Lecture 9-Origin of Variation
• Circular chromosomes exist in plant chloroplast and mitochondria
• Mitosis “checkpoint”= metaphase
• Cells in mitosis have 4 x C
Notes from Lectures:
• Variants in genomes is what makes people differ from one another
• Ventor individual genome sequence showed 1.2 million variants
• 25% of variant bases are SNPs (single nucleotide polymorphism); 75% are CNV (Copy
Number Variations), inversions, etc.
• Each person has about 1000 CNVs affecting 35% of genes
• Each person has about 300 variants in insertion of retro elements
• According to SNP data, people ofAfrican origin are much more different from each other than
any other race.
• SNPs arise from replication errors.
• This means thatAfrican origins are the oldest, and have the most diversity, which leads to the
theory that all humans evolved fromAfrican decent.
Mobile elements; the “junk” in the human genome
• Mobile elements are different in hundreds of locations in every person
• Bacteria elements code for their own mobility (transposase)
• Simplest known piece of DNAthat can move on it's own in known as an insertion sequence, or
an IS element
• Structure of IS element has inverted repeats on both ends, whereas the central sequence codes
for a gene known as transposase.
• Transposase makes “cuts” in the backbone of a target sequence, which initials the element being
able to move.
• If two IS elements are nearby, they can move everything between them.
• One of the reasons why bacteria are becoming resistant to antibiotics, is because of the genes
carried by transposons.
• Some elements move with, some without, making a copy
• The mobile element is called a “jumping gene”
• These mobile elements can move in a replicative way.
• IS elements are never free floating; they are always moving around, cutting and pasting from
gene to gene
• IS elements are considered “biological mutagens”, since they are naturally found in the genome,
and cause mutations. They are completely different from chemical mutagens (outside chemical sources which cause mutations in a sequence).
• Retrotransposons move via RNA; their mechanisms allows them to be “wandering” along the
How retrotransposons move;
1. Retrotransposon possibly including genes is transcribed by RNApolymerase, which produces
2. Retrotransposon encoded reverse transcriptase makes DNAcopy.
3. Transpositions occurs, whereas this DNAcopy is embedded in a different part of the genome
• Viruses that enter the genome are known as proviruses; i.e. HIV
• They have three kinds of genes.
• Most retroviruses are imbedded in our genomes, although they cannot get out
• These are known as endogenous retroviruses
• This increases the “junk” in our genome
• One of the most common retro elements in the human genome is Alu-elemennt
• Alu element insertion is an OPA1 intron sequence which comes with autosomal disorders.
• Mobile elements are attracted by plants
• We can determine how long an element has been in our system by the activity of that element,
and it's effects.
• Replication errors can be repaired
• Proofreading by DNApolymerase
• Sometimes, mismatches escape proofreading
• Motor proteins pull chromosomes across the spindle, rather than the spindle itself pulling the
• Micro filaments are structures which are responsible