Errors in Genes and Chromosomes

1. Errors in Genes and Chromosomes

2. 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

3. 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.

4. Causes of Mutations Mutagenic Agents • These are agents that cause mutations. These include: • Radiation (UV, X-rays) • Temperature extremes • Exposure to chemicals (pesticides

5. 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

6. Point or Gene Mutations: • Occur when DNA is transcribed into RNA • There are two types:

7. ‘The child walked down the street’ • What do you notice about…. • “The child wapked down the street”

8. 1. Substitution: • One nucleotide is being substituted or replaced with another. • End result is a different nucleotide sequence than the original DNA sequence

9. ‘The child walked down the street’ • What do you notice about…. • “The child walkxe ddow nth estreet” X

10. 2. Frame-shift Mutation • During transcription nucleotide base pairs may be inserted or deleted from the DNA sequence

11. 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

12. 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)

13. Ex. Sickle Cell Anemia

14. 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

15. 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:

16. ‘The child walked down the street’ • What do you notice about…. • ‘The child down walked the street’

17. 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.

18. ‘The child walked down the street’ • What do you notice about…. • ‘walked down the street’ • ‘It was a sunny day. The child’

19. 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)

20. ‘The child walked down the street’ • What do you notice about…. • ‘The down the street’

21. 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)

22. ‘The child walked down the street’ • What do you notice about…. • ‘The child child child walked down the street’

23. 4. Duplication: • Within a chromosome, repeated segments of DNA are seen.

24. 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

25. Nondisjunction in autosomal chromosomes p.173

26. 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

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

28. Nondisjunction in sex chromosomes

29. 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

30. 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

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

32. Ex. Seedless Watermelon • Breeders cross a diploid male with a tetraploid female (4n) • Result, • Sterile offspring (no seeds)

33. MUTATION REPAIR MECHANISMS 1) 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

34. 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.

35. 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.

36. Recap Errors or Mutations Gene/point mutation Chromosomal mutation 4 types: Inversion Translocation Deletion Duplication Nondisjunction Autosomal chromosomes Sex chromosomes Polyploidy 2 Types: Substitution Frame-shift mutation

37. 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

38. 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

39. Homework: Grade 12 Text: p. 263 #1-4, 6-7 (Use the genetic code found on p. 240 to answer #6 and 7)