diversity, classification topic 4 Flashcards
What is a gamete?
A sex cell (e.g. sperm or egg) with a haploid nucleus — contains half the number of chromosomes.
What happens when two gametes fuse?
They form a diploid zygote.
What are homologous chromosomes?
Pairs of chromosomes that are the same size, have the same genes at the same loci, but may have different alleles.
What is the purpose of meiosis?
To produce four genetically different haploid cells from one diploid cell — e.g. gametes.
Outline the key stages of meiosis.
DNA replicates → sister chromatids form
DNA condenses into chromosomes
Meiosis I: Homologous chromosomes pair up and are separated
Crossing over occurs
Independent segregation happens
Meiosis II: Sister chromatids are separated
4 genetically unique haploid cells result
What is crossing over?
During meiosis I, chromatids twist around each other and exchange sections — creates new allele combinations.
What is independent segregation?
Random arrangement and separation of homologous chromosomes into gametes — leads to variation in maternal/paternal chromosomes in each cell.
What is a chromosome mutation?
A change in the number or structure of chromosomes, usually during meiosis.
What is non-disjunction?
Failure of chromosomes to separate properly — results in gametes with an abnormal number of chromosomes.
What disorder is caused by non-disjunction of chromosome 21?
Down’s syndrome — individuals have 3 copies of chromosome 21.
What is a gene mutation?
A change in the base sequence of DNA.
What are types of gene mutation?
Substitution
Deletion
Insertion
Duplication
Inversion
Translocation
What is a substitution mutation?
One base is swapped for another. May be silent due to the degenerate nature of the genetic code.
What is a deletion mutation?
A base is removed — usually causes a frameshift, altering every codon downstream.
What is a frame-shift mutation?
A mutation (e.g. deletion or insertion) that changes the triplet grouping, drastically altering the protein.
What is a translocation mutation?
A section of DNA is moved to a different location in the genome — within the same or another chromosome.
What is a nonsense mutation?
A base change that creates a stop codon, prematurely ending translation.
What is a missense mutation?
A base change that results in a different amino acid being coded for.
What is a silent mutation?
A base change that does not alter the amino acid due to the code’s degeneracy.
What are mutagenic agents?
External factors that increase mutation rate.
Examples of mutagenic agents?
UV radiation
Ionising radiation
Some chemicals
Some viruses
How do mutagenic agents cause mutations?
Base analogs: substitute for actual bases
Chemical modifiers: alter or delete bases
Radiation: can cause cross-links (e.g. thymine dimers)
