Homeobox patterning of segments

Question 1

what is a homeodomain?

Answer 1

a family of transcription factors that are involved in the patterning of different parts of the body. they are coded for by the homeobox. different. different homeodomains (hox) encode for transcription factors that act on certain genes which need to be acted on in order to form the part of the body needed

Question 2

where are the homeotic genes found on a drosophila?

Answer 2

on chromosome 3.

Question 3

how many homeotic genes are there in the dros?

Answer 3

the antennapedia complex contains lab dfd scr antp and pb

the bithorax complex contain ubx, abd-a and abd-b

Question 4

what is the homeotic complex?

Answer 4

the chromosome region containing the antennapedia complex and the bithorax complex.

Question 5

what is the antennapedia mutation and how does it occur, what does this show?

Answer 5

when the antp gene is expressed in the head, legs rather tan antennae grow out of the head sockets. the antp protein binds to and represses the enhancers of homothorax and eyeless which are needed for antenna and eye formation.

Question 6

what is the result of the ubx deletion?

Answer 6

When the Ultrabithorax gene is deleted, the third thoracic segment (characterized by halteres) is transformed into another second thoracic segment.- four wings fly

Question 7

how are the homeobox genes regulated? give 2 examples?

Answer 7

the initial domains of homeotic genes are influence by the gap and pair rule genes.
abda and abdb is repressed by hunchback and kruppel, preventing them from being expresed in the head and the thorax.
antennapedia is activated by particular levels of huncback

Question 8

what happens if they entire bithorax complex is deleted? why is this?

Answer 8

antp is expressed in the entire region, all bithorax genes repress the expression of antp.

Question 9

how are the hoh genes expressed interegulated?

Answer 9

there are once again mutual repression between each

Question 10

how do homeotic genes have an effect?

Answer 10

e homeotic genes work by activating or repressing a group of “realisator genes”—those genes that are the tar- gets of the homeotic gene proteins and that function to form the specified tissue or organ primordia

Question 11

dictate an example of a homeobox gene in the drop controlling the patterning on a parasegment

Answer 11

the posteri- or spiracle—is well on its way to being elucidated. This organ is a simple tube connecting to the trachea and a pro- tuberance called the “Filzkorper” (see Figure 6.23D). The posterior spiracle is made in the eighth abdominal seg- ment and is under the control of the Hox gene AbdB. Love- grove and colleagues (2006) have found that the AbdB protein controls four genes that are necessary for posteri- or spiracle formation: Spalt (Sal), Cut (Ct), Empty spiracles (Ems), and Unpaired (Upd). The first three encode transcrip- tion factors; the fourth encodes a paracrine factor. None of them are transcribed without AbdB. Moreover, if these genes are independently activated in the absence of AbdB, a posterior spiracle will form.
Controlled by AbdB, these four regulator genes in turn control the expression of the realisator genes that control cell structure and function. Spalt and Cut encode proteins that activate the cadherin genes necessary for cell adhesion and the invagination of the spiracle. Empty spiracles and Unpaired encode proteins that control the small G proteins (such as Gef64C) that organize the actin cytoskeleton and the cell polarizing proteins that control the elongation of the spiracle

Question 12

what is the standard body plan of the fly?

Answer 12

3 head segments, 3 thorax regions, 8 abdominal segments

Question 13

how are segments and parasegments related?

Answer 13

the engrailed in the parasegments signals the anterior portion of a parasegment and the posterior portion of a segment.

Question 14

do segments or parasegments determine homeotic expression ?

Answer 14

parasegments

Question 15

how are segments divided between parasegments?

Answer 15

the anterior portion of one parseg and the posterior of another paraseg makes up a segment.

Question 16

how do parasegment gene expression relate to gene segment gene expression?

Answer 16

Segments are named based on visual examination of the adult fly
Parasegments correspond to the actual spatial domain along the AP axis over which a specific selector genes exert their patterning function

Question 17

describe the regulation of box gene expression

Answer 17

Regulation by segmentation genes: the control elements described above are likely to be complex integrators of a variety of information from the gap, pair-rule, and segment polarity genes. The domains of expression of the homeotic genes are parasegmental. Therefore the limits of expression must be controlled at least partially by the pair-rule and segment polarity genes. Mutations in pair rule genes do affect homeotic gene expression. However, since various pair-rule stripes are not molecularly distinguishable from one another, additional information is required to determine where along the AP axis particular homeotic genes are active. This type of information already exists at the time the homeotic genes are activated in the spatially ordered domains of gap gene expression. As an example, hunchback seems to set the anterior limits of Ubx expression. In hunchback mutant embryos, Ubx expression expands anteriorly. hunchback protein can bind to cis-acting control regions of the Ubx gene and mutations of hunchback binding sites in the enhancer region obviates hunchback mediated repression. (Gap genes)

Question 18

what did Lewis observe in his experiments that brought him to his model?

Answer 18

• based on the bithorax complex only-found that recessive mutations (loss of function of underlying gene) caused the transformation each parasegment to their anterior segment. he suggested that there was a gene which determine the patterning for each parasegment. that the most anterior one was expressed in all segments and as you moved along the ap axis, a new gene was expressed and that the identity of each para set was determined by the most ventral gene. therefore when a gene was knocked out the most ventral one was that in the paraseg anterior to it. he suggested there was a repressor molecule which was strongest anterior and weakest posteriorly and that the sensitivity to the repressor increased posteriorly- so that paraseg 14, the most senstive could only be expressed the furthest away

Question 19

How did lewis’ mutations match his model?

Answer 19

he found that when the entire bithorax regionall posterior segments was ps4 identity

• found 2 genes mutations which gave all ps14 identity- he postulated this was because the repressor was knocked out.
• dominant mutations (gain of function) which made genes more posterior- said they reduced the genes sensitivity to the repressor

Question 20

what proved lewis’ model wrong?

Answer 20

it predicted that there would be 9 transcription units in the bithoarx complex- but there were only 3.

Question 21

how was lewis’ model adjusted to the truth/

Answer 21

there are three transcription factor for three hox genes. but it is true that the most ventrally expressed dictates the identity. however it also based on the level of expression these hox genes. the mutations that affect single parasegment affect cis-regulatort region of ubx abd-a or abd-b- ubx expression is lower in ps5 than ps6

Question 22

what is the conserved part of homeotic genes?

Answer 22

the homeodomain or hox sequence is 60 amino acid DNA binding domain

Question 23

what is the relationship between hox proteins between organisms?

Answer 23

dros and vert hox proteins are very similar but vertebrate hox genes are found in 4 clusters on 4 different chromosomes

Question 24

what is colinearity?

Answer 24

The more anterior genes of each cluster are expressed earlier in development. This is called co-linearity