diagram represents only one pair of homologous chromosomes.
Meiosis I is unusual because the two homologs from each pair are
segregated into different daughter cells. (This is not so in mitosis.)
Trisomy 21 and Down Syndrome
Errors in meiosis may lead to odd sets of chromosomes, such as the human
trisomy 21 (three copies of chromosome 21). The presence of the extra
gene copies causes developmental abnormalities known as Down syndrome.
The Flow of Genetic Information
•  Cells Inherit Molecules, Energy, and Genetic Information
•  Genetic Information Is Encoded in DNA
•  DNA Has the Structure of a Double Helix
•  DNA Is Replicated Before Cell Division
•  Genetic Information Flows from DNA to RNA to Protein
•  Cells Express Their Genetic Information Selectively
•  Chromosome Structure Changes During the Cell Cycle
•  Chromosome Number Changes During the Life Cycle
Transfer RNAs have dual specificities
Transfer RNAs (tRNAs) have
two specific binding sites:
•  The anticodon binds to one
out of 64 codons.
•  The amino acid attachment
site binds to one out of 20
amino acids.
There are tRNAs with the
same amino acid attachment
site but different codons, but
not vice versa. Thus, the
genetic code is redundant but
not ambiguous.