This chapter looks at the molecular and cellular levels of life in order to address constraints on the 19th century version of natural selection.
Constraints on 19th century evolutionary theory
-Natural selection only works if there’s variation in populations.
*Darwin did not know the source of the variation or why it persisted across generations.
-Natural selection only works with heritable traits.
*Darwin did not know how traits were inherited too.
Cells are the basic units of life
3.5 billion years ago: the first cells emerged
1.7 bya eukaryotic cells (enclosed nucleus) emerged
All plants and animals today are made of eukaryotic cells.
Humans start as a single-celled organism and become an organism made of trillions of cells.
The nucleus contains the genetic information that codes for the heritable traits, ie, DNA
DNA structure and function are important to variation and inheritance
Genome: complete genetic makeup of an individual or species
Genes: units of inheritance making up the genome
-Genes code for hereditary traits
-Instructions for making proteins
-Determining biological traits
Human Genome Project mapped out all the genes for the human species.
Humans have 20-30,000 genes
Types of eukaryotic cells
-AKA body cells making up the structural components of an organism
-Eg, skin cells, hair cells, etc.
-All of your genes are housed in the nucleus of every somatic cell.
-AKA gametes are the specialized cells involved in reproduction.
-Eg, egg and sperm
-Only half of your genetic complement are found in the sex cells.
Cells have substructures called organelles and important ones anthropologists should know are:
Genes: segments of DNA controlling the order of amino acids
Amino acids are the building blocks of proteins. The DNA controls the creation of amino acid chains that we call proteins and the number and order of amino acids on a chain will code for different types of proteins.
DNA structure: stacked nucleotides form the strands of DNA. Nucleotides are composed of bases:
Adenine – Thymine
Cytosine – Guanine
Bases form complementary pairs with each other resulting in DNA’s double-helix structure (ladder analogy: sides of a ladder are the sugar-phosphate parts of nucleotides and bases pair to makeup up the rungs).
The pattern of bases on DNA can be copied by RNA three at a time (triplets) becoming codons that bind to specific amino acids
Protein synthesis: DNA and RNA copy and translate bases into proteins
Transcription: messenger RNA copies the pattern of DNA bases in the nucleus.
RNA is single-stranded and has a Uracil base instead of thymine
Translation: ribosome translates mRNA into codons and transfer RNA links an amino acid with the appropriate codon.
-amino acids are connected into a chain to become a protein and the number of amino acids and order determine what type of protein
Mutations: any change in DNA
-typos in the DNA message
-contribute to cancer, aging
-often have no effect because the code is redundant (multiple codons code for the same amino acid (DNA has four triplets – CGA, CGG, CGT, CGC – that code for one amino acid alanine).
-sometimes mutations have a significant effect – they result in new genetic material creating new variations
Types of mutations
C mutations can delete two bases changing the code: everything downstream shifts up
C Cystic fibrosis arises from this type of mutation
Chromosomes: DNA will coil tightly into temporary packages during cellular division
-chromosomes look like sausage links and carry genes
-animals will differ in the number of chromosomes they have
-humans have 46 chromosomes
Homologous chromosome pairs: chromosomes carrying genes coding for the same traits.
23 pairs of homologous chromosomes:
22 pairs of autosomes: the chromosomes carrying the bulk of the genes coding for most of the traits we inherit except…
1 pair of sex chromosomes: carry the genes that determine our sexual traits (autosomes do not) and other sex-linked genes.
Females pass on X chromosomes to their offspring
Males pass on X or Y chromosomes
Female sex determined by XX
Male sex determined by XY
Chromosomes are visible during cell division and we use karyotypes to study them
Normally, DNA is uncoiled which leads to another way to define a gene.