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Lecture 1: Intro & Principle of Segregation 09/03/2013 

Dr. Doreen Glodowski

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Office: B-246 Nelson Hall, Busch  Office Hours: Monday (1-2pm), Wednesday (10:30-11:30am) E-mail: [email protected]

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Required Textbook: Genetics: A Conceptual Approach (4th Edition) by

BA Pierce  http://bcs.whfreeman.com/pierce4e/#t_646189   Grading        

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Recitation: 15% Exam 1: 25% Exam 2: 25% Final Exam: 35% A: 90-100% B: 80-89% C: 65-79%

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D: 50-64% F: below 50%

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There is no curve

Exams

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Multiple Choice Short Answer

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Exam 1: October 8th (in-class) Exam 2: November 12th (in-class)

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Final: December 16th (8-11am)

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Recitation  New problem sets are posted every Friday morning on Sakai under resources o There will be 11 problem sets throughout the semester, the top 10 recitation grades are calculated in your final grade o Answer keys are posted every Friday

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Main points to consider:  Gregor Mendel o Who is he? o What did he do?  Terminology o Gene vs. allele vs. locus vs. chromosome o Genotype vs. phenotype o Dominant vs. recessive o Homozygous vs. heterozygous 

Experiments o Monohybrid cross

o Dihybrid cross o Test cross  

Some definitions  Gene: fundamental physical and functional unit of heredity that 

carries information from one generation to the next Chromosome: the physical structure that carries the genes o Chromosomes are arranged in order of largest to smallest in their karyotypes o All along the length of each of these chromosomes are

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thousands of genes Phenotype: the appearance or characteristics of an organism o The physical traits  Ex. Short vs. tall Genotype: the underlying genetic composition of an organism o Which genes are required to give the phenotype  Yy vs. YY vs. yy

Gregor Mendel is the father of genetics  Lived from 1822-1884  The work that he did laid up the field for what we now know as  

genetics Studied the inheritance in observable characteristics in the garden pea Published his work in 1866, but no one cared for it o Importance of his work was not recognized until about 1900, when it was rediscovered

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Before Mendel’s Work: Theory of Blending Inheritance was popular  Said that both parents carried something that held their traits and when the gametes would join the gamete would carry the essence of the parents and there was a blending of the two parents

o Sperm and egg contain a sampling of essences from throughout the body o Essences blends at conception to form pattern for new  

individual Mendel set up experiments to test this hypothesis

Mendel’s model organism: the garden pea  Advantages of peas o Cheap, easy to obtain o Easy to grow o Self-fertilizing o Can be cross-pollinated  Keys to Mendel’s Success o He used true-breeding strains  He grew each stain for several seasons to make sure they were true breeding (all offspring will always exactly resemble the parent) o Studied contrasting characteristics  Chose traits that were easy to score 

Ex. Stem length (tall vs. short), color (green vs.

yellow), round vs. wrinkled o He carefully quantified his experimental results  He would not just say a majority are tall, he would

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count exactly how many were tall vs. how many were short This helped him to see trends

Mendel’s 1st Experiment was a Monohybrid Cross  Mono: one 

Hybrid: individual formed from two true-breeding parents of

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different genotypes He would cross a green plant with a yellow plant to see what color the offspring would be

o First generation (P0: parental generation) o Second generation (F1: first filial generation) 

He was testing the theory of blended inheritance o So according to the blending theory, if he was mixing yellow with green he should have been expecting to see a mix between green and yellow color in the offspring (yellow-green plants)  Disproved it because the color of the offspring was always identical to one of the parents  There was no green mixed with yellow, it was

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either yellow or green He started realizing that there were 3 green individuals for every 1 yellow individual

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Results of the reciprocal cross were the same  Reciprocal Cross: Flipping the traits of the parents to see if you get the same result o Ex. If in the first experiment the mom had blue eyes while the

