On Cells, DNA, Proteins, and Populations

Cell Theory
• A set of explanatory principles used to understand cells
• Cells are the smallest unit of life
• Consist of smaller (non-alive) parts: organelles

A Cell

Two Types of Cells
Somatic cells Sex Cells (gametes)
• The vast majority of cells in our bodies
• Only found in ovaries and testes
• Reproduce through meiosis
• Reproduce through mitosis

Two Types of Cell Division
• Occurs in somatic cells
• Produces 2 daughter cells
• Genetically identical
• diploid
• Occurs in sex cells (gametes)
• Produces 4 daughter cells
• Genetically unique
• haploid

• Asexual reproduction
• Occurs in somatic cells
• Produces 2 genetically identical diploid daughter cells

• Occurs in sex cells
(gametes)
• Produces 4 daughter cells
• Genetically unique
• haploid

Crossing-Over
6 wet
A
5 # of fingers 6 dry cerumen dry
O ABO group A
5 wet
O

Cell Division
Mitosis Meiosis

Karyotype
• Species-specific set of chromosomes
• Differs from species to species in terms of
• The number of chromosomes
• The sequences of genes contained in the chromosomes

Human karyotype
• 46 chromosomes
• Arranged in
23 pairs
• 1 set from each parent

Human karyotype
• Autosomes
• Pairs 1-22
• Are homologous
• same length
• same sequence of genes (may be different alleles)

Human karyotype
• Sex chromosomes
• Pair 23
• X & Y
X longer than Y
• XX – homologous
• XY – partially homologous

So what’s in a chromosome?

Let’s take a look!

A chromosome contains genes

and genes contain…
Deoxyribonucleic Acid
• Present in all living organisms
• Amount varies from organism to organism
• Species can read each others’ DNA

DNA
• Sugar-phosphate backbone”
• Bases are “rungs” adenine = thymine cytosine = guanine

Genome the total DNA/genes of a species
• Homo sapiens
• app. 3,000,000,000 DNA bases
• 35,000 – 40,000 genes
• Honeybee – 300,000,000 DNA bases
• Fruit fly – 13,600 genes
• Bacteria – a few hundred to a few thousand genes

DNA Replication
• Produces two identical strands from one original strand
• Each side of the original is a template for making a new copy of its complement

The Three Problems
1.
How is the variation present within a species at any time affected by the environment?
Darwin
2.
How is variation passed on from one generation to the next?
Mendel
3.
Where does variation come from?
Watson & Crick

But what are genes used for?

Protein Synthesis
• A two stage process
• Transcription
• Translation
• Our players:
• Messenger RNA (mRNA) – the locks
• Transfer RNA (tRNA) – the keys
• Ribosome (“locksmith)
• Amino Acids

Protein Synthesis
1: Transcription
• messenger RNA (mRNA) copy of gene is made
• mRNA copy leaves nucleus and goes to cytoplasm

Protein Synthesis
2: Translation
• mRNA copy is “read” by ribosomes
• Ribosomes match tRNA to codons on mRNA

Proteins: the End Result
• One gene codes for one protein
• Differences between individuals due (in part) to differences in their proteins

• A two stage process
1) transcription
2) translation
• The process whereby the DNA message is converted into a protein product

http://en.wikipedia.org/wiki/ http://www.johnkyrk.com/ http://www.youtube.com/
• DNA replication
• Protein synthesis
• DNA translation
• Transcription

and now let’s turn to…

Population
• A (potentially) interbreeding group of organisms
• (usually) a subset of a species

“Populations” Differ

How Should We Describe Populations?
Monogenic Traits Polygenic Traits
• Qualitative • Quantitative
• Discontinuous graph • Continuous graph

Use Polygenic Traits?
• Advantage: often easily observable
• Disadvantage: can change over an individual’s life time
• Poorly suited for population studies

Racism
• A social/cultural phenomenon
• Attempts to distinguish between human populations on the basis of polygenic traits
• Used to justify hierarchies within society
• Is not based on scientifically valid criteria

Use Monogenic Traits!
• Disadvantage: not always easily observable
• Advantage: do not change over an individual’s life time
• Useful in the study of populations
• Genetic Markers = monogenic traits in which the genotype is known

Genetic polymorphism
When two or more phenotypes exist within a population that means that the population exhibits variation for that trait and variation is what makes evolution possible

Adaptation a trait that increases an organism’s reproductive success
What adaptation is being measured here?
How might this adaptation increase an individual’s reproductive success?

(a classic example)
The wing color of the Peppered Moth is a monogenic trait

Adaptation produced by natural selection in the context of a particular environment
A light variant mating with a dark variant
Light and dark variants on light and dark backgrounds

Adaptation produced by natural selection in the context of a particular environment
Smokestacks during the
Industrial Revolution

Evolution defined drum roll please…

This means that…

Population Genetics
• Studies how evolution occurs in populations of organisms
• Population – a (geographically localized) group of individuals in a species that share a common gene pool and tend to find their mates within this group

What are the factors that could influence the evolution of our population?
• Founder’s effect
• Natural selection
• Mutation
• Migration
•
• Assortative mating
• Positive
• negative
Genetic drift

Founder’s effect
• How well would we represent the population from which we are derived?
• SoCal!

Natural Selection
• Do blondes have more fun (and more early onset skin cancer)?
• Hereditary diseases, anyone?
• Other possibilities?
• Is it better to be redder?

Mutation
• Random mutations
• Heightened mutation rates?
• Cosmic rays?
Uranium ore

Migration
• Do some people leave?
• Do others arrive?
• Did some “visit”?
• Gene flow vs. migration
Hypothesized migration pattern of the
Pacific Islands

Assortative Mating
Positive Assortative Mating
• phenotypically similar organisms
• increases homozygosity
• decreases variation
Negative Assortative Mating
• phenotypically dissimilar organisms
• increases heterozygosity
• increases variation

Genetic Drift the random factor, or “joker in the deck”

A genetic bottleneck reduces a population temporarily to very low levels, removing much of its genetic diversity.

Mutiny on the Bounty by Robert Dodd (artist & engraver); 2 Oct., 1790

In 1790, 9 Bounty mutineers plus 18 Tahitians (six men, 11 women and a baby) fled to Pitcairn Island

The Pitcairn
Islanders today app. 60 people
(the smallest democracy in the world!)