Chapter 3 - Heredity, Cell Biology, and Evolution

cells, how they arose, and how similar their composition is from one another

• make up all living things

• arose from pre-existing cells through cell division

• contain DNA, which is passed on to new cells during cell division

• chemical composition of all cells is very similar

• metabolic processes associated with life occur within cells

components of eukaryotic cells

• cell membrane - holds the goop inside

• organelles - little “organs” which carry out the cell’s vital function

• cytosol - the goop inside the cell that the organelles float in

• nucleus - ball in the cell that holds all the DNA

types of cells

• nerve cells

• muscle cells

• bone cells

• gland cells

• blood cells

• reproductive cells

deoxyribonucleic acid (DNA)

• double helix molecule composed of two chains that form a ladder

• links in the chain are nucleotides

• 4 bases (A,T,C,G) - used to write the genetic code

3 components of nucleotides

• deoxyribose (sugar molecule)

• phosphate (salt molecule)

• nitrogenous base

codons

3 letter words made up of ATCG

• each codon represents an amino acid

how are amino acids formed together

connected together in chains to form proteins, which are the building blocks of body tissues

DNA replication

• cells need to divide and DNA needs to replicate itself in order to grow and reproduce

• Nucleotides bond in a specific way

  • A-T, C-G, two strands complimentary - makes sure there are no copying mistakes

• to replicate, the two strands split and the complimentary nucleotides are attached onto both strands

  • results in two exact copies of the chromosome

why does DNA replication happen

• growth and repair - mitosis

  • every cell needs a complete genetic code, so DNA has to replicate every time a cell divides

• reproduction - meiosis

  • organisms pass their DNA to their offspring

2 types of cell division

• mitosis - creates two identical copies of cells with same chromosome set as parent

• meiosis - creates two different cells with half chromosome set as parents

chromosome

• DNA molecule that coils up tightly into a sausage-shaped structure

• come in pairs (one from mom, one from dad)

• humans have 46 chromosomes (23 pairs)

mitosis steps

IPMAT

Interphase

Prophase

Metaphase

Anaphase

Telophase & Cytokinesis

Meiosis steps

Interphase

Prophase 1

Metaphase 1

Anaphase 1

Telophase 1

PMAT x 2

Cytokinesis

Crossing over

during prophase 1 homologous chromosomes (in their doubled state) swap info with each other

• reduces genetic linkage

  • ensures every daughter cell is unique

differences in mitosis and meiois

Mitosis

• one division

• 2 identical daughter cells

• 46 chromosomes (diploid number)

• produces somatic cells

Meiosis

• two divisions

• 4 unique daughter cells

• 23 chromosomes (haploid number)

• produces sex cells

Gregor Mendel and Mendelian Genetics

• crossed different strains of purebred plants and studied their progeny

• his work highlights the basic rules of inheritance

Gregor Mendel and his work with peas

• noticed that the plants could express many discrete traits, with no blending

• conducted hybrid experiments and studied the inheritance of these traits

  • when Mendel bred:

  • P gen - tall x short plant = all tall

  • F1 gen - self fertilized those tall offspring = ¾ tall and ¼ short

Mendels work on peas and how it relates to genetics

• the traits Mendel was studying was controlled by genes (sentences of codons)

• genes occur in pairs (one from mom, one from dad, and found on homologous chromosomes

genotype

genetic makeup of an individual

• exact genes in your chromosomes

phenotype

physical makeup of an individual

• result of interaction between allele’s to produce the organism’s physical appearance

alleles

variants of genes

gene locus

location for a specific gene on a chromosome

how do alleles interact?

traits are controlled by genes with two different alleles

one dominant, one recessive

dominant allele

• overshadows the recessive allele, so its dominant trait is expressed

recessive allele

overshadowed in the presence of a dominant allele

homozygous

when an organism has two copies of the same allele

• the allele is always expressed because it’s the only one there

heterozygous

two different alleles are present

• dominant trait is expressed, the recessive one is not

• heterozygous phenotype is identical to the homozygous dominant phenotype

Punnet Square

• used to predict the possible genotype and phenotype of offspring if both parents’ genotypes are known

principle of segregation

• when an individual reproduces, only half of allele pairs are passed to each offspring

  • the alleles were segregated

  • one allele was passed on at random

Principle of Independent Assortment

• how one allele pair is distributed does not influence how another allele pair will be distributed

• genes controlling different traits aer inherited independently of one another

• exceptions:

  • genes on the same chromosome

    • sex linked traits

    • but recombination solves this

codominance

both alleles are dominant and both expressed at the same time

• ex. spotted cows

incomplete dominance

neither allele is dominant and the phenotype is a blend of the two

• ex. red flower x white flower = pink flower

Mendelian Traits

traits are controlled by one gene (with two alleles) at a single locus

Polygenic traits

more than one gene contributes to a trait

• makes up most traits of humans and animals

• is the opposite of pleiotropy

pleiotropy

one gene contributes to several different traits

• opposite of polygenic traits

ABO blood types

blood type is controlled by 3 alleles (ABO)

