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

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 X-

linked 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).

Structure of DNA• DNA = deoxyribonucleic acid

– DNA gets copied during mitosis and each new cell gets a complete copy

• Chromosomes vs. ProteinsWhich 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 Hershey-

Chase– He determined that DNA is made up of nucleotides,

which are themselves made of three parts:1. Sugar – deoxyribose2. Phosphate group (phosphoric acid)3. 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 (Jumping Genes)– Specific segments of DNA that can move along the chromosome.– 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. This results in white streaks or mottling rather than a solid purple grain. The duration of a transposon in this "turned off" position affects the degree of mottling. If the pigmentation gene is turned off long enough by a transposon, the grain will be completely unpigmented. The reddish-purple patterns caused by transposons may be blotches, dots, irregular lines and streaks.

Transposons

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 DNARecombinant 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