Biological basis of life and Mendel

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Issues with Darwin's Evolutionary Theory

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Key terms and definitionsAmino acids - molecules that are the basic building blocks of proteinsChromosome - structures made up of compacted DNA and protein visible during cell replicationCodon - (also referred to as a triplet) a set of three bases coding for a specific amino acidNucleotide - the basic unit of DNA composed of a sugar, phosphate, and baseGenome - complete genetic makeup of an individual or species

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Cells - fundamental units of life in all biological organisms

Eukaryotic cells - composed of proteins, lipids, nucleic acids, and carbohydrates

Proteins - molecules (structures with ≤ 2 atoms) serving different functions due to their binding to other molecules.

Protein synthesis - DNA-directed manufacturing chains of amino acids to function into protein molecules

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Cells - General CompositionOrganelles - substructures in the cell performing various functionsE.g., protein synthesis, energy storage, waste disposal, etc.

Nucleus - contains DNA and RNA

Ribosomes - organelles in part made up of RNA; involved in protein synthesis

Mitochondria - powerhouse of the cell - produces energy; think: cell's engines; also has DNA: mtDNA

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Cells - Two typesSomatic cells - body tissues e.g., organs, brain, bone, and muscle

Sex cells (gametes) - unites with another gamete from each parent which forms a zygote. They transmit genetic information from parents to offspring.

Zygote - Potential to develop into a new organism, contains all the chromosomes (46 for us)

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ProteinsFunction - attach to various molecules to perform different functions

Hemoglobin (Hb) - binds w/ oxygen and transports it around the body

Collagen - (tissue composition) - most common protein in the body

Enzymes - regulate chemical reactions; E.g., digestive enzyme lactase breaks down lactose into simpler sugars.

Hormones - affect different tissues and organs. E.g., Insulin made by specialized pancreas cells causes the absorption of glucose in liver cells

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DNADNA (deoxyribonucleic acid) - Double-stranded host of the genetic code

RNA (ribonucleic acid) - single-stranded molecule messenger (mRNA)transfer (tRNA)

DNA + RNA contain the genetic information controlling the cell's functions

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DNAStructure-nucleotides stacked on top of each other form the double-stranded twisted ladder-like structure

Nucleotides: composed of a sugar, a phosphate (sides), and a nitrogenous base (rungs)

Bases - form complementary bonds

Adenine bonds with Thymine and Guanine bonds with Cytosine

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Protein SynthesisDNA determines the structure and function of proteins

Proteins: made of chains of amino acids

Function is determined by the number and arrangement of amino acids making up the chain

Amino acids - building blocks of proteins

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Protein Synthesis - Two stages1. Transcription (in the nucleus)-complementary strand of mRNA produced from DNA strand-m(essenger)RNA has Uracil instead of Thymine

2. Translation (in the ribosomes)-ribosomes 'read' mRNA three bases at a time (codons)-t(ransfer)RNA binds with matching codons creating amino acid chain

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DNA ReplicationGrowth and healing/repairing tissue is possible because cells are able to multiply and make exact copies of themselves.

The new copies (daughter cells) have their own DNA, which is also replicated

Process of DNA replication1. Enzymes 'unzip' bonds between the bases2. Separated nucleotide chains act as templates for the formation of a new nucleotide strand3. Unattached nucleotides pair with complementary bases

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ChromosomesAfter DNA replication, DNA becomes tightly coiled to form chromosomes

ChromosomesStructure - single-stranded during normal cell functioning and double-stranded during cell division

Types of chromosomesAutosomes - govern all physical characteristics minus sex determinationSex chromosomes - X and YXX - mammal femalesXY - mammal males

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ChromosomesHumans have 46 chromosomes, whereas gorillas and chimpanzees have 48.

Karyotype - Chromosomes are visible during cell division

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Mitosis - cell division for somatic cells-occurs during growth and repair of tissue

Phase 1: DNA replication - 46 single-stranded chromosomes become 46 double-stranded chromosomes.

Phase 2: Cell division - 46 double-stranded chromosomes are split and make up the genetic material in two daughter cells.-Daughter cells are identical - 46 single-stranded

Note: clone - organisms that are genetically identical to each other

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Meiosis - specialized cells found in the ovaries and testes

-two cell divisions resulting in four daughter cells which each carry half the original number of chromosomes

Recombination-genetic material is exchanged between partner chromosomes-daughter cells are no longer identical to their parents or each other-variation is introduced into reproducing populations of organisms

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Cell Division - Complications in meiosis and Sex Chromosomes

Complications with meiosis

98% of newborns have correct numbers of chromosomes50% of pregnancies end in miscarriages. 70% of those miscarriages result from abnormal chromosome numbers.

