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Genes

Topic 3
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loki) = a specific region on a chromosome

➢ Normal body cells carry 2 chromosomes of each pair, one from the mother and one
from the father
Alleles

➢ Alleles are different variations or versions of a gene
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A specific form of a gene differing from other alleles by one or a few bases

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Example: attached or detached earlobes

➢ They are also hereditary
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New alleles can occur through mutation

Mutations
➢ A mutation is a random, rare change in the genetic material
➢ One type involves change of the base sequence
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Does not happen if DNA replication works correctly

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A base substitution mutation results in a single letter being changed
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A different amino acid is placed in the polypeptide chain

➢ Can be positive or negative for an organism
➢ A mutation that provides an individual or a species with a better chance for survival
is considered to be a beneficial mutation
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Good chance to be passed on

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Historically humans have not been able to digest lactose as adults

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The last 10 000 years humans have held cattle which produce dairy products

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Another mutation enabled the digestion of lactose
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Good source of energy = survival

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Most common among people from N
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America

➢ A mutation that causes disease or death is detrimental mutation , therefore is less
likely to be passed on
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Because it decreases chance for survival

➢ There are neutral mutations that do not have any effect on a species survival
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Genes

Topic 3
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1

BIology SL

➢ Those with 2 genes for sickle cells have only sickle-shaped red blood cells
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They suffer from severe anaemia

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Have the highest resistance to malaria

Sequencing DNA

➢ Genome is the complete set of an organism's base sequence
➢ Human Genome Project’s goal was to determine the order of all the bases the
genome possesses
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In 2003 it achieved its goal

➢ Now, scientists are working on figuring out which sequence represents genes and
which gene does what

➢ Sanger technique
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We take a sample of DNA, cut it into fragments and copy these fragments

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We add a primer to start the process

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A DNA polymerase enzyme starts to add free nucleotides to the fragments
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Some of the nucleotides are ddNTPs and are terminators (stop
elongation) and have been previously marked with fluorescent

markers so they can be identified
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We get strands of various lengths and using electrophoresis we place them
from longest to shortest

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A laser activates the fluorescent markers on the nucleotides during
electrophoresis in order to see on what base the elongation has stopped
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A computer analyses the wavelength of the light and determines if it
is A, C, T or G

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Replicated many times

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When they are done with fragment 1, they progress to fragment 2, 3 & so on

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The fragments get mixed up and using computers, scientists can put them
back in the order they were originally

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