dna and genes. dna is a double helix discovered by watson and crick in 1953 they won a nobel prize...
TRANSCRIPT
DNA is made of the same repeating subunits called……Nucleotides.
The basic chemical makeup of DNA:
Phosphate
5 carbon sugar deoxyribose
Nitrogenous Base
Forms the “backbone” and always the same
Variables:A, T, G, C
Chargaff’s Rule
Adenine must pair with Thymine Guanine must pair with Cytosine because they always pair together, their
amounts in a DNA molecule are the same
G CT A
Answer:
there would be 20% Cytosine:
Adenine (30%) = Thymine (30%)
Guanine (20%) = Cytosine (20%)
(50%) = (50%)
Genes
DNA is not a sequence of random information, it is broken up into areas with different functions
sections of DNA with particular functions are called genes
Alleles
different versions of the same gene are called alleles
e.g. the gene for eye pigment in fruit flies (Drosophila) has a red allele, a white allele, and a brown allele
chromosomes are always paired, so it is possible to have two different alleles for any given gene
Gregor Mendel: father of genetics
discovered heredity – the passing on of characteristics from parent to offspring
bred pea plants until they were true-breeding, the offspring always resembled the parents
Mendel’s experiment
he took one plant of each type, such as purple and white flowers, and bred them together: all the offspring were purple
took two of these offspring and bred them to make a second generation: 3 purple offspring / 1 white offspring
cross-fertiliseParental generation
First generation
(F1)
true-breeding purple flowers
true-breeding white flowers
all offspring purple
First generation
(F1)
self-fertilise
Second generation
(F2)
all offspring purple
3/4 purple 1/4 white
Mendel’s Conclusions
1. parents pass on ‘elements’ to their offspring; these ‘elements’ remain unchanged during life
2. offspring receive one ‘element’ from each parent; therefore offspring have a pair
3. each parent can pass on a different version of the ‘element’
Mendel’s ‘elements’ are now called GENES.
Number of genesCarsonella ruddii (bacterium) 180
Streptococcus pneumoniae (bacterium) 2,300
Escherichia coli (bacterium) 4,400
Saccharomyces cerevisiae (yeast) 5,800
Drosophila melanogaster (fruit fly) 13,700
Caenorhabditis elegans (nematode) 19,000
Strongylocentrotus purpuratus (urchin) 23,300
Homo sapiens (human) 27,000
Mus musculus (mouse) 29,000
Oryza sativa (rice) 50,000
Dominant vs. recessive
dominant = when one allele overwrites the expression of another allele
recessive = when one allele is masked by the expression of another allele
the dominant allele is given a capital letter (P), the recessive allele a lower case letter (p)
Human dominant/recessive traits:Cleft in chin No cleft dominant, cleft recessive
Hairline Widow peak dominant, straight hairline recessive
Eyebrow size Broad dominant, slender recessive
Eyebrow shape Separated dominant, joined recessive
Eyelash length Long dominant, short recessive
Dimples Dimples dominant, no dimples recessive
Earlobes Free lobe dominant, attached recessive
Freckles Freckles dominant, no freckles recessive
Tongue rolling Roller dominant, nonroller recessive
Tongue folding Inability dominant, ability recessive
Finger mid-digital hair Hair dominant, no hair recessive
Bent little finger Bent dominant, straight recessive
Interlaced fingers Left thumb over right dominant, right over left recessive
Hair on back of hand Hair dominant, no hair recessive
Heterozygous vs. homozygous
Heterozygous = having two different alleles for a particular gene (e.g. Pp)
Homozygous = having two of the same allele for a particular gene (e.g. PP or pp)
Genotype vs. phenotype
genotype = the genetic characteristics of an organism
eg. PP or Pp
phenotype = the visible physical features of an organism
e.g. white flower
Phenotype Genotype
PP or Pppurple flower
white flower pp
The Punnett Square
In three steps, it’s an easy way to determine the probability of offspring:
Step 1: Make a 2 X 2 Square gridStep 2: Put the alleles of each parent on the
outside of the squareStep 3: Combine alleles to make potential
offspring in the middle of the square
Sample Punnett problem
The gene for free ear-lobes is dominant (E), and the gene for attached earlobes is recessive (e). If a heterozygous male mates with a homozygous recessive female, what is the probability that their child will have attached earlobes?
Solution:
50% chance heterozygous (Ee – free lobes) 50% homozygous recessive (ee – attached lobes)
Male
Female E e
e Ee ee
e Ee ee
Fruit fly genetics lab
the common fruit fly, Drosophila melanogaster is a staple of genetics research
it reproduces quickly and is easy to keep in the laboratory
we know more about the genetics of this organism than any other (even humans)
Today’s procedure in brief:
1. carefully pour out your bottle of flies
2. sort them by sex and by wing phenotype (normal or vestigial) into four categories
3. count and record the number of flies in each category
4. share and record the fly counts from the other groups