-nondisjunction - homologous chromosomes [strands] fail to separate during meiosis - results: monosomy or trisomy E.g., Down syndrome (trisomy 21) occurs when chromosome 21 is copied three times in an individual, a phenomenon occurring 1/1000 births.

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Mitosis and Meiosis - types of cell division RecapMitosis-Somatic cells (i.e., body cells - skin, hair, muscle, etc.) are duplicated-ONE division produces cells with all 46 chromosomesResults: two daughter cells, genetically identical to parents and siblings

Meiosis-Gametes (sex cells like sperm and ova in humans)-TWO divisions produces cells with only 23 chromosomesResults: four daughter cells, not genetically identical

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Unlike mitosis, meiosis involves...-Gametes transmit genetic info from parent to offspring

Crossing over: chromosomes break and reconnect onto different chromosomes which results in...

Recombination: new combinations of genetic information is created-Every generation has new genetic combinations = additional variation

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MutationsAny change in DNA e.g., point mutations - (DNA bases-changes), changes in chromosome number or structure,...

Potential effects-stop the creation of a protein-result in the creation of a defective protein, ultimately corrupting the function of a cell.

-important evolutionary consequences if inherited

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NEW genetic information?Recombination generates new combinations of genetic information

But…

Mutations - changes in the nucleotide sequence of DNA-Only way NEW genetic variation is introduced

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Genes - structures and functions"a sequence of DNA bases that specifies the order of amino acids in an entire protein, a portion of a protein, or any functional product [like RNA]" (58p)

Structure

Composition: 100s-1000s of nucleotide basesOrganization: coding and noncoding segments

FunctionRegulatory genes - to switch on/off other DNA segments.E.g., Hox genes - direct body plan development and embryonic tissue segmentation. 22

Take home quizHow it works-Friday, June 30 at 5pm the quiz will be emailed and available on the course website

DUE DATE: Sunday, July 2 at midnight-students must email their answers (in the email body or as an attachment) before the date above

Format20 questions total - 15 multiple choice and 5 short answer (must answer all five)

ContentChapters 3 and 4

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Mitosis and Meiosis - types of cell division RecapMitosis-Somatic cells (i.e., body cells - skin, hair, muscle, etc.) are duplicated-ONE division produces cells with all 46 chromosomesResults: two daughter cells, genetically identical to parents and siblings

Meiosis-Gametes (sex cells like sperm and ova in humans)-TWO divisions produces cells with only 23 chromosomesResults: four daughter cells, not genetically identical

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Natural selection and geneticsNatural selection - Traits making reproductive success more likely given environmental pressures will appear in higher frequencies from generation to generation.

Sources of Variation*Mutations - new genetic information*Meiosis (recombination) - new combinations of genetic information

Result: new variation in every generation of sexually reproducing populations

Now we know how the variation NS needs to act on is created. (Thanks science!)

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Mendel - 1860s Monk experimenting with peasRecall: Cross-breeding - artificial selection, farmers...metaphor for natural selection

Missing in Darwin's theory of NS: a mechanism governing how traits were inherited

Background

-Mendel cross-bred pea plants-Observed the traits individual plants possessed over thousands of generations

Inferred the Principles of Inheritance

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Mendel - Cross-breeding pea plantsMendel inferred: regularity governed by a mechanism of inheritance; specifically,

*Inheritance is determined by a 'unit' (gene) offspring receive from their parents

*Individuals inherit one 'unit' from each parent for each trait

*Traits might not be expressed in an individual but can still be passed on to the next generation

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Mendel - overview-Cross-bred pea plants for thousands of generations-different trait expressions controlled by discrete units (genes)

alleles - the different expressions of a gene

The principles of inheritance

1. Segregation - for a trait, the pair of alleles from each parent separate and only one passes from parent to offspring.Meiosis - NOW we know this principle is Meiosis

2. Independent Assortment - different pairs of alleles are inherited by offspring independent from one another.

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Mendel - inferences from observationsNote: Parent plants = purebred = homozygous for pod color

I.e., each parent has identical expressions of the 'unit' (gene) for this trait

E.g., trait: seed color; expression: yellow or green. Y = yellow allele and G = green allele.

Parent 1 = YY (this is the allele pair)

Parent 2 = GG

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Mendel's InferencesGenotype - genetic makeup of an individual (e.g., YY, GG, YG, etc)Phenotype - physical expression of an individual's genotype (e.g., yellow, green, tall, short, smooth, wrinkled, etc.)

Mendel observed: Some allele expressions dominate others.

E.g., Pea pod genotype = YG resulted in phenotype yellow so the dominant expression/form/allele = yellow (green is recessive)

E.g., Trait: height; alleles: tall T, short t

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Dominance and recessivenessRecessive - traits that are not expressed in heterozygotes

Dominance - traits that are expressed in heterozygotes AND homozygotes-prevent the expression of recessive alleles in heterozygotes.