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dad had green eyes you would switch it so that the second time the mom had green eyes while the dad had blue eyes  This was done in order to see if the trait was sex-linked Wanted to test if the sex of the plant was involved Last time he had green as the father, and yellow as the mother so

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he flipped it NOTE: female is written in first in a cross so green x yellow means

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mom is green He got the same result (3:1 in favor of green) o So he proved that sex did not play a role

Mendel Concluded:  Male and female parents contribute equally to the characteristics you see in the progeny o Proved this because reciprocal crosses give the same results 

Inheritance is not blended, so he rejected the null hypothesis o F1 progeny always resembles only one parent (the dominant)  NOTE: progeny means children, offspring

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Mendel Proposed:  He came up with two theories o 1. Dominant vs. Recessive Traits  Dominant trait is always the one displayed in F1 generation  Recessive trait is displayed starting in the F2 generation o 2. Theory of Particulate Inheritance  Traits are determined by discrete units that are inherited intact through generations  This looks a lot like our definition of a gene  He was hinting of what gene function would be,  

having no knowledge of chromosomes or anything The term gene was coined in about 1910, way

after Mendel Each individual has 2 particles in genotype, but passes only 1 particle on to progeny  Because the progeny is getting one from mom and one from dad they will end up with two

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Steps for setting up a Punnett Square  Determine gametes for parents  Write gamete and frequency along sides of square o Doesn’t matter if mom or dad is on top or side  Combine gametes  Translate genotype into phenotype More Definitions  Allele: alternate forms of a gene o Like green (Y) vs. yellow (y)

o Different forms of the same gene that is coding for the same 

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exact characteristic Locus: the place on the chromosome where the gene is located o Plural: loci o Ex. Where the allele for hair color is located on the chromosome vs. where the allele for eye color is located Homozygous: has two identical alleles o Two copies of the same exact gene o Ex. YY or yy o True breeding individuals are homozygous

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Heterozygous: has two different alleles o Ex. Yy

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Dominant: the trait you see in the F1, phenotype observed in

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heterozygote Recessive: not seen in F1, allele is hidden is heterozygote

If Mendel’s two particle theory of inheritance is true, we predict:  All yellow F2 plants (homozygous) will breed true o So the recessive trait will only be present in a homozygous 

(true-breeding) individual Some green F2 plants (homozygous) will breed true o 1/3 of the green F2s

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Some green F2 plants (heterozygous) will produce green and yellow progeny o 2/3 of the green F2s

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Mendel’s 1st law: the principle of segregation  During gamete formation, members of a pair of alleles segregate 

into separate gametes The idea of the particle segregating into the gametes

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Such gametes form in equal frequencies Fertilization should be random o 50% of Y being used, 50% of y being used in Yy

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If parent is diploid (has 2 alleles for all genes): o Phenotypic ratio will be 3:1 in F2 o Genotypic ration will be 1:2:1 in F2  The above are the results of the monohybrid cross

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Phenotype does not always reveal genotype  Recessive phenotype = homozygous recessive genotype (yellow: 

yy) Dominant phenotype = homozygous dominant or heterozygous genotype (green: YY or Yy) o So, how do we know what we’re working with in terms of genotype?  We use a test cross, also called a back-cross  A test cross is always crossing an individual with dominant phenotype to homozygous recessive

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individual  Cross would be: o Y_ x yy If YY x yy  Yy (100% dominant phenotype) If Yy x yy  (50% dominant and 50% recessive)  If we get this we know our initial individual was heterozygous: Yy

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Answers for Lecture 1 Practice Problems 1.

B

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C

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A

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C

Lecture 2: Mitosis & Meiosis 

Main points to consider:

09/03/2013

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Mitosis vs. meiosis Centromere, telomere, haploid vs. diploid, homologous

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The cell cycle o How does this relate to mitosis and meiosis?

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Chromosomes movement during each phase of mitosis and meiosis o How does chromosome number change with each phase? o How does chromosome position change with each phase?

