Topic 4.1: Chromosomes, genes, alleles, and mutations

Assessment Statements: 4.1.1: State that eukaryotic

chromosomes are made of DNA and proteins

4.1.2: Define gene, allele and genome 4.1.3: Define gene mutations 4.1.4: Explain the consequence of a base

substitution mutation in relation to the processes of transcription and translation, using the example of sickle cell anaemia

How is DNA organized Chromosomes are bundles of long

strands of DNA› If you could unwind a chromosome, it

would be like unraveling a ball of string

In eukaryotes that reproduce sexually, chromosomes always come in pairs› Humans have 46 chromosomes in 23

pairs› The DNA un eukaryotes is associated

with proteins which helps to keep the DNA organized

Gene A gene is a heritable factor

that controls a specific characteristic› Heritable means passed on

from parent to offspring and characteristic refers to genetic traits such as your hair color or your blood type

› The estimated 30,000 genes which you possess are organized into chromosomes

Gene The genes which determine

eye color have more than one form› Variations (like in eye color) of a

gene are called alleles An allele is one specific form of a

gene, differing from other alleles by one or a few bases

Alleles of the same gene occupy a corresponding place (locus) on each chromosome of a pair

Gene In order to find out which gene does

what, a list must be made showing the order of all the letters in the DNA code (sequence of bases)› A complete set of an organism’s base

sequence is called its genome› The complete genome of a few organisms

have been fully written out.

Mutations

A mutation is a random, rare change in genetic materials› One type involves a change of the

sequence of bases of DNA If DNA replication works correctly, this should

not happen

Mutations Base substitution mutation

› The consequence of changing one base could means that a different amino acid is placed in the growing polypeptide chain This may have little or no effect on the

organisms or it may have a major influence on the organisms physical characteristics

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter15/mutation_by_base_substitution.html

Mutations In fruit flies

› One base substitution mutation can cause: Extra pair of wings Change in eye color The number of legs Shape of the wings

Mutations In humans:

› one base substitution mutation can cause: Mutations is sometimes found in the gene

which creates hemoglobin for red blood cells This mutation gives a different shape to the

hemoglobin molecule The mutated red blood cell with the characteristic

curved shape made its discoverers think of a sickle (a curved knife used to cut tall plants) Called sickle cell anaemia

Mutations Sickle cell anemia

› One base is substituted for another so that the codon GAG becomes GTG

› During translation instead of adding glutamic acid it added valine instead This results in a different

shape of the polypeptide—hemoglobin molecule has a different shape which causes the red blood cell have a different shape

Mutations Symptoms of sickle cell anemia

› Weakness, fatigue, and shortness of breath› Oxygen cannot be carried as efficiently by

the irregularly-shaped red blood cells› Hemoglobin tends to crystallize within the

red blood cells, causing them to be less flexible The affected red blood cells can get stuck in

capillaries so blood flow can be slowed or blocked

Mutations People affected by sickle cell anima

have a risk of passing the mutated gene to their offspring› Mutated gene is mostly found in

populations originating from West Africa or from the Mediterranean