DNA and genes

DNA is a double helix

discovered by Watson and Crick in 1953

they won a Nobel prize for their work

The structure of DNA

uncoiled diagram diagram space-filling model

Chromosome – condensed DNA

Chromatin– extended DNA

Chromosome – condensed DNA

Chromatin– extended DNA

The structure of DNA

P

P

P

P

P

P

G C

T A

S

S

S

S

S

S

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

The base pairs

CytosineC

GuanineG

Hydrogen bond

ThymineT

AdenineA

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

Question:

If there is 30% Adenine in a strand of DNA, how much Cytosine is present?

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

start of human chromosome 1

These are all 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

eye colour alleles in Drosophila

Genes and alleles

You will need: 1 pencil crayon or pen

(preferably purple)

Ishihara Colour Blindness Test

everyone sees 12

normal = 8

red-green = 3

total = nothing

normal = 74

red-green = 21

total = nothing

normal = 6

red-green = nothing

total = nothing

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)

Dominant and recessive traits

in peas

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

Drosophila chromosomes

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)

wildtype (normal) Drosophila

♀ (female) ♂ (male)

Vestigial wing mutant

Wing phenotype varies from almost absent (left) to shriveled (right)

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

How to tell males from females:

sex combs

♀ (female)♂ (male)

small, dark

abdomen

large, striped abdomen

no sex combs