Recap on Mendel’s Theory of Particulate Inheritance  Said that parents had two particles and those particles were somehow giving rise to the progeny that we see and those progeny 

somehow also end up with 2 particles Traits are determined by discrete units that are inherited intact

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through generations Each individual has 2 particles in their genotype, but they pass only

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1 particle to progeny Mendel came up with this theory just by using his peas in the garden

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Using light microscopy to watch cells divide helped to identify the

“particles”  These people put together his theory with what was really happening in the cell o Walther Flemming (1843-1905)  Famous for studying cells as they divide using light 

microscopy Looked at cells as they divided and drew pictures of

what he saw  Very accurate pictures o Walter Sutton (1877-1916) and Theodor Boveri (1862-1915)  Credited with putting together Mendel’s theory with 

what’s happening in the cells Looked at what’s happening in the cell as they divide

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Suggested that the particles were chromosomes from Mendel’s theory

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Some important observations from microscopy  When all eukaryotic cells divide, the nucleus divides first o They said, “ok the most important component in the cell for the cell to divide must be in the nucleus”  They were right, chromosomes are in the nucleus  

Chromosomes are present in the nucleus Cells from different species contain different total chromosome

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numbers All somatic (body) cells from one species have the same chromosome number whereas germ cells (gametes) have half o Gamete is providing the particle, but only one particle per parent even though the parents have two o Somatic cells are body cells o Germ cells are the gametes

Conclusion: each individual has 2 copies of each chromosomes in their genotype, but they pass only one copy of each chromosome to their progeny  Now we’re not calling them particles anymore, we know they are chromosomes  

What is a chromosome?  Physical structure that carries genes  DNA molecule containing genetic information arranged in a linear 

sequence In eukaryotes, chromosomes contains DNA, RNA and proteins o Chromosome is DNA that is wrapped along many different

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proteins Prokaryotes have a circular chromosome

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Chromosomes are shown to us in karyotypes Most of the time when a cell is not dividing, when it is just existing and maintain the chromosomes are diffuse, the DNA is not condensed into the structure of chromosomes because it is not undergoing cell division

o This is what happens in most of the cell cycle o It only is in chromosomes when the cell is getting ready to divide, otherwise the DNA and proteins are dispersed not condensed  Chromatid: dispersed DNA  Most of the time  Chromatin: DNA that has the protein and the RNA associated with it  

Every eukaryotic chromosome has a unique shape and size but they all

have:

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Telomeres o Every eukaryotic chromosome has 2 telomeres o Telomere are at the top and bottom (the poles) o Telomeres are the ends of the chromosomes

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Centromeres o Every eukaryotic chromosome has 1 centromere o Constricted area where microtubules bind o Located in the center of the chromosomes

Chromosomes are distinguished by:  Size  Genes  Location of centromere in relation to telomeres o They have a shorter arm and a larger arm (except in telocentric and metacentric)

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Short arm: p  HINT: remember, p=petite  Longer arm: q o Metacentric  Centromere is in the middle o Submetacentric  Centromere is closer to one end o Acrocentric  Even closer to the end o Telocentric  All the way at the end

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Total chromosome number per cells differs among species  Humans: 46 chromosomes  Drosophila (flies): 8 chromosomes  

Cows: 60 chromosomes No correlation between chromosomes and complexity of the

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organism Most eukaryotes have 2 copies of every chromosome in somatic cells (diploid) o Diploid: two complete sets of chromosomes  Diploid number is referred to as 2n 

Haploid number is referred to as n  Number of chromosomes in a gamete  Number of chromosomes in a distinct set  We have sets

o Each set has 23 (haploid number) so diploid is 46 o The 2 copies of each chromosome are called homologous chromosomes  Each member is a homolog  Homologous chromosomes have the same length, same centromere placement and the same genes  Difference: alleles or forms of genes  Ex. One codes for green the other codes for yellow  