• not a polygenic trait

  • even though there’s 3 possible alleles (ABO), you can only inherit 2 out of those 3, one from each parents

• A and B are codominant

• O is recessive

• O is only expressed in homozygotes

• AB heterozygotes - both A ad B antigens are expressed

what antigen/antibodies is each bloodtype made up of/produce

Type A - A antigens, Anti-B antibodies

Type B - B Antigens, Anti-A antibodies

Type AB - AB antigens, neither Anti-A and Anti-B antibodies

Type O - neither A or B antigens, both Anti-A and Anti-B antibodies

polygenic traits and humans

• very few Mendelian traits are actually physically observable in humans

  • most of them are polygenic

  • ex. height, skin colour, eye colour

  • expression of these traits is continuous (spectrum), not discrete as opposed to Mendel’s peas

  • more complicated to study

polygenic traits and skin colour

• pigment (melanin) - determines skin colour and is produced by specialized cells called melanocytes

• two types of melanin (brown and red)

  • amount of each type produced determines how dark or light skin will be

• also many different genes that controlling melanin production, adding an effect

  • more genes that code for melanin production = darker skin

what is the most popular blood type

type O

epigenetics

study of non-genetic traits that are inherited like in Lamarck’s principles

• epigenetic changes caused by different chemical environments that parents went through can pass onto offspring

do genes control all of an individual’s phenotypic expression

no, genes do not control 100% of an individual’s phenotypic expression

• culture and where you grew up can influence phenotypes slightly

• ex. height is a polygenic trait

mitochondria

membrane bound organelle that converts energy into a form usable by the cell

• each contains copies of a ring-shaped chromosome that is unique from the cell’s DNA (mtDNA)

mtDNA

mitochondrial DNA

• animals (male or female) inherit their mtDNA and all mitochondrial traits from their mothers

• mutations cause all variation in mtDNA

  • makes it useful for studying genetic change over time

Darwin and Mendel

• lived and worked at the same time, but never met or read each other’s work

  • Darwin knew natural selection acted on variation in populations, but didn’t know how that variation was inherited

  • Mendel knew how variation was inherited, but didn’t apply it to evolutionary theory

• not until the 1930s that their work was combined into the Modern Synthesis of evolutionary theory

The Modern Synthesis (of evolutionary theory)

Evolution is a two-stage process

1. production and redistribution of variation (Mendel)

2. selective forces acting on this variation affect their ability to successfully reproduced (Darwin)

Current definition of evolution

“a change in allele frequency from one generation to the next”

• allelic frequencies - indicators of the genetic makeup of a population, members which share a common gene pool

allele frequencies

percentage of all the alleles at a locus accounted for by one specific allele

4 forces of evolution

• mutation

• gene flow

• genetic drift

^ random ways allelic frequencies can change

• Natural selection (and other selective factors)

Mutation

copying mistake in the genetic code

• can hurt individual if during mitosis (ex. cancer)

• can lead to evolution if during meiosis

can be good, bad, or neutral depending on environment

• bad - gene is broken and affects health of individual

• neutral - noncoding DNA is affected, or the new codon codes for the same amino acid (no change)

• good - new gene works better within the environment than the old one

apoptosis

• programmed cell death

• gene tells cell when to die

recombination

• occurs during meiosis

• doesn’t change allele frequencies or cause evolution

• changes composition of parts of chromosomes

• decreases genetic linkage and increases variability

• affects how some genes act and slight changes of gene function can become material for natural selection to act upon

Gene flow

exchange of genes between populations

can be caused with or without migration

• with - individuals move from one population to another, introducing their genes to the gene pool through interbreeding

• without - two separated populations may interact at their boundary, passing genes between gene pools

genetic drift

random change between generations as a result of statistical sampling error

• sexual reproduction and recombination result in random inheritance of genes

• in very small populations (and through random chance), alleles passed on may not reflect the frequencies of those in the previous generation

2 very important concepts related to Genetic Drift

• Founder effect

• population bottleneck

founder’s effect

• small group leaves and becomes “founder” of new population

• gene frequencies do not reflect average of the whole population since the group is small

• genes are passed onto next generation

• new population has different gene frequencies from the old one

bottleneck effect

• type of genetic drift that occurs when a population’s size is severely reduced due to a sudden, random environmental event or human activity

• affects smaller populations as it drastically reduces genetic diversity compared to the original

• remaining gene pool may not reflect the original population

Natural selection and how it affects allele frequencies and populations

• natural selection can be defined as nonrandom reproduction

• produces directional change in allele frequency relative to specific environmental factors

• variation in a population means some individuals are better at reproduction and survival than others

Artificial selection

humans selectively breeding livestock and crops for generations to obtain favoured traits

sexual selection

• non-random mating resulting from the preferences of the opposite sex

• selected traits may not be adaptive in an environmental context even if potential partners seem more attractive

• mechanism is identical to natural selection