Recall : the 4 types of organic molecules
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Carbohydrates
Lipids
Proteins
Nucleic Acids ****
Monomer: Nucleotide
Polymer: DNA/RNA
What is DNA?
• DNA : Deoxyribose Nucleic Acid
FUNCTION : It’s primary function is to
store and transmit information
• Specifically: How and when to make a protein
STRUCTURE :
• Nucleotide (monomer) has 3 parts:
- Sugar
- Phosphate
- Nitrogen Base
Discovery of DNA
• During the mid-1900’s, scientists all over
the world were competing to see who could
discover the structure of this genetic material
• In 1953, James Watson and Francis Crick
published a paper suggesting a model for
the structure of deoxyribonucleic acid
• They relied heavily (and somewhat unethically) on
scientific evidence reported by other scientists
Example: Rosalind Franklin’s X-ray
crystallographic images
• “We’ve found the secret of life!”
~ Francis Crick (1916-2004)
Watson and Crick determined DNA to have
the shape of a Double Helix (twisted ladder)
Watson (left) and Crick (right)
More On Nucleotides…
There are 4 different types of N-bases in DNA
Therefore, there are 4 different DNA nucleotides
Adenine (A)
Guanine (G)
↑
PURINES
These bases contain 2
carbon rings
Cytosine (C)
Thymine (T)
↑
PYRIMIDINES
These bases contain 1
carbon ring
The 4 different nucleotides differ only in their nitrogen-bases
The sugar and phosphate groups are identical in each nucleotide
DNA vs. RNA
DNA
-Deoxyribose Sugar
-Double-stranded
- Bases A, T, G, C
RNA
- Ribose
Sugar
- Single-stranded
- Bases A, U, G, C
Molecular Modeling: NUCLEOTIDES
Chargaff’s Data
• Originally it was thought that the bases
A, T, G, and C occurred in all living things
in the same repeated pattern, such as:
… ATGC ATGC ATGC ATGC ATGC …
• If this were true, then DNA could not be
the hereditary molecule because it would
not be able to provide the variety needed
for a molecule containing the genetic code.
• Following WWII, the research of biochemist
Erwin Chargaff revealed the percentage of
each base (A, T, G, and C) found in the DNA
of various organisms.
The following table contains some of his actual data…
Nitrogen Base Make-Up of Different Organismal DNA (by %)
ORGANISM
A
Mycobacterium tuberculosis 15.1
Yeast
31.3
Wheat
27.3
Sea Urchin
32.8
Marine Crab
47.3
Turtle
29.7
Rat
28.6
Human
30.9
T
14.6
32.9
27.1
32.1
47.3
27.9
28.4
29.4
G
34.9
18.7
22.7
17.7
2.7
22.0
21.4
19.9
Question: Do you observe any patterns in this data?
C
35.4
17.1
22.8
17.3
2.7
21.3
21.5
19.8
Complementary Base-Pairing
• DNA is said to have a “sugar-phosphate” backbone
• The rungs of the “ladder” are made of the N-bases
• The makeup of the rungs follows some specific rules:
– Each rung contains 1 purine and 1 pyrimidine
– A always pairs with T
– G always pairs with C
• The pairing is the result
of hydrogen bonding
between the N-bases
TRY IT!
• Carefully copy the single-stranded sequence of
DNA into your notebook. Then, directly below it,
write the sequence of the complimentary strand
that would bind to it.
A C T G G C C T A T T
→
T G A C C G G A T A A
G G T A C A A T G T A
→
C C A T G T T A C A T
DNA Replication
• Cells reproduce by dividing (in half) through the process of MITOSIS
• Before it divides, a cell must copy all of its DNA … WHY?
• How can this be done efficiently? (so that there are NO mistakes!!!)
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The enzyme HELICASE unwinds the
DNA molecule (hydrogen bonds break
apart, creating 2 single DNA strands)
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The enzyme DNA POLYMERASE
binds to the single stranded DNA
DNA polymerase adds available
nucleotides, one at a time, to the free
strands (following Chargaff’s rules)
This results in two DNA molecules
with identical base sequences
This does not happen at just
one site, but at many sites
simultaneously on the DNA molecule
Occasionally, the polymerase may
make a mistake, changing the
original base sequence (MUTATION)
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CHROMOSOMES
During mitosis (cell division), the DNA of eukaryotic cells is
packaged into compact structures called CHROMOSOMES
Chromosomes are:
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- Right before division, a chromosome
Rod-like structures
consists of two identical halves
Easily visible by microscopy - Each half is called a (sister) chromatid
DNA coiled around proteins - The sister chromatids are joined
by a structure called a centromere
Histones : Proteins that help
maintain chromosome shape - When the cell divides, each daughter
and tight packaging of DNA
cell will receive one chromatid
Chromosome Numbers
•
Each species has a characteristic number of chromosomes in each of its cells
- Humans have 46 chromosomes
- The fruit fly has 8 chromosomes
- Horses have 64 chromosomes
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Diploid (2n) cells have 2 copies of each chromosome (1 from each parent)
Haploid (1n) cells have only 1 copy
• In sexually reproducing organisms, a chromosome is either:
Sex Chromosome
Autosome
In humans, there are 2:
In humans, there are 44
XX = female
XY = male
Actually, they are 22 pairs of
homologous chromosomes
Chromosome Analysis
• KARYOTYPE
• A photomicrograph of
all the chromosomes
in a (dividing) cell
• Homologous pairs
(the 2 copies of each
chromosome a cell has)
are arranged by size
• Used to analyze the
genetic condition of an
individual (that is, their
sex and the possible
presence of a disorder)