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BIOB11H3 Study Guide - Midterm Guide: Punnett Square, Chromosome Segregation, Gynoecium

Biological Sciences
Course Code
Dan Riggs
Study Guide

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Lecture 1
History of Genetics
1856 discovery of unit of inheritance
o Mendel and his principles of hereditary
1880 discovery of chromosomes
o With the use of the microscope
1903 discovery of homologous chromosomes
1909-1911 discovery of crossing over
o During meiosis the mom and the dad’s chromosomes cross over their different genes
1911-1913 genes can be mapped in order along the length of the chromosome
1944-2952 DNA is the genetic material, not proteins
1953 DNA structure by Watson and crick
In 2018 many genomes are completely sequenced and can be done quickly
o Human genomes are 3x 10 ^9 bp, and now take a day to do
o Bioinformatics/computational genomics
Most organisms are this
There are two copies of each gene and chromosome
Alternative forms of the same gene
The genetic composition of the organism
The observable traits or morphology of seeds
Based on which trait is dominant
When Mendel was establishing the basis of heredity, he used
height, colour, and shape alleles
He was able to determine that some traits were dominant over
He used peas that had complete flowers with both male and
female reproductive structures
o Anthers shed Pollen that contains the sperm
o Carpel contains ovules which contain eggs
o There can be multiple fertilization events
o In peas multiple ovules an be fertilized by a pollen grain
The pea pods contained 8-10 seeds/progeny
The plants were allowed to self fertilize, he used a punnet square
to determine the traits of sperm vs egg
He observed that two alleles are segregating and that one has
dominance on the other
When both alleles are the same (dominant or recessive)
Wildtype the homozygous dominant
o The most prominent in the population, purebred for dominant phenotype
Nulls Homozygous recessive

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o This is generally associated with a mutant trait
o Pure breeding that gives rise to the recessive/mutant trait
When the alleles are different (dominant and recessive)
Not pure breeding, the next generation can have both phenotypes
Heterozygote Example
Suppose mother donates normal dominant genes and father donates recessive mutant genes, the heterozygous
progeny will often exhibit normal activity
The mutated gene will be translated and transcribed and will lead to an inactive protein, but the dominant allele
will lead to an active protein
o As long as you have one good copy of the gene of the protein, you are ok
o But if both alleles have mutations you have problems and cannot produce the right protein and then you
are in trouble
You are technically a carrier of the dads mutant gene even if you don’t express it
Biochemists Characterize cellular macromolecules
DNA is a polymer that has four nitrogen bases
Proteins are polymers made of 20 amino acids
o Since proteins were more complex, they thought that was the genetic material
They were both abundant in the cell
Evidence that DNA is the Genetic Material
Avery, McCarthy and McLeod (1940)
o Studies of virulent strains of bacteria showed that a transforming principle could be passed from one
bacteria to another and that DNA may be the transforming principle that carries the information
o They said the transforming principle was DNA based on:
Chemical properties of the transforming principle were consistent with DNA
No other material could be detected in the preparation
o Experiment using a Direct Test
1. Encapsulated strain
2. Lyse cells, filter to obtain cell free extract (had all the insides)
3. Use enzymes that degrade a specific macromolecule (DNAse, proteinase etc.) into the extract
4. Inject mice with the altered extract to determine if it retained the transforming property
o The result was that only the DNAse enzymes could inactivate the transforming principle, so DNA is
therefore the genetic material
o If you used the protease, the mouse still got the infection, therefore the transforming principle was still
present so it couldn’t have been proteins
Hershey and Chase (1950)
o Studied viruses that infect bacteria and proved that phage DNA was responsible for the viral infection
o Bacteriophage virus that infects bacteria
Contains a protein coat that encases the DNA
It is released from a host cell and finds another new host cell that it can inject the DNA into and
it uses that cell to reproduce until it bursts and releases more phage
o Experiment
The bacteriophage displays heritable properties

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They wanted to know if the protein or DNA
directed the replication of the new
They radiolabelled the protein or DNA of a
bacteriophage and infected the bacteria
When the DNA was labelled, there was
radiolabel found in the bacteria/ new phage
When the protein was labeled, the
radiolabel was found in empty phage
o They too concluded that DNA was the genetic material because that is what caused the infection
o Came before the Avery et al experiment
o Showed that heat killed lysed streptococcus pneumonia bacteria contained a transforming principle
o The encapsulated(s) strains were virulent, the capsuleless (r) strains are not virulent
Inject live s cells mouse dies from infection
Inject live r cell mouse lives
Inject heat killed s cells mouse lives
This shows that heat can kill the contagious aspect
Heat killed s cells and live r cells mouse dies from infection
When you combined the two non contagious trials, the mouse still died…. Why
There must be a soluble factor that is released from the s cells to the r cells to transform
them into a virulent cells
o The transforming principle transformed harmless cells into virulent cells
o Therefore there must be some factor that can be taken up by the live r cells to transform them into the
virulent s cells transforming principle
Watson, Crick, Wilkins and Franklin (1950)
o Used x-ray crystallography elucidate the structure of DNA
You make crystals of the molecule, shine an x ray and the detraction of the x ray produces a
picture of the structure
DNA Structure
It has a double stranded, antiparallel structure, 3’-5’ vs 5’3’, these refer to the carbon positions in the sugar
molecule that link adjacent bases in DNA
Complementary base pairing occurs by hydrogen bonding of arginine to thymine and guanine to cytosine
o In RNA uracil replaces thymine
Nucleotide phosphate group, sugar, and nitrogenous base
Nucleoside the structure without the phosphate group
The phosphate and sugar are connected by phosphodiester bonds
Nitrogenous Bases
o Purines arginine and guanine
o Pyrimidines cytosine, thymine and uracil
Early Work on DNA: Physiochemical Approaches
DNA is a polynucleotide chain
Estimates made of genome size
Values were tiny yet enormous
o Humans: 3.5 pictograms of DNA/haploid (3x 10 ^-12 grams)
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