9: Genetics

Prokaryotes

• Single-celled organisms, no nucleus

• Bacteria, some types of algae

Eukaryotes

• Multicellular organisms

• Nucleus containing genetic material and specialized organelles

• Multiple cells function together in one organism

• Cell types:

  • Gametes (sex cells) - divide through meiosis

  • Somatic (other cells) - divide through mitosis

Plasma membrane

• Physical barrier surrounding cells to maintain homeostasis

• Regulates exchange with environment

• Cell communication

• Structural support

Cytoplasm

• Material enclosed by plasma membrane

Ribosomes

• Used to create amino acid chains during protein synthesis

Mitochondria

• Supply energy

• Have their own DNA

• Mitochondrial DNA is maternal

Nucleus

• Control center

• Contains genetic material (DNA)

• Most cells have one except:

  • Skeletal muscle (multiple); Red blood cells (none)

A chromosome is composed of a DNA molecule and associated proteins

Humans have 46 (23 pairs)

• 22 pairs: autosomes

• 1 pair: X and Y (sex chromosomes)

Diploid

• Contain 2 sets of chromosomes, 1 from each parent (paired chromosomes)

Haploid

• Sex cells are haploid (half of each chromosome pair, 50% of genetic material)

Each nucleus contains a set amount of chromosomes (depending on what kind of organism it is)

• Chromosomes contain DNA code

Amino acids

• Structural building blocks of proteins

Proteins make up body tissues and body structures

• Proteins are polymers of amino acids linked together by peptide bonds

• The precise structure of a protein is determined by genetics

DNA is a double-helix structure of nucleotides

• 2 strands

• Nucleotides consist of various molecules (we won’t go into too much detail in this class)

• Sugar, phosphate group, and a nitrogenous base

• Nitrogenous base - includes rings of carbon and nitrogen

Bases

• Two types: purines (adenine and guanine) and pyrimidines (cystosine and thymine)

Each strand: sugars, phosphates

• Bases in the middle

• The 2 strands bond together, with the 2 bases making up the “rung” of the ladder

• The combination of the bases that bond together is called a base pair

  • The base pair sequence is the genetic code

Proteins are made up of amino acids (basically building blocks)

Essential amino acids are amino acids that cannot be synthesized in the body and must be obtained from protein in the diet (i.e. consuming other animals and plants)

Each amino acid can be produced by several possible codons.  This redundancy is useful, in case errors in DNA replication occur and the wrong base pair is transcribed.  

Codon supposed to be GCU, but GCC is transcribed?

• Not a big deal since it still codes for alanine (A)

Codon supposed to be GCU, but GUU is transcribed?

• Wrong amino acid (glycine instead of alanine)!

Errors where DNA base pairs are removed, substituted, or inserted are called mutations.  

• Most mutations are neutral and do not positively or negatively affect survival.  

• Some mutations may be positive and improve survival.  

• Others may be negative and hinder survival.  

Mutations can be affected by natural selection and get passed down to offspring.

Gene

• Unit of heredity; part of DNA that contains genetic information for one protein 

Trait

• Character or expression which is carried by genes (there may be an eye color gene on a chromosome; having brown eyes is a trait)

Locus (plural: loci)

• Location of a gene on a chromosome

Allele

• Alternative form of a gene (ex: if there is a gene for pea plant height, alleles could be "tall height" and "short height")

Homozygous

• Same allele at the locus on both chromosomes

Heterozygous

• Different alleles at the locus on both chromosomes

Dominant

• Prevents the expression of recessive alleles in heterozygotes; 

• Does not mean that it is more common in the population.  Achondroplasia is inherited through a dominant allele, but it is not common.

Recessive

• Not expressed in heterozygotes; for a recessive allele to be expressed, there must be two copies of the allele

Monogenic genes

• One gene controls one trait

• Ex: widow’s peak, earlobe attachment, etc.

• Ex: some genetic disorders are affected by one gene (sickle cell anemia, Huntington’s disease, etc.)

Codominance

• Both affect the phenotype of a heterozygous genotype

• Neither is dominant over the other

• Ex: calico cats, blood types (A, B, O)

  • AB blood type - A and B are codominant

Polygenic genes

• Multiple genes control trait

• Ex: height, weight, body shape, eye color, skin color, and hair color

Pleiotropic genes

• A gene pair that influences the expression of a number of a different characteristics and traits

• Ex: Albinism, phenylketonuria, schizophrenia, sickle cell anemia

• Ex: phenylketonuria

  • PKU is determined by a single gene and can lead to cognitive impairments, reduced hair and skin pigmentation, and seizures