inheritance and genetics Flashcards
- Why do we learn about inheritance and genetics?
In order for natural selection to be a viable mechanism for evolution, the traits of parents must be transmitted to their offspring. That is, there must be heredity. Parental traits must be inherited by their offspring.
In The Origin of Species, Darwin used observations from selective breeding to establish that parents
do pass traits to their offspring.
But Darwin had no detailed understanding of hereditary patterns, did not know the hereditary material, and had no explanation for the hereditary mechanisms.
From 1856 to 1864, GregorMendel, an Austrian scientist and Augustinian friar,
conducted breeding experiments using pea plants.
- On the basis of his observations, Mendel proposed that there must be some discreteunchanging particles of inheritance. That is, that the genetic material carrying the traits from the parents
does not blend or merge together, but remains distinct and separate. This is Mendel’s principle of segregation.
- We now call Mendel’s discrete particle of inheritance a gene, its
position on a chromosome its locus,and alternative forms of a gene alleles.
- Individuals who have two copies of the same allele at a locus are
homozygous
individuals who have two different alleles at a locus are
heterozygous.
- Mendel hypothesized that some alleles are dominant, while others are recessive. This is Mendel’s
principle of dominance. The dominant trait is expressed whenever the dominant allele is present, but the recessivetrait is expressed when only recessive alleles are present.
Mendel’s predictions can also be expressed as an expectation that the ratio
of yellow peas to green peas should be close to 3:1.
Mendel also observed that different traits of the parental true-breeding lines did not remain associated in the F2 generation. From these observations, Mendel hypothesized that the genes for different traits are inherited
independently.
Today, we call this Mendel’s principle of independent assortment. We now know that genes that are located on different chromosomes1 segregate independently (as Mendel observed).
- The discovery of heredetary material:
In 1928, Frederick Griffith demonstrated that non-pathogenic pneumococcus bacteria could be transformed
into pathogenic forms.
In transformation, some of the traits of one bacterial form are passed to another. This suggested (and we now know) that it is the transfer of
the hereditary material from one bacterium to the other that effects the transformation.
Many researchers of the time were initially skeptical of Griffith’s claim, but his experiment was soon replicated by
several different groups, including one headed by Oswald T. Avery at the Rockefeller Institute for Medical Research1 here in NYC.
Having isolated the transforming substance, the next step was to identify which of the cellular components (protein, carbohydrate, lipid, RNA, or DNA) was
present and active. Further experiments and chemical tests of the transforming substance indicated that it was almost pure DNA.
- Having identified DNA as the hereditary material, the important question became
“ What is the structure of DNA?”
In 1950, Maurice Wilkins began working on capturing an X-ray image of DNA in order to deduce its structure. He had early successes and was optimistic that it would be possible to capture an image that would give enough information to identify the structure of DNA.
Then, beginning in 1951, Rosalind Franklin was recruited to take over the DNA X-ray imaging project. The head of the research unit neglected to inform Wilkins that this aspect of his research was being assigned to someone else.
At the same time,
James Watson (trained in zoology) and Francis Crick (trained in physics) proposed a triple helical structure with the sugar-phosphate backbone on the inside and the bases on the outside.
Neither Watson nor Crick had a strong background in biochemistry. When Franklin saw their first model, she pointed out that they made a basic error in placing the hydrophilic sugar-phosphate backbone on the inside and the hydrophobic bases on the outside.
- In 1952, Franklin was able to capture what been called the critical image of the
DNA molecule. This image is known as “Photo 51.”
Watson was shown Photo 51 by Wilkins and both Watson and Crick were able to read internal progress reports on Franklin’s research. Having access to this information was certainly useful; it is not clear if her research results were the pivotal component to their second, and ultimately correct, DNA model.
- In April 1953, Watson and Crick published their model of
the structure of DNA.
In 1962, Wilkins, Watson, and Crick were awarded the Nobel Prize “for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material.”
- Components and helix structures of DNA
There are four DNA nucleotides:
adenine (A), guanine (G), cytosine (C), and thymine (T).
- Franklin had died in 1958 of ovarian cancer at the age of 37. Since the Nobel Prize is not deliberately given posthumously, she could not be considered for the prize.
Each is composed of a sugar, a phosphate, and a nitrogenous base.
The sugar and phosphate groups are the backbone of the molecule.
Two strands are wrapped around each other in a helix.
The rungs of the helix/ladder are composed of the complimentary base pairs:
A=T: adenine pairs with thymine
G=C: guanine pairs with cytosine
