Section 2 Lectures 1-3

Cell Division, The Cell Cycle, Mitosis, Meiosis, Mendelian Genetics

What is the mystery of heredity?

* All organisms come from other organisms
* All organisms resemble their parents
* Siblings are not identical

What did ancient greeks think about parents?

* That males and females must share some “essence” that mixes to produce progeny
* Aristotle thought the “essence” came solely from the male and the female was just a fertile “field” for the essence to grow

What do we know now that contradicts the ancient greeks theory about heredity?

* Parents share sperm and egg (2 individual cells)
* The sperm cells attack one egg, as soon as one gets in the others are blocked

Classical Cell Theory

* All organisms consist of cells
* Cells divide to produce new cells
* Higher organisms fuse their cells (sperm & egg, pollen & egg) to produce a new organism

Overview of Cell Division and Growth

* One mother cell divides into 2 daughter cells
* Bothe daughter cells must inherit everything needed to survive
* Including the information of what properties the cell should have

Chromosome

* A single strand of DNA (a polymer)
* Comes in 2 types":
* Circular (bacterial)
* Linear (most organisms)

Karyotype

* A way of staining/organizing chromosomes that makes the structure clearer
* Chromosomes used for a karyotype only come from dividing cells (only time you can see them)

Chromatin

* Mixture of DNA (chromosome) and proteins as cell is getting ready to divide

Why can we see chromosomes during cell division?

* Can be seen because as the cell is getting ready to divide, the chromosome condenses by associating with proteins (ex. histones)
* DNA double helix wraps around histones
* Histones keep the DNA tightly packed instead of tangles

How do you make a karyotype?

* Take a cell preparing for division
* Squish it with a microscope slide
* Stain the chromosomes with a dye, take a photo

What can you say about the number of chromosomes in different species?

Number of chromosomes is the same for a species but varies between species

* Bacteria typically have 1, humans have 46

What are identical pairs of chromosomes in eukaryotes called?

In eukaryotes, chromosomes often come in identical pairs called homologs 

* Humans have 22 homologs, 2 sex chromosomes 

What are the duplicates of chromosomes during cell division called?

At cell division each chromosome is duplicated once, the duplicates are chromatids, held together by proteins called the centromere 

* One of each pair of chromatids goes to each daughter cell

What are mitotic chromosomes?

* Normally a chromosome is a single piece of DNA
* When looking at a karyotype we see 2 identical pieces of DNA (2 chromatids bound by a centromere)
* This is considered a mitotic chromosome

Why 2 sister chromatids?

* 2 identical chromatids means the genetic material was duplicated
* So giving each daughter cell one chromatid from each mitotic chromosome means each one will have a complete copy of the genetic material

Segregating Mitotic Chromosomes and Probability

* Unlikely it happens by choice since that would mean both chromatids would have an equal chance of entering either of the two daughter cells
* Thus the probability of each daughter cell getting one chromatid is 50%
* Assuming the segregation of chromatids from different chromosomes is independent, the probability that both cells inherit one chromatid from each of the 46 chromosomes in a human cell, only by chance, is 0.5 ^ 46

Probability Theory

The probability of 2 independent events is the product of the probability of each event.

Why is chromosome segregation so exact?

* Organisms need at least one of each chromosome, they carry essential genetic material
* Organisms usually need exactly one of each chromosome

Down Syndrome

* Only case in humans where you can survive past infancy with an extra chromosome
* Results from an extra copy of chromosome 21

Cell Division - The Cell Cycle Steps

1) S Phase: Chromosome (DNA) Replication

2) M Phase

* Mitosis: Process by which somatic cells make identical copies of themselves, create 2 daughter cells that inherit one copy of each chromosome
* Meiosis: Process by which germ cells make non-identical copies of themselves by creating daughter cells that inherit one copy of each homolog. (i.e. daughter cells end up with half the DNA of the mother cell)

3) Cytokinesis (optional): dividing the cytoplasm in two

Interphase: all phases of the cell cycle excluding M phase

* Between M & S: G1 - cell is resting
* Between S & M: G2 - cell prepares to divide

What are Cell Checkpoints

* Checks for the completion of each step before the next occurs
* Ensures that certain conditions are fulfilled before the next phase starts

Explain the Checkpoint After S Phase

* Between G2 and M
* Ensure a cell does not proceed from G2 unless all DNA has been replicated

What happens if you add caffeine and hydroxy urea to the checkpoint after S phase?

