Chromosomes, Genes, Alleles and Mutations

What is a chromosome?

What is a gene? A section of DNA that codes for a polypeptide

Allele A version of a gene. Trait = tongue rolling Allele= roller or non roller

genome All your genes!

Humans have 23 pairs of homologous chromosomes

Mutations

Mutations Changes to DNA are called mutations TACGCACATTTACGTACG change the DNA changes the mRNA may change protein may change trait DNA TACGCACATTTACGTACG mRNA AUGCGUGUAAAUGCAUGC aa protein trait

What Causes Mutations? Mistakes when DNA is copied Inherited mutations (only occur when the mutation was in the gametes (sperm or egg) Environmental factors: Mutagens Chemical: Pesticides, chemicals in cigarette smoke Physical: x-rays, uv rays from the sun

Are Mutations Helpful or Harmful? Mutations happen regularly Almost all mutations are neutral Many mutations are repaired by enzymes

Are Mutations Helpful or Harmful? Some type of skin cancers and leukemia result from somatic mutations Some mutations may improve an organism’s survival (beneficial)

Types of Mutations

Gene Mutation Animation

Gene Mutations Change in the nucleotide sequence of a gene May only involve a single nucleotide May be due to copying errors, chemicals, viruses, etc.

Types of Gene Mutations Point Mutations Substitutions Insertions Deletions Cause a frame shift

Point Mutation Change of a single nucleotide Includes the deletion, insertion, or substitution of ONE nucleotide in a gene

Point Mutation Sickle Cell disease is the result of one nucleotide substitution Occurs in the hemoglobin gene

Frameshift Mutation Inserting or deleting one or more nucleotides Changes the “reading frame” like changing a sentence Proteins built incorrectly

Frameshift Mutation Original: The fat cat ate the wee rat. Frame Shift (“a” added): The fat aca tat eth ewe era t.

Amino Acid Sequence Changed

Gene Mutation Animation

HHMI sickle cell video

Describe the relationship between the following terms… Gene, Allele, Chromosome, Genome and DNA

Explain how sickle cell anemia came about as a result of a mutation and relate that to the process of transcription and translation. Explain why the sickle cell mutation is so common in populations of African decent.

Meiosis Overview Major Understandings • One diploid nucleus divides by meiosis to produce four haploid nuclei. • The halving of the chromosome number allows a sexual life cycle with fusion of gametes. • DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids. • The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation. • Orientation of pairs of homologous chromosomes prior to separation is random. • Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number. • Crossing over and random orientation promotes genetic variation. • Fusion of gametes from different parents promotes genetic variation

Chromosome Mutations May Involve: Changing the structure of a chromosome The loss or gain of part of a chromosome

Chromosome Mutations Five types exist: Deletion Inversion Translocation Nondisjunction Duplication

Chromosome Mutation Animation

Deletion Due to breakage A piece of a chromosome is lost

Inversion Chromosome segment breaks off Segment flips around backwards Segment reattaches

Duplication Occurs when a gene sequence is repeated

Translocation Involves two chromosomes that aren’t homologous Part of one chromosome is transferred to another chromosomes

Translocation

Nondisjunction Failure of chromosomes to separate during meiosis Causes gamete to have too many or too few chromosomes Disorders: Down Syndrome – three 21st chromosomes Turner Syndrome – single X chromosome Klinefelter’s Syndrome – XXY chromosomes

Chromosome Mutation Animation

KARYOTYPE ANALYSIS

Chromosome Mutations Down Syndrome Chromosome 21 does not separate correctly. They have 47 chromosomes in stead of 46. Children with Down Syndrome develop slower, may have heart and stomach illnesses and vary greatly in their degree of inteligence.

Chromosome Mutations Cri-du-chat Deletion of material on 5th chromosome Characterized by the cat-like cry made by cri-du-chat babies Varied levels of metal handicaps

Sex Chromosome Abnormalities Klinefelter’s Syndrome XXY, XXYY, XXXY Male Sterility Small testicles Breast enlargement

Sex Chromosome Abnormalities XYY Syndrome Normal male traits Often tall and thin Associated with antisocial and behavioral problems*

Sex Chromosome Mutations Turner’s Syndrome X Female sex organs don't mature at adolescence sterility short stature

Sex Chromosome Mutations XXX Trisomy X Female Little or no visible differences tall stature learning disabilities limited fertility

Normal Male 2n = 46

Normal Female 2n = 46

Male, Trisomy 21 (Down’s) 2n = 47