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DNA Structure and Replication –
Chapter 5
Nucleotides, Genes, Errors,
Recombinant DNA
Cytology and Genetics
• Why do you think these two ideas are connected?
– Aristotle  heredity was related to the power of male
semen
– Bloodline  hereditary traits were located in the blood
• Nucleus (1831) helped to clear up some of the confusion
• Microscope allowed
– Fleming (1882) ~ separation of “threads”
– Van Benden (1882) ~ roundworms had 4 chromosomes,
sperm and egg of roundworm only had 2 chromosomes
– Weisman (1887) ~ meiosis
Chromosomes and Heredity
• Mendel’s laws were created without an understanding
of meiosis or an understanding of chromosomes.
– His findings went largely unnoticed until the early 1900’s.
• Sutton and Boveri (1902) were the two who realized
that the behaviour of chromosomes during meiosis
was related to the behaviour of traits in Mendel’s
experiments.
• The combined research of the three formed the
chromosome theory of inheritance
– Genes are located on chromosomes and chromosomes
provide the basis for segregation and independent
assortment of genes.
Morgan’s Discovery
• Thomas Morgan (1910) crossed two red eyed flies and
produced a white eyed offspring.
– Thought it was related to the law of dominance in a
monohybrid cross.
– However, when the white eyed male was crossed with a red
eyed female, he could not produce a female with white eyes.
• The coding for eye colour was determined to be on the X
chromosome.
• p. 166, fig 3
Morgan’s Discovery
Sex-linked Inheritance
• Some traits that are passed on depend on the sex
of the parent carrying the trait.
– Transmission of genes that are located on one of the
sex chromosomes, X or Y
– Gene that is located on X chromosome is called Xlinked and Y is known as Y-linked.
– X-linked is more common then Y linked as the X
chromosomes is larger.
• Examples of X-linked are: colour blindness, hemophilia.
• Y linked disorders are rare, and almost always cause
infertility
Chromosomes and Gene
Expression
• Females have two copies of the X
chromosome however the amount of proteins
in males and females is equal.
– What happens is that one of the X chromosomes
is inactivated randomly, it is referred to as a Barr
body. It is important to remember that different
X chromosomes are inactivated in different cells.
• Tortoiseshell coat colour in cats is an example (black
and orange).
Chromosomes and Gene
Expression
Structure of DNA
• DNA = deoxyribonucleic acid
– DNA gets copied during mitosis and each new cell gets a complete
copy
• Chromosomes vs. Proteins
Which one carried genetic information?
- This was not figured out until the 1950’s
- Alfred Hershey and Martha Chase – 1952.
- They completed an experiment with bacteriophages
- Viruses that attack bacteria and use the bacteria’s structures
to quickly reproduce new viruses in the cell
- Outer coating made of protein and inside carries DNA
- 35S on protein and 32P on DNA
- DNA went inside the cell, therein this is what was being transmitted therein it must
be the genetic information.
Nucleotides
•
P.A. Levine – work done earlier then HersheyChase
–
He determined that DNA is made up of nucleotides,
which are themselves made of three parts:
1.
2.
3.
Sugar – deoxyribose
Phosphate group (phosphoric acid)
Nitrogenous base –
•
•
•
adenine, guanine = purines
Thymine, cytosine = pyrimidine
Chargaff = ratios
•
•
A=T
C=G
Double Helix
• The Race Was On!
• Certain things were known at this point:
–
–
–
–
DNA is made of nucleotides
Nucleotides are linked together
Ratios
Compression (1m if stretched out)
• Rosalind Franklin – X ray diffraction
– DNA was a giant spring or coil (2 nm)
• Watson and Crick put all the information together
– Double Helix
– Complementary Base Pairs
Nucleotide Sequences
• Only 4 different nitrogenous bases
– Similar organisms have similar DNA
– Remember a computer uses only 2 pieces of
information – electrical impulse or no
• Genes and Proteins
– 20 different AA that are linked together to
create polypeptides
– Sequence of AA is determined by the sequence
of nucleotides in the DNA.
Nucleotide Sequences
• Genetic Code
– If each nucleotide coded for one amino acid, we would
only need 4 amino acids
– If two nucleotides coded for one amino acid, we still
would not have enough combinations.
– So we have three nitrogenous bases to code for one
amino acid (although there are now 64 different
combinations).
• However, in some cases two or more codons code for the same
amino acid
– Stop codons = indicate that no more amino acids should
be added
– Start codon (AUG) – codes for methionine and it is a
start codon
Transposons
• Transposons (Jumping Genes)
– Specific segments of DNA that can move along the chromosome.
– Flint or Indian Corn
• The individual grains are purple with white streaks or mottling. This
mottling effect defies Mendel's basic principles of genetics because
individual grains may be multicolored rather than a single color.
• In the pigmented layer of corn grains, the position of transposons
may inhibit or block pigment production in some cells. For example,
if the transposon moves to a position adjacent to a pigment-producing
gene, the cells are unable to produce the purple pigment. The reddishpurple patterns caused by transposons may be blotches, dots,
irregular lines and streaks.
Transposons
• The different cards represent a linear sequence of genes on a
chromosome. The ace of spades represents a transposon that moves
to different positions on the chromosome. The jack of diamonds
represents the gene for purple pigmentation in the corn grain. When
the transposon (ace of spades) moves to a position adjacent to the
gene for pigmentation (jack of diamonds), the pigmentation gene is
blocked and no purple is synthesized (white area)
• 1940’s  recognized in 1983…why?
Recombinant DNA
Recombinant DNA
•Bacteria often provide
the appropriate
machinery (enzymes
and ribosomes) for us
to produce proteins
from a specific gene 
insulin
•Bacteria have small
circular pieces of DNA
called plasmids within
their cytoplasm
Genetic Research & Tech
• Human Genome Project – to locate all of the believed
80 000 – 100 000 genes on our 46 chromosomes 
international flavour
– Actually 25 000-30 000
– The human genome contains 3164.7 million chemical nucleotide
bases (A, C, T, and G).
– The average gene consists of 3000 bases, but sizes vary greatly,
with the largest known human gene being dystrophin at 2.4 million
bases.
• With this knowledge if a genetic disorder is known, drugs
could be used to treat the disorder instead of just the
symptoms or gene therapy could be utilized
• What about the problems associated with this?
DNA Fingerprinting
• Isolate DNA from biological materials
from the crime scene (hair or skin).
• Compared to that of the suspect
– Try p.183 = Reading a DNA Fingerprint.
• Can also be used to determine relatedness
– Pedigrees
• Conservation Biologists
– Important to managing populations of
endangered or threatened species
Errors in Replication
• Mistakes are few and far between
– There are special enzymes that read the DNA for errors.
• Mutations can be:
– Useful = positive mutation
– Harmful = negative mutation
– Inconsequential = neutral mutation
• Mutagens cause mutations to occur at a higher
frequency.
Gene Therapy
• Treatment vs. Cure
• Insertion of new (working) gene to replace
one that is not working
– Cell specific
– Issues with this therapy?
– This is the area of hope