Chromosomes, genes, alleles, and mutation

Topic 4.1

Assessment Statements

4.1.1 State that eukaryote chromosomes are made of DNA and proteins.

4.1.2 Define gene, allele and genome.

4.1.3 Define gene mutation.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 anemia.

Organization of chromosomes

• Chromosomes are bundles of long strands of DNA associate with proteins

• Occur in pairs (except in sex cells or gametes) in sexually reproductive eukaryotes

• Humans have 23 pairs• Prokaryotes only have

one chromosome and DNA is not associated with proteins

Gene

• A heritable factor that controls a specific characteristic or genetic trait

• Passed on from parent to offspring

• Humans possess an estimated 30,000 genes

Alleles

• A specific form of a gene, such as eye color or hair color

• Differ from other alleles by only one or a few bases

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

Genome

• The complete set of an organism’s base sequences

• Used to find out which gene does what

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Gene mutation

• A random, rare change in genetic material

Base substitution mutation

• Changing of one base could result in a different amino acid placed in a growing polypeptide chain

• May have little or no effect on the organism

• May have a major influence

• Ex. Extra pair of wings, change in eye color, number of legs, etc.

Sickle cell anemia• Base substitution found in

the gene which creates hemoglobin for red blood cells

• GAG becomes GTG• During translation instead

of adding glutamic acid, valine is added instead

• Since valine has a different shape and properties, the shape of the polypeptide chain is changed

• Mutation gives a different shape to the hemoglobin molecule

• Difference leads to rbcs having a curved shape

Symptoms of sickle cell anemia

• Weakness• Fatigue • Shortness of breath• Causes

– Oxygen cannot be carried as efficiently

– Rbcs tend to crystalize causing them to be less flexible

– Rbcs get stuck in capillaries

Critical thinking

What has ensured that the frequency of the gene that causes sickle cell anemia in a population

is balanced with the normal gene’s frequency?