Dividing to multiply: Mitosis and Meiosis
Why it’s important...
What do you think?
Ensures everyone gets a complete copy of genetic material (and only 1 copy) Allows for growth Allows for diversity Basis for evolution (recombination) Determines genetic diseases
A gene is ~1,000-100,000 basepairs A chromosome is tens or hundreds of thousands of genes A genome is 1-100s of chromosomes A genotype refers to the alleles present in a given genome A human genome is ~3,000,000,000 basepairs A human genome is (currently estimated at) ~20-30,000 genes A human genome is ~1 meter of DNA
Some vocab to start
Gene – A stretch of DNA representing all the information for a product as well as when and where to make the product Allele – A version of a gene; two alleles of the same gene may differ by a nucleotide or dozens of them— generally a small number Dominant/recessive – Determines phenotype of individuals (expression of alleles)
Dominant does NOT necessarily mean most common
Eye color – brown dominant, blue recessive
I have brown eyes, what alleles do I have?
Poison ivy – Immunity dominant, susceptibility recessive Hemoglobin – Sickle cell recessive Digits – extra dominant, normal number recessive
Touching mitosis & meiosis
Every row gets a set of ‘chromosomes’ Grab a piece of chalk and make yourself a nucleus
Meet the Chromosomes
Compare our plastic models with image on the left What do our ‘beads’ represent?
What cells ‘do’ mitosis? What is the goal/purpose of mitosis? What is our first step? Now what?
How do we pick the appropriate half?
How do your final results compare with your original cell?
Meiosis: the other cell division
What is the benefit of sexual reproduction?
Suppose you have a red flower and a white flower, how long would it take to make a pink flower if they reproduced:
Sexual reproduction allows for variation
What cells ‘do’ meiosis? What is the goal/purpose of meiosis? What is our first step? Now what?
Why bother ‘breaking’ DNA?
How much are you ‘like’ your mom & dad (on average)? How much of your genome should you give your child if he/she is has 2 parents? Just shuffling chromosomes leads to all genes on every chromosome to be inherited together Recombination allows two sets of genes on a chromosome to be inherited independently of each other
Homologous Recombination I
Where should the circled site on Chromosome1 recombine with Chromosome2?
Recombination: crossing over
How should we line our chromosomes up in the cell to allow for recombination? Recombine!
Spreading your genes
Now that we’ve recombined, how do we get to a gamete? How do your gametes compare to your original cell? Once you’ve got your gamete, go fuse with a classmate Stop by and show me the genotype
Genetic diversity provides the foundation upon which natural selection works Understanding chromosome replication has important implications in medicine
Cancer – uncontrolled growth of cells Down syndrome – extra chromosome (21)
Check it out...
Mitosis – Observation of onion root tip
Procedure on page 5-9 in lab manual
Meiosis – Prepared grasshopper testes on slides
Linked genes – 2 genes that ‘travel’ together by virtue of being ‘close’ together on a chromosome (eg: eye color & wing length in fruit flies)
Linkage in action... The closer two nucleotides are the (less/more) likely a recombination event takes place between them The closer two GENES are on a chromosome, the (less/more) likely they will look like the parent chromosome
Homework – Gameter
Clearly explains relationship between location on chromosome (relative to another gene) and likelihood of alleles being inherited together (linkage). Discusses results in terms of two genes (A & B) and their alleles (A, a, B, b). Relevance to gamete production and variability in offspring discussed. Examples provided to show relationships. Points off if: Relationship between location and linkage not clearly explained, gamete production and variability in offspring not discussed, multiple examples not included. Words should be spelled correctly and appropriate grammar should be used.