14. unit 2b mutations

8/3/2019 14. Unit 2B Mutations

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Unit 2B

Human Diversity & Change

Inheritance

Mutations


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A mutation

A mutation is a permanent structuralalteration in DNA.

In most cases, DNA changes either haveno effect or cause harm - occasionally amutation can improve an organism'schance of surviving and passing thebeneficial change on to its descendants.


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NIH - National HumanGenome ResearchInstitute


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Somatic mutations

A somatic mutation is a DNA change that

occurs in body cells, other than the egg orsperm (germ cells) they cannot bepassed on to offspring.


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Germ line mutations

A germ line mutation is a DNA change

that occurs in the egg or sperm.Germ line mutations can be passed on tooffspring.


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Point mutations

The simplest type of mutation is a point

mutation.This involves a change in a single basepair.


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An analogy

The following short sentence represents asequence of nucleotides in a fragment ofDNA:

THEBOYATETHEBUN-----

This is read in codons (3-letter segments)THE BOY ATE THE BUN------


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Deletion

THEBOYATETHEBUN------

THEBOYTETHEBUN------

THE BOY TET HEB UN------

A


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Duplication

THEBOYATETHEBUN

THEBOYATETOYATETHEBUN

THE BOY ATE TOY ATE THE BUN


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Inversion

THEBOYATETHEBUN

THEBTETAYOHEBUN

THE BTE TAY OHE BUN


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Insertion


THEBOYAXTETHEBUN-----

THE BOY AXT ETH EBU N-----

In the above cases part, or all, of the sentence is nolonger readable.

X


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Substitution


THEBOYATETHEBUG------

THE BOY ATE THE BUG------

In this case, the meaning is changed but sentence is stillreadable.

G


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Non-disjunction

A non-disjunction occurs if homologouschromosomes fail to separate correctlyand migrate to opposite poles during

meiosis.This results in too many chromosomes insome daughter cells and too few in others.The most common disease resulting from

a non-disjunction is Downs syndrome, ortrisomy 21, where there are three copiesof chromosome 21.


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Cytogenic map

This is how chromosomeslook when stained andexamined under amicroscope. The light and

dark bands give eachchromosome a uniqueappearance. This feature isuseful for the study of

karyotypes, which allowsscientists to look forchromosomal alterations.


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Normal karyotypes

Normal female - XX Normal male - XY

Wessex Reg, Genetics Centre, Wellcome Images


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Autosomal mutations

Downs syndrome trisomy 21 Edwards syndrome trisomy 18



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Downs syndrome trisomy 21

People with Downs

syndrome have anextra copy of

chromosome 21. Affected people have

varying degrees ofmental retardation,

short stature, and aflattened facial profile.

Wellcome Photo Library


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Sex chromosome mutations

Klinefelters syndrome - XXY Turners syndrome - XO



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Turners syndrome - XO

The Turners syndromekaryotype has only one Xchromosome and no Ychromosome.

Women with Turnerssyndrome are usuallyshort and have a numberof abnormal physicalfeatures. Their ovariesnever develop so theyare infertile, and nevermenstruate.



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Klinefelters syndrome - XXY

The Klinefelters syndromekaryotype has an extra Xchromosome (XXY).

Men with Klinefelters

syndrome do not produceenough testosterone. Thisresults in underdevelopedsecondary sex

characteristics, breastenlargement, small testesand infertility due to lack ofsperm.



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Factors affecting the outcome of amutation

The type of cell affected (somatic or germline)

The type of mutation (e.g. substitution,insertion)

The extent of the mutation (e.g. point, non-disjunction)

The area of the gene affected (coding ornon-coding)


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Mutagens

A mutagen is anything that causes amutation (i.e. causes a permanent changein a cells DNA).

Most mutagens are either chemicals orphysical agents such as radiation.

Some mutations are spontaneous,occurring as a result of errors during DNAreplication or repair.

14. unit 2b mutations

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