Lab Exam 4

This was with Dennis and goes over Mitosis, Meiosis, Tetrad, and Transmission Genetics

G0

Resting phase of the cell cycle. Cells can enter this phase if there are not enough nutrients/space to grow and divide. Some may enter and stay there, but they can go back and enter the cell cycle again

Interphase

Makes up the majority of the cell’s life cycle. DNA is not ordered into compact chromosomes; it looks like a “ball of yarn.” Three main phases

You cannot differentiate individual chromosomes since the DNA is in its relaxed form, called chromatin.

G1

Cell growth and metabolism

S Phase

DNA replication

G2 Phase

Preparation for mitosis and reproduction of organelles

Prophase

Nuclear envelope degrades, chromosomes condense into characteristic shapes, and sister chromatids are bound at the centromere

Metaphase

Chromosomes align along the metaphase plate with the help of microtubules (aka spindle fibers). Some microtubules stretch from pole to pole (nonkinetochore microtubules).  Sister chromatids are still attached at the centromere

Anaphase

Microtubules separate the two chromatids. The process forms two separate daughter chromatids in what will be the two daughter cells that arise from one cycle of mitosis. Separated chromosomes move towards opposite ends of the cell. Separation is a result of the pushing and pulling of nonkinetochore and kinetochore microtubules.

Telophase

Individual chromatids are pushed to opposite ends of the cell pole. Nuclear membrane reforms. Cytokinesis generally occurs during telophase, rather than at the end of it.

Role of mitosis 

Cell division that results in cell duplication 

Allows organisms to grow and add body mass 

How asexual organisms reproduce 

Allows organisms to replace damaged or old cells

Chromosome Structure

Chromosomes contain very tightly wound DNA. You need to unwind the DNA, which is wrapped around histones. 

Homologous chromosomes are a pair of chromosomes

Sister chromatids are two identical copies of a single chromosome 

Mitosis plant vs. animal cells

Animal cells form a cleavage furrow during cytokinesis

Plant cells form a cell plate, which becomes part of the structure of the cell; no centrioles are present 

How does cancer impact the cell cycle?

The cell cycle is out of control, and the cells go under mitosis way more than they need to. Cancer often disables the multiple checkpoints, so they continue to divide despite the errors. 

Chemo drugs target cells in mitosis. The collateral damage is that cells that normally divide rapidly are targeted as well 

Function of meiosis

Produce new gamates

Sexual vs asexual reproduction

Sexually reproducing organisms need meiosis to reproduce

Asexually reproducing organisms can reproduce on their own via mitosis 

Prophase 1

Synapsis and recombination/crossing over

Prophase 2

Chromatin condenses into visible chromosomes, like mitosis

Metaphase 1

Homologous chromosomes align along the metaphase plate

Metaphase 2

Chromosomes align along the metaphase plate, like mitosis

Anaphase 1

Homologous chromosomes are separated, sister chromatids still joined at the centromere

Anaphase 2

Sister chromatids are separated, like mitosis

Telophase 1

Homologous chromosomes are pulled to either pole of the cell. Cytokinesis results in the formation of two daughter cells

Telophase 2

Single chromatids pulled to either pole. Cytokinesis results in the formation of four daughter cells

When do synapsis and recombination/crossing over occur?

During prophase 1

Synapsis 

The pairing up of homologous chromosomes

Recombination/Crossing Over

The exchange of parts of homologous chromosomes increases genetic variation. Exchange of chromosome pieces

Leptotene

Prophase begins, chromosomes start to condense 

Zygotene

Synapsis begins, synaptonemal complex forms

Pachytene 

Crossing over, DNA exchanged by non-sister chromatids

Diplotene

Synapsis ends, chiasma visible within bivalent

Diakinesis 

Prophase ends, nuclear membrane disintegrates

Non-Disjunction

Abnormal segregation of chromosomes during meiosis 1 or 2, produces gametes with abnormal number of chromosomes copies 

Mitosis duplication and cells division vs meiosis

Mitosis has 1 duplication and 1 cell division 

Meiosis has 1 duplication and 2 cell divisions

How does crossing over increase genetic variation?

Crossing over is part of meiosis and helps produce genetic variation in sexually reproducing organisms

What is a tetrad and how are they produced?

A general term for things in a group of four 

We are using it to describe the four spores produced after meiosis in fungi 

Eight spores are seen since mitosis occurs after meiosis is complete 

In some species of fungi, tetrads are ordered. 

Perithecium 

Flask-shaped or spherical, closed structure with a small opening at the top where the spores are released. Protects the developing asci and ascospores and provides a controlled mechanism for their release and dispersal 

Ascus

A Sac or tube-like-shaped cell that helps the ascospores. This is where the fusion of nuclei and meiosis happens. This is where sexual reproduction culminates. Often plays a role in actively discharging the mature ascospores into the environment.

Ascospore

A haploid spore produced within an ascus. They have various shapes but have a unique, thick, and highly resistant cell wall. They are the sexual reproductive units. When released, they are responsible for dispersal, surviving harsh conditions, and germination to begin a new fungal colony

Color of ascospore

The color is encoded by a gene that, when mutated, produces a mutant color 

Wildtype → Dark brown 

Mutant → Tan

There is also a spore killer allele that eliminates ascospores in the ascus

First division segregation

No crossing over has occurred between homologous chromosomes 

Second division segregation

Crossing over has occurred, which can generate several different patterns

Equation for tetrad analysis

r= 0.5 (# of 2nd division / total number) 

r stands for recombination frequency

The further the gene is from the centromere, the more likely recombination will be

The recombination frequency is divided by two since only two of the four chromatids in a tetrad are involved in crossing over

Large r

The gene for spore color is far from the centromere

Small r

The gene for spore color is closer to the centromere

Gene

A Segment of DNA that encodes instructions for a general trait (eye color)

Allele

A variant of those instructions (brown eye allele for the eye gene, there can also be blue or green eye alleles

Homozygous

Contains identical alleles of the gene on both homologous chromosomes. (BB or bb)

Heterozygous 

Contains two different alleles of a gene (Bb)

Genotype

The particular combination of alleles someone has for a given trait

Phenotype

The physical manifestation of the combination of alleles

Monohybrid Cross

A cross between two individuals for one gene

Dihybrid Cross

A cross between two individuals for two genes

Back Cross

The crossing of an F1 generation offspring with its parent. Brings the newer hybrid generation closer to one parent (either dominant or recessive) 

Amplifies the trait of a parent in future offspring generations 

Test Cross

Taking an unknown and known species alleles to determine the genetope of the unknown. Sheep in the slides

Dominant

Only shows dominant color (red)

Recessive

Only shows recessive color (white)

Codominance 

Shows both colors separately (red in middle, surrounded by white)

Incomplete dominance

Mix of the colors (pink)

Sex-linked Inheritance

Carrier: A parent that has a recessive allele but doesn’t express the recessive trait 

A male is hemizygous for traits on the X chromosome  

Only males can inherit Y-linked traits 

Males and females can inherit X-linked traits but males are more likely to express recessive X-linked traits 

Males cannot pass on X-linked traits to male offspring