Errors in Genes and Chromosomes

Genes are portions of DNA at a specific site called a locus within a chromosome.

The genes at a specific locus encode for a particular function.

The genetic sequence could encode for Enzymes Hormones Structural proteins

At times, errors or mutations in a gene or chromosome may occur during: Transcription Chromosome separation during Mitosis or Meiosis

Mutations that occur in gamete cells: will be present in the organism and be passed on to the next generation.

Causes of MutationsMutagenic Agents These are agents that

cause mutations.

These include: Radiation (UV, X-rays) Temperature extremes Exposure to chemicals

(pesticides

Mutations are divided into two categories:

1. Point mutations At a single gene Alterations may occur in the sequence or number of

nucleotides

 

2. Chromosomal mutations: More extensive alteration with a part of or entire

chromosome

Point or Gene Mutations:

Occur when DNA is transcribed into RNA There are two types:

‘The child walked down the street’

What do you notice about….

“The child wapked down the street”

1. Substitution: One nucleotide is

being substituted or replaced with another.

End result is a different nucleotide sequence than the original DNA sequence

‘The child walked down the street’

What do you notice about….

“The child walkxe ddow nth estreet”

X

2. Frame-shift Mutation During transcription

nucleotide base pairs may be inserted or deleted from the DNA sequence

Point mutations can lead to the following outcomes:Silent Mutations Have no effect on the operation of the cell (do not

change the amino acid sequence) Usually occurs in the noncoding regions of DNA Why are they silent? Introns are cut out of the mRNA transcript during

transcription, thus mutations never surfaces. Genetic code has a redundancy in nature (Ex.

UUU and UUC both code for phenylalanine

Missense Mutations Occurs when a change in the base sequence

of DNA alters a codon, therefore a different amino acid is placed in the protein sequence.

E.g. sickle cell anemia (see next slide)

Ex. Sickle Cell Anemia

Nonsense Mutations Arises when a change in the DNA sequence

causes a stop codon to replace a codon specifying an amino acid

Causes translation to stop short of the end of the full mRNA.

Therefore, only the part of the protein that precedes the stop codon is produced (the fragment may be digested by cell proteases)

Are often lethal to the cell

Chromosomal Mutations Portions of a chromosome may break off

and rejoin leading to an interruption in the sequence of genetic information.

There are 4 types:

‘The child walked down the street’

What do you notice about….

‘The child down walked the street’

1. Inversion A segment of DNA

will break off and be re-inserted in the same location but ‘flipped’

This could result in a drastically changed nonfunctional protein.

‘The child walked down the street’

What do you notice about….

‘walked down the street’ ‘It was a sunny day. The child’

2. Translocation A segment of DNA

breaks off a chromosome and is inserted into another chromosome.

At times, portions of DNA can be exchanged between two chromosomes (not only one a one way process)

‘The child walked down the street’

What do you notice about….

‘The down the street’

3. Deletion Loss of a chromosome

segment The effects could be

lethal if the deleted segment codes for vital proteins.

Ex. Cri-du-chat (The loss of a portion of chromosome 5, causes an abnormally developed larynx; makes the affected infant’s cry sound like the meowing of a cat)

‘The child walked down the street’

What do you notice about….

‘The child child child walked down the street’

4. Duplication: Within a

chromosome, repeated segments of DNA are seen.

Nondisjunction Improper separation of chromosomes

during: Meiosis I

(homologous chromosomes do not separate) Meiosis II or Mitosis

(sister chromatids do not separate) Result?

Excess or lack of chromosomes

Nondisjunction in autosomal chromosomes

p.173

During gamete formation, if nondisjunction occurs with a chromosome pair the resulting gametes will have one extra or one less chromosome.

Thus, when fertilization occurs, the zygote will have: 3 copies of one chromosome called TRISOMY or 1 copy of a chromosome called MONOSOMY

Ex. Down syndrome occurs because of an extra chromosome 21 (trisomic condition)

Nondisjunction in sex chromosomes

Nondisjunction in sex chromosomes during Meiosis will lead to an additional X or Y chromosome in the offspring.

This may result in disorders such as Turner and Klinefelter syndrome

Sometimes an entire set of chromosomes do not separate during Meiosis.

Result? The gamete will be diploid

Upon fertilization the zygote will have 3 sets of chromosomes (3n)

Rare in animals, but common in plants

Polypoidy: A cell or an organism in which the number of complete sets of chromosomes is greater than two.

Ex. Seedless Watermelon

Breeders cross a diploid male with a tetraploid female (4n)

Result, Sterile offspring (no

seeds)

MUTATION REPAIR MECHANISMS1) DIRECT REPAIR / PROOFREADING DURING

REPLICATION

During DNA replication, an incorrect base may be added to the growing polynucleotide chain. DNA Polymerase I performs a proofreading function.

When a mispairing of bases occur during the replication process, then the enzyme will remove the improperly placed base and try again. (Helicase, DNA ligase and other proteins also play a role in this mechanism

MISMATCH REPAIR If a mispairing of bases occurred during

DNA replication and ‘Proofreading’ wasn’t effective at correcting this error, then mismatch repair will take place.

Proteins will excise the mismatched base and DNA polymerase will add the correct bases.

3) EXCISION REPAIR During the life of a cell, DNA may become damaged due

to hazards such as high-energy radiation, chemicals that induce mutations, and random spontaneous chemical reactions.

Therefore, the cell will rely on excision repair, where certain enzymes will ‘inspect’ the cell’s DNA.

When they find mispaired bases, chemically modified bases or points at which one strand has more bases than the other, these enzymes cut the defective strand.

Other enzymes will cut away at the adjacent bases and DNA polymerase and DNA ligase synthesize and seal up a new piece to replace the excised one.

Recap

Errors or Mutations

Chromosomal mutation Gene/point mutation

4 types:

Inversion

Translocation

Deletion

Duplication

Nondisjunction

Autosomal chromosomes

Sex chromosomes

Polyploidy

2 Types:

Substitution

Frame-shift mutation

Questions 1. A diploid cell (2n) undergoes Meiosis I and II.

Nondisjunction of one pair of chromosomes occurs during Meiosis I. What are the number of chromosomes that result in the new gametes?

Answer: diploid (46) haploid (23 + 1) = 24 chromosomes Diploid (46)haploid (23 – 1) = 22 chromosomes

Question: 2. When fertilized, which gamete will produce a

trisomic condition and a monosomic condition? Explain

Answer: Trisomic Condition - Gamete with 24 chromosomes will have 3 of the same chromosomes.

Monosomic Condition – Gamete with 22 chromosomes will have only one chromosome

Homework:Grade 12 Text: p. 263 #1-4, 6-7 (Use the

genetic code found on p. 240 to answer #6 and 7)