Lecture 22a

Question 1

As animals go, mice are relatively closely related to humans. Why is this?

Answer 1

Rodents (mice, rats) and lagomorphs (rabbits, hares) are the orders evolutionarily closest to the primate order.

Question 2

What is a main advantage to using mice as subjects? What is a limitation of this though?

Answer 2

Mice can handle being completely inbred, meaning that they are genetically identical to each other.

However, the mouse are dopey and STUPID AF.

Question 3

T/F: The mouse cycle is long for a mammal.

Answer 3

False! The mouse cycle is rapid for a mammal. They give birth after 21 days of pregnancy, and can reproduce when 5.5 weeks old.

Question 4

In relation to mice, what is a “plug”?

Answer 4

When a female mouse has mated, she develops a white candle-wax substance in her vagina, which prevents other male mice from accessing her.

Question 5

If a female mouse has a plug, what is the likelihood that she is pregnant?

Answer 5

A plugged mouse has a 50% chance of being pregnant.

Question 6

Why is it a huge help that “plugs” exist in mice?

Answer 6

We know when a female has mated and when she mated so we know when to euthanize her and dissect out the embryos.

Question 7

What are Forward Genetic Screens?

Answer 7

Animals are FIRST mutated, then we observe the offspring bearing the mutant phenotype and find the gene responsible for it.

Question 8

What are Reverse Genetic Screens?

Answer 8

FIRST, the mutation is made, then we have to find the mutant phenotype that results from that mutation.

Allows us to determine how the mutant allele affects the phenotype of the mouse.

Question 9

Why are Forward Genetic Screens rarely done in mice?

Answer 9

It is too expensive and can be slow.

Question 10

In what organisms are Forward Genetic Screens performed in?

Answer 10

Model organisms such as fruit flies or C. elegans or zebrafish

Question 11

Describe the 3 steps of Reverse Genetics with mouse and flies.

Answer 11

1) The mouse Hox genes were first cloned using Drosophila homeotic genes are probes.
-This is where we find the gene that has the mutant phenotype we are interested in in Drosophila. Then, we look for mouse genes that cross-hybridize to this gene.

2) A mutant version of a mouse Hox gene was then created in mouse ES cells using Homologous Recombination.

3) Mutant mice were derived from the mutant ES cells and we look to see what the mutant phenotype was.

Question 12

In mammals, how many clusters of Hox genes are there?

Answer 12

All mammals have 4 clusters of Hox genes.

Question 13

Mammals have _______ Hox genes than fruit flies.

Answer 13

more

Question 14

More complex animal = _______ Hox genes.

Answer 14

more

Question 15

The fact that mouse chromosomes share several genes that come from the Drosophila suggests what?

Answer 15

This suggests that there is to some degree a “universal body plan” for animal development.

Question 16

How many different types of homeotic genes are found in mice? How many of the complexes contain all of them?

Answer 16

There are 13 different types of homeotic genes in the mouse. NONE of the four Hox complexes contain all 13.

Question 17

Name the Hox clusters and what chromosomes they are on.

Answer 17

HoxA = chromosome 6
HoxB = chromosome 11
HoxC = chromosome 15
HoxD = chromosome 2

Question 18

In mammal embryos, there is a _______________ orientation.

Answer 18

segmental

Question 19

Describe the relationship somites have for their vertebrae.

Answer 19

There is a segment-parasegment kind of relationship. This means that a vertebra comes from the posterior half of one segment and the anterior half of another segment.

Question 20

In regards to Hox gene expression, what does Polycomb heterochromatin do?

Answer 20

Polycomb heterochromatin limits Hox gene expression to specific sets of segments.

Question 21

T/F: At the tail, no Hox genes are expressed.

Answer 21

False! At the tail, all Hox genes are expressed.

Question 22

Where does expression of HoxC-6 begin?

Answer 22

At the posterior boundary of the neck vertebrae (basically, begins starting after the neck).

Question 23

What does HoxC-6 gene silencing correlate with?

Answer 23

The number of neck vertebrae.

Question 24

What is unique about snake HoxC-6 gene expression?

Answer 24

The HoxC-6 gene is expressed all of the way through, so there is no neck or HoxC-6 limb, cuz it makes up the whole body.

Question 25

At what vertebra in mice does HoxC-6 expression begin at?

Answer 25

Expressed starting at the 8th vertebra in mice

Question 26

What do limbs start off as in the embryo?

Answer 26

Limb buds (4 limb buds, two for legs, and two for arms).

Question 27

Over time, what do limb buds do?

Answer 27

Over time, limb buds get bigger to develop into limbs and then digits will appear.

Question 28

What type of approach do developmental biologists use to find out what a gene does?

Answer 28

Find it, Lose it, Move it.

Question 29

Describe “Find it”.

Answer 29

This is where we describe the gene location, and correlate it.

Gene expression that is in the right place at the right time.

Question 30

What are two methods we use to “Find it”?

Answer 30

In situ hybridization: We hybridize a probe into an embryo to the mRNA.

Immunofluorescence: Antibodies with a fluorescent protein bind to the protein from the mRNA to show the location of the protein.

Question 31

Describe “Lose it”.

Answer 31

This is when we do loss-of-function to figure out what it does.

Basically, we remove it to deem why it is necessary.

Question 32

What can we perform to “Lose it”?

Answer 32

Genetic mutations/deletions (knock out)
siRNA (knock down)
antisense morphilinos oligos (knock down)

Basically, mRNA cannot be translated.

Question 33

Describe “Move it”

Answer 33

This is when we move something to give a gain-of-function and observe sufficiency.

Question 34

What is an example of “Move it”?

Answer 34

Zone of Polarizing Activity (ZPA).

A limb bud has a ZPA that gives rise to a normal chicken wing. A limb bud with a second ZPA that got moved onto it gives rise to a wing with a second set of digits.