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Mutations

A mutation is a mistake in the sequence of DNA bases.

Since the pattern of these DNA bases are instructions for building everything in a cell, mutations lead to unexpected changes in cells and tissues.

When do mutations matter?

In regards to evolution by natural selection mutations won’t matter if they occur in your body cells.

Body cell mutations can’t be passed down to your children, or their descendants.

Mutations only matter in evolution by natural selection if they occur in gametes – a sperm or egg cell.

That is because gamete mutations are passed on to the next generation.

Here is a visible mutation: Polydactyly/polydactylism

Etymology: From the from Greek πολύς (polys),  ‘many’, and δάκτυλος (daktylos),  ‘finger’.

Four types of base mutations

Base substitution

Base insertion – creates a frameshift

Base deletion – creates a frameshift

Base substitution mutation

One nucleic acid base is substituted for another.

Original strand: TAC GCA TGG ACG …

Mutated strand: TAC GTA TGG ACG …

This affects only one amino acid in a protein.

{ https://islaslab.wikispaces.com/Mutation }
substitutionjpg

Base insertion mutation

One nucleic acid base is accidentally inserted into the sequence.

Original strand: TAC GCA TGG ACG…

Mutated strand: TAT CGC ATG GAC G…  (see the extra “T” ?! )

This messes up how the cell reads these genes.

The cell always reads three bases at a time – so this insertion messes up every single amino acid after it.

This is a frameshift mutation.

Example of how this works, if we used English words:

Normal DNA THEREDFOXATETHEBIGRAT…

Read 3 at a time:
THE RED FOX ATE THE BIG RAT

Insertion mutation:  (See how there is an extra “E” ?!)
THEEREDFOXATETHEBIGRAT…

Read the words 3 letters at a time:
THE ERE DFO XAT ETH EBI GRA T…

See how the resulting message is all messed up?

Base deletion mutation

One nucleic acid is left out.

Original strand: TAC GCA TGG ACG…

Mutated strand: TCG CAT GG ACG… (See how the first “A” is missing here?)

This messes up how the cell reads these genes.
Another frameshift mutation

Normal DNA: THEREDFOXATETHEBIGRAT…

Read 3 at a time: THE RED FOX ATE THE BIG RAT

Deletion mutation: THREDFOXATETHEBIGRAT…

Read the words 3 letters at a time: THR EDF OXA TET HEB IGR AT

Since the first “E” is missing the rest of the message becomes nonsense.

{ https://islaslab.wikispaces.com/Mutation }
deletionmutation

Good, bad or indifferent?

Mutations are not “good” or “bad” – they are just random changes.

A very tiny percent of mutations are helpful. In biology this means that the mutation makes it more likely for the organism to grow, find a mate, and have fertile offspring.

A larger percent of mutations are harmful. In biology this means that the mutation makes it less likely for the organism to grow, find a mate, and have fertile offspring.

Many mutations are “silent”. They don’t have much of a visible effect, so we don’t notice them.

Mutations : silent, beneficial, harmful

Silent mutations

“Silent mutations occur when the change of a single DNA nucleotide within a protein-coding portion of a gene does not affect the sequence of amino acids that make up the gene’s protein.

That’s possible because proteins are encoded by “triplets” of nucleotides, each responsible for adding a particular amino acid to the protein chain.

A change in one nucleotide, however, doesn’t always change the triplet’s meaning; the mutated triplet may still add the same amino acid. And when the amino acids of a protein stay the same, researchers believed, so do its structure and function.”

The Sound of a Silent Mutation, ScienceMag.org, 12/22/2006

In the image below the leftmost column is what the DNA is supposed to be.

In the next column (“silent) we see that one of the base pairs was mutated from C to T, yet by dumb blind luck the resulting amino acid is the same. So the mutation is “silent.”

Types of Mutations Silent Nonsense Missense by Jonsta247 Wikipedia

Image, Mutations by Jonsta247, Wikipedia

Beneficial mutations

A. Mutations leading to extra skin pigmentation. Helpful for people who live closer to the equator

1. This reduces the amount of UV light that penetrates one’s skin; which also reduces one’s risk of skin cancer.

B. Mutations leading to less skin pigmentation. Helpful for people who live further from the equator

1. Humans need some UV light to process vitamin D. Lack of vitamin D processing leads to disease.

a. Rickets – childhood disease, impeded growth, deformity of the long bones

b. Osteomalacia – bone-thinning disorder that occurs exclusively in adults

C. Mutations leading to ability of adults to digest lactase. This is why humans with significant European or Kenyan ancestry can digest milk as adults.

Harmful mutations: genetic diseases

A. Sickle cell anemia

B. Cystic fibrosis

C. Huntington’s disease

D. Tay-Sach’s diesase

Chromosome-level mutations

So far we have only looked at mutations of just one gene. What if an entire chunk of a chromosome got mixed up, deleted, or repeatedly copied? That could affect hundreds of genes at the same time.

Deletion

cri-du-chat syndrome – intellectual disability, delayed development, small head size, low birth weight, weak muscle tone in infancy, and distinctive facial features

22q11.2 deletion syndrome – cleft palate, heart defects, recurrent infections, unique facial characteristics, feeding problems, kidney abnormalities, scoliosis, hearing loss, developmental delay, learning disabilitie

Duplication – partial trisomies, occur when there is an extra copy of a segment of a chromosome.

MECP2 duplication  – occurs almost exclusively in males. Creates intellectual disability, weak muscle tone in infancy, feeding difficulties, poor or absent speech, or muscle stiffness (rigidity).

https://rarediseases.info.nih.gov/guides/pages/73/faqs-about-chromosome-disorders

Chromosome Mutations

Source presently unknown.

Having an entire extra chromosome

Triple-X syndrome occurs in women who inherit three X or more chromosomes.

taller than average with unusually long legs and slender torsos; have normal development of sexual characteristics and are fertile but tend to have some ovary abnormalities; may have slight learning difficulties, especially in speech and language skills

from https://www2.palomar.edu/anthro/abnormal/abnormal_5.htm

 

Examples

“Diagram of the human chromosome set, showing the location of some genes whose mutant forms cause hereditary diseases. Conditions that can be diagnosed using DNA analysis are indicated by a red dot.”

Human chromosome genetic diseases by Ігор Пєтков

From Wikimedia, Human chromosome diseases

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