A&P - Week 6: Meiosis and Genetics

What is the largest chromosome?

Chromosome 1 is the largest.

Haploid/diploid?
n=23

Haploid

Most human cells are (haploid/diploid)

Diploid

Haploid/diploid?
2n=46

Diploid

Gametes are (haploid/diploid)

Haploid

Somatic cells are (haploid/diploid)

Diploid

You have a _____% chance of each chromosome coming from either parent

50% chance

What is gamete formation a result of?

Meiosis

How many cell divisions are there in meiosis?

2 (meiosis I and meiosis II)

What are the 2 functions of meiosis?

- Reduce the number of chromosomes in half (2n to n)
- Introduce genetic diversity (all daughter cells are genetically different from each other and the mother cell)

No single virus would be able to wipe out the entirety of the human population. Why is this?

This is because of genetic diversity. There has to be at least one person with immunity to each disease (due to genes)

Unique traits of Prophase I

- Synapsis: Homologous replicated chromosomes pair up and form tetrads (consist of 4 chromatids).
- Crossover: Exchange of genetic material from maternal and paternal chromosomes.

What are chiasmata?

A chiasma is where crossover occurs between 2 non-sister chromatids during prophase I

What is the main step of metaphase I?

Tetrads line up randomly at the metaphase plate.

What happens during anaphase I?

Each tetrad is pulled apart into 2 replicated chromosomes.
(Each chromosome is made of 2 sister chromatids)

What happens during telophase I?

- The nuclear envelope may or may not reform (depending on cell).
- DNA replication (S phase) does not occur before meiosis II, so the cells remain haploid.
- Some cells enter a brief rest period between meiosis I and meiosis II (interkinesis).

What is interkinesis?

The rest period that some cells exhibit between meiosis I and meiosis II

What are characteristics of cells once meiosis I is completed?

- Each cell has 2 copies (sister chromatids) of one member of each homologous pair (either maternal or paternal chromosome), and none of the other chromosome
- Each cell contains the haploid chromosomal number (n) (still-united sister chromatids are considered chromosomes)

What is unique to meiosis II?

Sister chromatids from meiosis I are separated and pulled towards opposite poles (each cell gets one of the daughter chromosomes).

Mitosis vs. meiosis

- Mitosis has one round of division, meiosis has two.
- Synapsis and crossing over occurs in meiosis but not mitosis.
- Mitosis produces two diploid (2n) daughter cells identical to the mother cell, meiosis produces four haploid (n) daughter cells that are different from one another and the mother cell.
- Mitosis produces somatic cells, meiosis produces gametes.

What forms of cell death were covered in this chapter?

Autophagy and apoptosis

What is autophagy?

"Self-eating".
The process of disposing nonfunctioning organelles and cytoplasmic bits by forming autophagosomes. These can then be degraded by lysosomes.
Unneeded proteins can be marked for destruction by adding ubiquitins.
Proteasomes disassemble ubiquitin-tagged proteins, recycling the amino acids and ubiquitin.

What are ubiquitins?

Little protein tags can indicate when a protein is ready to be destroyed.

How are proteins recycled after they're broken down by proteasomes?

Proteins are broken down into amino acids which then can be used by ribosomes to create other forms of proteins.

Apoptosis

"Programmed cell death".
Causes certain cells (cancer, infected, old, ...) to neatly self-destruct.
Occurs when mitochondrial membranes leak chemicals that activate caspases (enzyme). These degrade the cell's DNA and cytoskeleton (causing cell death).
The dead cell shrinks and is phagocytized by macrophages.

What are macrophages? (Brief)

A type of immune cell.
White blood cell which is responsible for "cleaning things up"/"getting rid of junk"

What are some theories of cell aging?

- Wear and tear theory: A lifetime of chemical damage and free radicals have cumulative effects.
- Mitochondrial theory of aging: Free radicals in mitochondria diminish energy production in each cell.
- Immune system disorders: Autoimmune responses, progressive weakening of immune system, etc. leads to loss of homeostasis.

What are telomeres and what can they tell us about cell aging, according to a genetic theory?

Telomeres are strings of nucleotides ("caps") over the ends of chromosomes to prevent damage during cell division.
Each time a cell divides, telomeres get shorter, signifying how many times a cell can divide.

What is telomerase?
Where is it found?

