LIFESCI 7A Lecture Notes - Lecture 7: Chromatin Remodeling, Histone Methylation, Histone Code
15.1 Genotype and Phenotype
● Mutations result from genetic variation; can be harmful, neutral, or beneficial
○ When favorable, allows organisms to evolve and become adapted to environment over time
● Genotype is the genetic makeup of a cell or organism; phenotype is its observed characteristics.
○ genotype The genetic makeup of a cell or organism; the particular combination of alleles
present in an individual.
■ Variation of genotypes of a population depend on its gene pool
○ polymorphism Any genetic difference among individuals sufficiently common that it is likely to
be present in a group of 50 randomly chosen individuals.
■ Result of mutation; at one point every individual had same genotype
■ Ex: many have C-G where others have A-T
○ phenotype The expression of a physical, behavioral, or biochemical trait; an individual’s
observable phenotypes include height, weight, eye color, and so forth.
■ Result of genotype and environment
● The effect of a genotype often depends on several factors.
○ alleles The different forms of a gene, corresponding to different DNA sequences
(polymorphisms) in each different form.
■ homozygous Describes an individual who inherits an allele of the same type from each
parent, or a genotype in which both alleles for a given gene are of the same type.
■ heterozygous Describes an individual who inherits different types of alleles from the
parents, or genotypes in which the two alleles for a given gene are different.
○ Harmfulness or benefits of sickle cell anemia S allele show us
■ It depends on homozygous/hteterozygous inheritance
■ Effect of a genome depends on the environment
19.1: Chromatin to Messenger RNA in Eukaryotes
● All cells of all types of an individual carry same genome; different types=result of different sets of genes
being expressed
● Gene expression can be influenced by chemical modification of DNA or histones.
○ chromatin remodeling The process in which the nucleosomes are repositioned to expose
different stretches of DNA to the nuclear environment.
■ When DNA is coiled up in chromatin, it is unacessible for transcription; must be loosened
up to have enough space for transcription to happen
■ Can occur on histone tails
● histone tail A string of amino acids that protrudes from a histone protein in the
nucleosome.
○ Can add or take away methyl or acetyl groups to amino acids,
activating/repressin transcription
○ histone code The pattern of modifications of the histone tails that affects
the chromatin structure and gene transcription.
■ Happens at key times in development to signal which genes are expressed as well as in
response to environment
● Chromatin remodeling complexes and histone modification enzymes regulate eukaryotic transcription.
○ Histone acetylation by HAT enzyme causes chromatin to be less dense, allowing genes to be
more accessible
■ Some transcription factors even bind to histone acetyl tail (further activation
transcription)
■ Deacetylation by HDAC removed acetylgroup and causes chromatin to return to
condensed state
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Document Summary
Mutations result from genetic variation; can be harmful, neutral, or beneficial. When favorable, allows organisms to evolve and become adapted to environment over time. Genotype is the genetic makeup of a cell or organism; phenotype is its observed characteristics. Genotype the genetic makeup of a cell or organism; the particular combination of alleles present in an individual. Variation of genotypes of a population depend on its gene pool. Polymorphism any genetic difference among individuals sufficiently common that it is likely to be present in a group of 50 randomly chosen individuals. Result of mutation; at one point every individual had same genotype. Ex: many have c-g where others have a-t. Phenotype the expression of a physical, behavioral, or biochemical trait; an individual"s observable phenotypes include height, weight, eye color, and so forth. The effect of a genotype often depends on several factors. Alleles the different forms of a gene, corresponding to different dna sequences (polymorphisms) in each different form.
