topics: dna structure, replication & central dogma genetics : introduction to iem

Topics:DNA Structure,

Replication & Central Dogma

GENETICS : Introduction to IEM

Studies of Heridity

• By geneticists - describe patterns of inheritance• traits (phenotypes)• heritable (passed from parents to offspring)• cytogeneticists knew that trait inheritance is associated with the cell nucleus and with chromosomes• biochemists knew that chromosomes are composed of DNA and protein

Q. What is the molecular/biochemical basis ofinheritance?

How is it known that DNAcontains genetic information

???

Parent trait Offspring trait

• Gene:- segment of DNA that contains all the information needed for regulated synthesis of an RNA or protein product.

• Genome:- the entire DNA sequence content of an organism (nuclear DNA)

Some Important Definitions

DNA Structure: Double Helix

• 1953 - Watson and Crick 3-D structure of DNA

• DNA is a double helix (ll-stranded)

• Polymer of nucleotides (phosphate, sugar, base)

• DNA has 4 base types (adenine, thymine, guanine, cytosine)

phosphate

sugar

ADENINE (A) THYMINE (T)

CYTOSINE (C)GUANINE (G)

base

4 DNA Nucleotides

Strands have different polarity

&antiparallel

A - T

G - C

Base pairing

1 bp

or or

equal (randam) base

composition AT rich GC rich

5’

5’3’

3’

parentalstrand as a template

daughterstrand

has complement

bases

DNA Replication

Clones due to replication from 1 cell

How does DNA relate to proteins?

1908: Garrodinborn errorsof metabolism

(hereditary disease)

Alkaptonuria (AKU): accumulation of homogentisic acid1:200,000

Phenylalanine/Tyrosinedegradativemetabolicpathway

blocked inAKU1:200,000

blocked inPKU1:8,000

blocked inTyrosenemia

II

III

A defective enzyme results from a mutant gene

HOW????

Central Dogma of Genetics

DNA

RNA

Protein

Replication

Transcription

Translation

Reverse Transcription

aa aa aa aa aa aa

Genes and Proteins

• Inborn Errors of Metabolism shown by Garrod to cause hereditary disease.

• Study of Biochemical Pathways lead to understanding that mutant genes result in defective proteins (enzymes).

Biochemical Genetics

Archibald Garrod (1902) - an English doctor

Described “alkaptanurea” disease

Symptom: urine turns black when exposed to air

Found it was due to oxidation of homogentisic acid in urinehomogentisic acid = an intermediate in Phe degradation

Phe Tyrhomogentisic

acidfurther

metabolites

Accumulation of homogentisic

acid

Biochemical Genetics

Archibald Garrod : important contributions

Described “alkaptanurea” disease

Deduced that it is due to a defective metabolic enzyme

Disease is a hereditary condition (ran in his patients’ families)

Led to concept of “inborn errors of metabolism”

A novel phenotype may reflects a discrete biochemical difference

Biochemical Genetics

“Real-World Biochemistry”

Aspartame

= a dipeptide: aspartyl-phenylalanine methyl ester

Aspartame is metabolized in the body to its components: aspartic acid, phenylalanine, and methanol. Like other amino acids, it provides 4 calories per gram. Since it is about 180 times as sweet as sugar, the amount of aspartame needed to achieve a given level of sweetness is less than 1% of the amount of sugar required. Thus 99.4% of the calories can be replaced.

Look on your diet soda cans and read the warning

Biochemical Genetics

Archibald Garrod : important contributions

Proposed that inheritance of a defective metabolic enzyme leads to inheritance of a phenotype (disease)

Parent trait Offspring traitdefectiveenzyme

• born in Wahoo, Ne• undergraduate degree at UNL• did graduate work at Cornell• got a faculty position at CalTech• ended up as the president of the Univ of Chicago

• did work in the 1930’s & 40’s on Drosophila eyesand on Neurospora (bread mold)• “one gene - one enzyme” hypothesis (1941)• awarded Nobel prize in 1958 (with research colleagues J. Lederberg and E. Tatum)

George W. Beadle

George W. Beadle

• Bread Mold: Neurospora crassa• can grow on minimal media

sucrose Inorganic salts biotin

• Beadle selected for nutritional mutants (auxotrophs)• irradiated fungal spores, grew these up on completemedia, and transferred part of the stock to minimal media

• He looked for mutants that can grow on complete media but NOT on minimal media

•These mutants are lacking an enzyme for the synthesis of an essential nutrient

Beadle’s Experiment Summary

•Beadle could identify mutants in specific steps of a pathway

•Assuming each mutant was defective in a single gene, Beadle postulated that the different mutant classes each lacked a different enzyme for Arg biosynthesis

•Therefore, he could show a one-to-one correspondance between mutation and absence of an enzyme.

• one gene specifies/encodes one enzyme

Beadle’s experiment gave riseto a new field called

Biochemical Genetics

defectivegene

Parent trait

Offspring trait

defectiveenzyme

Mutations

• Mutation = change in the base sequence of DNA

• Any mutation that causes the insertion of an incorrect amino acid in a protein can impair its function

• Base substitutions alter the genetic code which specifies amino acid placement in proteins

Genes and Environment

• One gene can affect more than one trait: pleiotropy

• Any trait can be affected by more than one gene: epistasis

Pedigree Analysis

• The analysis of an unknown trait from a family history (pedigree) 

 – Is the trait dominant

• does every affected offspring have an affected parent

– Is the trait determined by a single gene• affected progeny born to heterozygous parents (carriers) should occur

with a frequency of 3:1 (unaffected to affected)

– Is the trait sex-linked• is it expressed more frequently in males • not, if expressed at equal frequency in male and female progeny

Autosomal Dominants• At least one parent of the index case (proband) is

affected, both male and female progeny are affected equally and can transmit the condition, when an affected person has offspring, they have a one chance in two of inheriting the trait

•  For dominantly inherited traits:– generations not skipped

– some patients do not have affected parents, the result of a new mutation

– clinical features:• reduced penetrance - reduced fraction of individuals show the phenotype

• variable expressivity – trait is expressed to different extents among affected individuals

– for many dominant traits the age of onset is delayed beyond reproductive age 

Mechanisms of dominant disorders•  Usually a loss of function mutation

– Loss of a component of an enzymatic, regulatory or signaling pathway– Loss of a structural protein such as collagen

•  These can produce a dominantly active phenotype by: – reducing function below a level necessary to maintain a normal phenotype

(familial hypercholesterolemia, LDL receptor) – acting as a “dominant negative” (Marfan Syndrome, fibrillin-1 or some

forms of Ehlers-Danlos Syndrome, collagen) which prevents the function of the normal allele in the heterozygous state

•  Gain of Function Mutation– Huntington disease results from a mutation in the Huntington gene which

gives rise to an over-expression of an altered protein that is toxic to neural cells

Autosomal Dominant Genetic Disease

Autosomal Recessive• The trait does not usually affect the parents, but

siblings may show the disease

• Siblings have a 1-in-4 chance of inheriting the disease

• The majority of the mutant genes in the gene pool are in heterozygous “carriers”

• If the mutant gene occurs with a low frequency in the population, there is a likelihood the proband is the product of a consanguineous marriage

Mechanisms of Recessive Disorders

• Features of autosomal recessive disorders– Complete penetrance– Early age of onset – Molecular change usually results in a loss of function

• In the heterozygous carrier the presence of 50% of the protein is sufficient to provide a normal phenotype– Essentially all inborn errors of metabolism are inherited as

autosomal recessive traits

Autosomal Recessive Genetic Disease