* Adding hydroxy urea blocks DNA replication, so the cell never begins M phase because the G2/M checkpoint stops it
* Adding caffeine disables the checkpoint, which normally does not cause an issue and the cell can divide/continue the cycle
* But, if you add hydroxy urea and caffeine the cell enters mitosis and the 2 daughter cells die

Cyclin and Cdk

* Proteins that send signals to the cell
* Does this because when they are joined the cell knows its in G1
* When they are destroyed the cell knows it is not
* This occurs for every step of the cycle

G1/S Checkpoint

* Most cells are at rest in G1
* The checkpoint looks for chemical signals from external cells to determine if the cell should go on to divide (since most cell past G1/S divide)

Benefits and Negatives of G1/S Checkpoint

* Benefits: cyclin E in mammals become active after pregnancy from hormonal signals and results in the proliferation of breast cells necessary for lactation
* Negatives: something goes wrong with the checkpoint and cells divide at the wrong time (causes cancer)
* For ex. an over expression of cyclin E when not pregnant contributes to breast cancer

Mitosis Key Point

Process that divides up chromosomes equally

Explain G2 in Mitosis

* Replication of the centrosome and centrioles in some cases

Prophase in Mitosis

* Centrosomes move to the poles
* Spindle forms
* Chromosome condensation - chromatids become evident
* Note: here each chromatid is a chromosome even though we refer to the pair of chromatids as a mitotic chromosome
* Kinetochores form

Prometaphase in Mitosis

* Step 2
* Nucelar envelope breakdown
* Polar microtubules and kinetochore microtubules form
* Kinetochore microtubules probe the cytoplasm and attach to kinetochores
* Chromosomes begi

Metaphase in Mitosis

* Sister chromatids bound to kinetochore microtubules on opposite spindles
* Tension from kinetochore microtubules pulling on the sister chromatids, causes them to line up on the metaphase plate

Anaphase in Mitosis

* Centromeres separate
* Kinetochore microtubules shorten
* Spindle elongates

Telophase in Mitosis

* Spindle breaks down
* Chromosomes decondense
* Nucelus reforms

Making a Metaphase Mitotic Spindle

* Keep chromatids paired until all are attached to a kinetochore microtubule
* Have unstable kinetochore microtubules that can grow and probe the cytoplasm and only become stable once captured by a kinetochore
* Make sure 2 kinetochore microtubules from the same spindle can’t capture both chromatids of a chromosome
* Have a checkpoint that senses when all the chromatids have been captured and only then allow the chromatids to separate by dissolving the centromere

Cytokinesis

* Animals: actin and myosin form a “purse string” that constricts and divides the cell
* Plants: vesicles fuse to make cell membrane and cell plates which eventually becomes the new cell wall
* Process is optional as some cells don’t divide their cytoplasm
* Ex. muscle cells have many nuclei (called syncytial) because they go through mitosis without cytokinesis

When is mitosis beneficial and why is it considered asexual reproduction?

* Good for clones, i.e. you believe you are perfectly adapted to your environment
* If the environment changes, and you can’t survive neither will your progeny
* No genetic material is mixed, thus asexual reproduction

Sex

* Mixing of two organism’s genetic material

Genetic mixing in humans & eukaryotic cousins

* First each chromosome creates a gamete cell that has half the genetic material
* Then the gametes are combined to get an organism with a complete genome

What characteristics do gametes have?

* Only one homolog of each chromosome
* Ex. sperm an egg cells

Ploidy Terminology

* We define n = a set of chromosomes that includes exactly one homolog of each chromosome
* Multiples of n get the names…
* 1n = haploid
* 2n = diploid
* 3n = triploid
* 4n = tetraploid

What is the ploidy of somatic cells and gametes in humans?

* Somatic cells = diploid
* Gametes = haploid

How does entropy tie into chromosomes?

Entropy is responsible for mixing haploid cell chromosomes.

Meiosis Key Point

* Process of splitting chromosomes in half
* First the diploid cell replicates its DNA like mitosis
* Consists of Meiosis 1 and 2

Prophase 1 (Meiosis 1)

* DNA beings to compact
* Synapsis: pairing of homologous chromosomes
* Chiasma form; crossing over
* Yields 2 recombinant chromatids

Prometaphase 1 (Meiosis 1)

* Nuclear envelope breakdown
* Spindle fiber forms

Mitosis vs Meiosis timing

* Unlike mitosis, meiosis can take a very long time
* Ex. a human female’s eggs begin prophase 1 in utero and finish at ovulation

Homologous Chromosome

* Two chromosomes in a pair
* One from the mother, one from the father
* Have been exchanges by recombination during synapsis

Metaphase 1 (Meiosis 1)

* Microtubules attach to kinetochores, one per homolog (not per chromatid)
* Chromosomes line up at the metaphase plate, held together by chiasma

Anaphase 1 (Meiosis 1)

* Separation of homologous chromosomes into 2 separate cells
* Now each ell has two copies (2 chromatids) of each homologous chromosome
* Note: chromatids not identical because of crossing over
* Telophase after this is optional

Meiosis 2 Key Points

* Like mitosis but with half the number of homologs

Metaphase 2 (Meiosis 2)

* Chromosomes line up on plate and chromatids separate to end up in different cells
* Note: the rest of the steps are the same as mitosis

Telophase 2 (Meiosis 2)

* Cells split to have 4 haploid cells where each chromosome is just one chromatid
* Results in 4 sperm or egg cells

Down Syndrome and Meiosis

* Down syndrome is caused by a problem in MEIOSIS not mitosis
* During anaphase 1 of meiosis a non-disjunction event occurs where both chromosome 21 homologs end up in a single gamete (other gamete has no chromosome 21)

Why does Down syndrome increase in older women?