Chromosomes must be copied and separated equally during cell

division  Chromosomes must be copied before the cell divides, if we don’t copy it then we’ll have fewer chromosomes  

Mitosis: 2n cell creates two 2n cells, one division occurs, one cell

becomes two cells which are clones of the parent cell (they are genetically identical)  This is the division that happens when we make more somatic 

(body) cells Meiosis: the process of making gametes  Go from one single diploid cell to 4 unique haploid cells (they are 

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not identical) There are two divisions that occur

The Cell Cycle  Stages that a cell goes through as it grows and divided o G0: non-dividing phase  The cell is keeping its size, maintaining itself 

Cell is in stable state at constant size o G1: 1 gap or “growth” phase  Proteins needed for cell division are made st

o S: DNA Synthesis

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All chromosomes are duplicated o G2: 2 gap or “growth” phase  Any damaged DNA must be repaired nd

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Repairing any problems that occurred during S phase o M: mitosis or meiosis phase  All the phases leading to M are preparing to go through cell division Cell division o G1, S and G2 are collectively known as interphase 

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Mitosis  REMEMBER: cell goes through S phase (where chromosomes are replicated before mitosis)

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NOTE: 2 chromatids (sister chromatids) make up a chromosome Prophase o Chromosomes condense  They are starting to come together and form the distinct, linear structure o Mitotic spindles form (outside nucleus)

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Little structures that are outside in the cytoplasm in

either side of the cell o Nuclear envelope breaks down

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Metaphase o Microtubules from spindle pole attach to each chromosome at centromere  Microtubules extend from the centromere to the outsides of the cells o Chromosomes are moved to metaphase plate in center of cell

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Anaphase o Proteins holding sister chromatid centromere together are degraded o Sister chromatids separate (disjoin)  They go to opposite ends of the cells  The proteins that were holding the sister chromatids together degrade

o Sister chromatids move to opposite spindle poles o At this point, the chromosome number in the cell has doubled  But, the number of sister chromatids in the molecule remain the same

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Telophase o Nuclear envelope reforms around each daughter nucleus o Chromosomes uncoil o Spindle disappears o Cytokinesis (division of the cytoplasm) occurs at the same time as telophase  So, she counts cytokinesis as part of telophase

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Results in two identical cells

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Meiosis  Process that leads to gamete formation  4 haploid cells are produced from 1 diploid cell  2 divisions o Meiosis I (reduction division)  After meiosis I we have half the number we began with o Meiosis II (equation division)

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After meiosis II we have the same number we had after

meiosis I NOTE: cell goes through S phase (where chromosomes replicate) before meiosis Meiosis I o Prophase I  Chromosomes condense  Exactly like mitosis 

Prophase I is actually divided into 5 stages  Names describe appearance of chromosomes as 

they condense, pair and recombine Crossing over occurs within these steps  Thin Thread: Lepotene o Condensations begins 

Paired Thread: Zygotene o Synapsis begins

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Thick o o o

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Thread: Pachytene Condensation continues Recombination Synaptonemal complex  Where the proteins hold

together the sister homologs before crossing over occurs Double Thread: diplotene o Synapsis is breaking down  The homologous chromosomes

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are starting to come apart from each other Proteins help to keep the chromosome together  Chiasmata keep homologs

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together Moving Apart: diakinesis o The two homologs move apart but they are still joined at every place where crossing over happened

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Homologous chromosomes pair (synapse) Crossing over may occur  Unique to meiosis  Also called genetic recombination  Definition: Equal exchange of genetic material from the sister of one homolog to the sister of the other  Two sisters are involved, they are simply 

exchanging material Crossing over is a continuous process

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Spindle forms Nuclear envelope breaks down

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Only difference between this and prophase in mitosis is crossing over

o Metaphase I  Each pair of homologs takes up a position in metaphase 

plate They line up along the middle of the cell

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Homologous pairs are next to each other, unlike in mitosis where they were in a straight like with

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nothing next to them Orientation of each pair of homolog...