An enzyme that lengthens telomeres.
Found in germ cells of embryos (but absent in adult cells except for cancer cells). In short, most cells don't have telomerase.

Genome

All of the genetic information encoded in all of your chromosomes

Homologous chromosomes

Pairs of chromosomes (one paternal and one maternal) which carry the same genes, but do not necessarily express the trait in the same way.
Slightly different versions of chromosomes (Form tetrads)

_____ of the 46 chromosomes are the sex chromosomes

Two

What are autosomes?
What do autosomes do?

The 44 chromosomes that are not sex chromosomes.
(22 homologous pairs of chromosomes).
Autosomes guide the expression of most other traits.

What is a karyotype?
How is it organized?
What can it be used for?

The counted pairing of chromosomes in a eukaryotic cell.
They're organized from largest chromosome to smallest chromosome.
They can be used to detect chromosomal abnormalities in an individual, or to compare 2 different, related species.

How do a biological male's sex chromosomes interact on the metaphase plate?

A biological male typically has XY chromosomes.
XY chromosomes will line up on the metaphase plate together, but they cannot cross over since they are not both an X.

What stage (of meiosis I) are replicated chromosomes visible on a karyotype?

Prophase and metaphate

What stage (of meiosis II) are chromosomes visible on a karyotype?

Anaphase and telophase

True or False:
In each chromosome, the placement of centromeres remains the same.

False.
Centromere placement varies, this is apparent when looking at a karyotype.

Genotype

Refers to the genes of an individual that determine a particular trait

Phenotype

The physical expression of the genotype

Locus

The position of a gene on a chromosome (physical location).

Alleles

Different versions of the same gene at the same locus on the same chromosome (affects the same trait, but codes for a different phenotype).

What type of genotype is this:
AA

Homozygous dominant

What type of genotype is this:
aa

Homozygous recessive

What type of genotype is this:
Aa

Heterozygous

How are dominant alleles written?

Capital letters (ex. R)

How are recessive alleles written?

Lowercase letters (ex. r)

What is a hybrid phenotype?

When both alleles in a heterozygous individual are expressed?

What are the 2 forms of hybrid phenotypes?

- Incomplete dominance
- Codominance

Incomplete dominance
(Heterozygous for red and white phenotype)

Produces pink

Codominance
(Heterozygous for red and white phenotype)

Produces red and white patches

True or False:
Hybrid phenotypes are common

True

Is sickle-cell anemia a dominant or recessive disease?

Recessive (to fully have sickle cell anemia, you are homozygous recessive ex. tt).

When being tested for sickle cell anemia, your allele results show as TT. What does this mean?

You have normal haemoglobin with the nucleotide, thymine, in a key location.
(You don't have sickle cell, and you aren't a carrier).

Why is sickle cell anemia dangerous?

Red blood cells aren't in their typical round shape, they're folded into crescent-moon shapes.
This means that they can't navigate well around blood vessels, and they don't carry enough oxygen.

How could someone be a carrier for sickle-cell anemia?
Describe traits of being heterozygous to this disease.
Are there benefits?

Two versions of haemoglobin are made, some based on thymine and some with a mutation to adenine.
Someone who's heterozygous to this disease is healthy, but could suffer a sickle cell crisis after prolonged reduction in blood oxygen.
Someone heterozygous to sickle-cell is more resistant to malaria.

How is malaria caused?

By a bloodborne parasite (typically mosquitos).

True or False:
A dominant allele will always have a better outcome than a recessive one.

False.

Evolution works on (phenotypes/genotypes)

Phenotypes

Why are Punnett squares used?

To predict possible gene combinations from parents of known genotypes.
(The probability of offspring inheriting a particular genotype and thus, phenotype).

What are sex-linked traits?

When genes are carried on the X-chromosomes.

Which sex is more likely to be affected by recessive alleles?
Why?

Biological males (XY chromosomes).
This is because they only have one X chromosome, so they cannot be carriers - they either get the trait or they do not.

What symbol is used when discussing blood types?

I or i.
The blood type is written as an exponent (Iᴬ for blood type A, Iᴮ for blood type B, …)
This rule applies unless the blood type is O. Then, it is written as: ii.

What symbols do these represent in a pedigree:
a) ◯
b) ⚫
c) ⬜
d) ⬛

a) Female (trait not present)
b) Female (trait present)
c) Male (trait not present)
d) Male (trait present)