The frequency of Down Syndrome increases in older women due to how long the eggs have been arrested in meiosis.

Two Ways to Think of Meiosis

1) A diploid organisms divides its chromosomes in the haploid gametes so that when the haploid cells fuse you get back a diploid organism (us)

2) Two haploid cells combine to form a diploid so that the chromosomes can be shuffled before undergoing meiosis to produce another haploid (yeast, molds, etc.)

Importance of Ploidy

* The absolute number of chromosomes is not important, the ratio of homologs is
* Odd ploidys (n, 3n, 5n) tend to be sterile because of problems in meiosis as there are unpaired homologs on the metaphase 1 plate
* Odd numbered ploidy is okay if the cell undergoes mitosis
* Note: Tetraploid organisms are balanced and all homologs can pair with a partner during meiosis 1 (ex. X. laevis frog)

Crop Varieties and Ploidy

* Crops with bigger ploidy are preferred as higher ploidy means bigger fruit
* Ex. strawberries that are octoploid

Character and Traits - Language of Heredity

* For a given character (height, colour, shape, etc.) offspring share traits (tall, red, oval) with their parents
* Siblings do not have identical traits for a given character

Gregor Mendel

* Showed heredity has predictable patterns that result from laws (rules of heredity)
* He did this before the discovery of chromosomes and karyotypes

Discrete Variation

* There are only two or a few traits for a given character
* Ex. fur colour in mice

Discrete pea traits, true breeding, and Mendel

* Mendel used peas to test inheritance
* Started with a trait that bred true
* Means that when bred with itself, all the progeny will have the same traits of the characters he was interested in
* Note: true bred/true breeding/ inbred strains are all synonymous

Mendel’s First Cross

* Crossed a round pea and a wrinkled pea ( w x R ), that always bred true
* The F1 (first filial generation) produced all round
* The F1 x F1 (R x R) bred 3/4 Round and 1/4 Wrinkled
* The result of the second cross was known as the F2 (second filial generation)

Conclusion of Mendel’s First Cross

* The recessive trait (w) disappeared in the F1
* But showed up in the F2 (second generation) in 1/4 of the progeny
* This applied to other traits too

Key Takeaway From Mendel’s First Cross

* Always one trait is recessive and reappears in the F2 in 1/4 of the progeny

Are all F2 round the same?

* Mendel crossed the F2 round pea with a wrinkled pea and saw that sometimes the progeny were all round or 50:50 round to wrinkled
* Thus some F2 round were like the parents and some harboured the round trait

Hereditary Units

* Mendel proposed that heredity comes in 2 discrete units
* 2 per organism, per character
* The dominant S and recessive s
* Gametes only get one of these at random from there parent

Genetic terms for unit of heredity, different flavours, having two of the same allele, have two of different alleles

Law of Segregation

* Only one of the 2 gene copies present in an organism is distributed to each gamete that it makes
* Allocation of the gene copies is random

Second Rule of Probability

* The probabilities of mutually exclusive events sum
* Ex. probability homozygous round: 1/4, probability of heterozygous round: 1/2, so probability round: 3/4

Independent Events

If the 1st coin is heads it has no influence on whether or not the 2nd coin will be heads or tails.

Mutually Exclusive Events

If both can’t happen at the same time then the probability of either event occurring is the sum of their individual probabilities.

Genotype

The set of alleles an organism has.

Phenotype

The set of traits an organism displays.

Genotype vs. Phenotype

Genotype determines phenotype, but because of dominance phenotype may not uniquely determine genotype.

How can you tell if two plants with the same PHENOTYPE have the same GENOTYPE (Ss or SS)?

Test cross: Cross the plant with the dominant phenotype with the plant with the recessive phenotype (i.e. must be ss)

* This will give you two different results based on the genotype of the plant exhibiting the dominant phenotype

Result: If it is homozygous you get all progeny with the dominant phenotype, if it is heterozygous you get progeny with 50% recessive and 50% dominant phenotype

Dihybrid Cross and Mendel

* Used to find if colour is linked to roundness
* Result: They assorted independently, which further supported his idea that heredity consisted of discrete units that moved into the gametes independently

Link Between Meiosis and Heredity

* Discovery of meiosis validated Mendel’s ideas
* Genes are on chromosomes, one gene on each homolog, 2 homologs in each cell, thus 2 genes in each cell
* Homologs similar not identical, could contain different alleles of a gene
* Homologs separated randomly at meiosis 1, so 1 to each gamete
* Independent assortment makes sense if the genes were on different chromosomes since there’s 2 ways 2 pairs of homologs could line up on the metaphase plate of Meiosis 1

Sex Chromosomes

* Women are XX men are XY
* Males are heterozugous and females are homozygous recessive
* Maleness is dominant over femaleness

Proof that maleness is dominant

* People with too many or too little sex chromosomes because of non-disjunction are XO (female) or XXY (male)
* The gene for maleness is called SRY, on the Y chromosome

Number of Gamete Genotypes in a Punnet Square

Number of Gamete Genotypes in a Punnet Square = 2^n

n = the number of different genes under consideration