Alleles - the different expressions of a gene

Genes - segments of DNA -direct protein synthesis-found at different locus or loci of a chromosome

E.g., H = tall = dominant allele and h = short = recessive allele

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Mendelian traitsMendelian traits - discrete traits determined by alleles at a single genetic locus

Dominant: cleft chin, dwarfism,...

Recessive: Phenylketonuria (PKU), albinism, sickle-cell anemia

-Recessive disorders manifest if homozygous - if heterozygous, a person = unaffected but carrier

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Polygenic TraitsPolygenic - traits influenced by genes at 2 or more loci

E.g., skin, eye, and hair color

Continuous traits - gradation of difference in several expressions

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Mendelian vs PolygenicMendelian: discrete categories of variationPolygenic: continuous

Both -determined by Mendelian principles at specific loci-Dominance and recessiveness still a factor

NOTE: Mendelian traits = less likely affected by environmental factors

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Modern Evolutionary TheoryModern synthesis in the later 1920s-early 30s.

Evolution now defined in two stages

1. Variation - inherited differences among organisms is produced and redistributed through various processes

2. Natural selection acts on variation resulting in differential reproductive success (85p).

Current definition of Evolution: Change in allele frequency from one generation to the next.

Allele frequencies: indicators of a group/population's genetic composition -Described as proportions or percentages of a total 35

Modern Evolutionary TheoryFactors that produce and redistribute variation

1. Mutation - any change in DNA

-"the only way to produce new genes (that is, variation)" (86p).

2. Recombination - exchange of DNA segments b/w chrom. pairs during meiosis

3. Gene flow - Migration and Nonrandom mating - movement of alleles w/in and between popns

4. Genetic Drift - Founder effect and Bottlenecking - change due to change to due small population = reduced genetic diversity

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Modern Evolutionary TheoryFactors that produce and redistribute variation

1. Mutation - any change in DNA

-"the only way to produce new genes (that is, variation)" (86p).

2. Recombination - exchange of DNA segments b/w chrom. pairs during meiosis

3. Gene flow - Migration and Nonrandom mating - movement of alleles w/in and between popns

4. Genetic Drift - evolutionary change due to small population size-evolutionary change due to random factorsFounder effect - smaller population of founders leaves parent group to form a colony elsewhere-reduction of genetic variation

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Modern Evolutionary Theory5. Natural selection - acts on the variation produced and distributed by 1-4-Directs change in the allele frequencies of a population relative to environmental factors

Microevolution - small genetic changes that occur w/in a species

Macroevolution - large-scale changes that occur in populations over many generations-result in speciation

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Current theory of natural selectionNatural selection provides directional change in allele frequencies relative to specific environmental factors

If the environment changes, then selective pressures change too

If there's long-term directional change, then allele frequencies will shift gradually each generation

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Ch 5 Macroevolution

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Taxonomy and Species ConceptsBiological Species Concept (BSC) - isolated populations gradually change over time and become distinct taxonomic groups-Taxonomic grouping heavily influenced by genetic drift and natural selection

Kingdom: AnimaliaPhylum: Chordata

Class: Mammalia Order: Primates

Family: HominidaeGenus: Homo

Species: sapiens

We are Homo sapiens (also H. sapiens for short).41

Macroevolution - synonymous with speciation

Focus: large-scale evolutionary processes

Synthesize our understanding of modes of evolutionary change, geologic time, and taxonomic classification

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Macroevolution aka speciation

HomologiesHomologies - Structures shared by species due to common descent

E.g., Humans, birds, and bats: same basic bone structure but modified

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AnalogiesAnalogies - similar structures arise in other lineages in response to different functional demands

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Classification schemes: Systematics and Cladistics

Evolutionary Systematics: -use homologous traits to trace evolutionary relationships over time

-focus: identify common ancestry between groups

Cladistics:-uses homologous traits to separate organisms into taxonomic groups

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Similar: Both use homologies to trace evolutionary relationships

Differ: Systematics uses homologies to trace common ancestry over timevs.

Cladistics uses homologies to separate organisms into groups

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Classification schemes: Systematics and Cladistics

Cladistics more explicit and rigorous Ancestral traits - similarities shared by many distantly-related groups that are inherited from a remote ancestor

E.g., Grasping hand in humans-Mice, bears, and lizards all have lungs-Remember the similar bone structures between whales, bats, and humans?

Derived traits - reflect specific evolutionary lineages-modified traits from last common ancestor unique to a given group

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Adaptive radiation and ecological niche

Adaptive radiation - is the rapid expansion and diversification of new life forms into open ecological niches.

Speciation results in as many variations as allowed by

(1) its adaptive potential; and

(2) adaptive opportunities

E.g., reptilian egg evolution spawned an adaptive radiation event by opening new adaptive niches on land.

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