CELLULAR BIOLOGY MENDELIAN GENETICS Cellular Biology THE CELL

CELLULAR BIOLOGY & MENDELIAN GENETICS

Cellular Biology

THE CELL n n n Smallest functional unit of living organisms Each cell maintains its own homeostasis Each cell contributes to the homeostasis of the tissue, organ and/or organism

THE CELL n All living organisms consist of cells n Humans have trillions of cells (Yeast one cell) n n Cells are of many different types (blood, skin, nerve), but all arose from a single cell (the fertilized egg) Each cell contains a complete copy of the genome (the program for making the organism), encoded in DNA

SOME DEFINITIONS Chromosomes = made up of a protein core and strands of DNA in the nucleus of a cell (46 chromosomes make up 1 human cell) DNA (deoxyribonucleic acid) = Molecule that carries the genetic code, ladder with rungs made of base pairs (“letters”: A, C, T, G) Codon = Sequence of 3 bases. Each 3 -letter word stands for an amino acid. (“word”: “GCA” = Alanine) Genes = Portions of the DNA molecule that code for specific proteins (“sentence”) Protein = chain of amino acids (see p. 54 for protein synthesis) Replication = Copying of genetic code during cell division

The Question of Inheritance Darwin was very concerned with the question: How are traits inherited? • Variation within populations was the raw material that powered the evolutionary process • Could not explain, however, the source of this variation or how it was distributed across generations • Towards the end of his life, Darwin admitted that the greatest challenge to his theory was its inability to explain the mechanism of inheritance

Theories of Inheritance During Darwin’s Time Blending Inheritance • Belief of how inheritance operated: offspring received a combination of all characteristics of each parent through the mixture of their “bloods” • If true, unique traits would be diluted, and… • Individuals would become more uniform If not, how are individual adaptive traits maintained and selected for within populations? Czech monk, Gregor Mendel, working in obscurity, came up with the answer: particulate inheritance

Gregor Johann Mendel (1822 - 1884) • 1865: Mendel published the results of his experiments with garden peas in which he crossed parents and examined offspring, became the “father” of genetics. • Concluded that each parent contributes “particles” or genetic units to their offspring • The particulate nature of inheritance and the laws of inheritance = final pieces in the puzzle that fulfilled Darwin’s vision of evolution by natural selection • Mendel’s contribution DID NOT become known until early in the 20 th century

Some Definitions Allele = Variants of a gene. Every gene has 2 alleles with different instructions for developing a certain phenotype Monogenic = Trait coded for by a single gene (ex. Taster trait for “PTC”) Polygenic = Trait coded for by more than one gene (ex. Skin color)

Characteristics of Good Model Organisms Mendel’s work with garden peas was successful because they had: • • • a well-known genetic history short generation time large number of progeny small size and easy to handle wide availability of mutants - genetic variation within garden peas

Mendel had true-breeding varieties of peas for 7 different traits including:

One of Mendel’s Monohybrid Crosses Gametes =Cells of sexual reproduction (sperm & egg), contain only ½ the chromosomes of normal cell Genotype: alleles of an organism 1/4 SS : 1/2 Ss : 1/4 ss = 1 SS : 2 Ss : 1 ss Phenotype: physical expression of genetic code 3/4 Smooth seeds : 1/4 wrinkled seeds

Mendel’s Model • Genes do not blend together • Pea genes have two versions or alleles • Each gamete gets one allele • Males & females contribute equally • Some alleles are dominant, while others are recessive

What Accounts for Species Variation? • Independent Assortment: Alleles segregate into gametes independently (Mendel’s Law) • Crossing Over: Sections of chromosomes switch between chromosome pairs during meiosis • Recombination: Exchange of genetic material between pairs of chromosomes during meiosis