Campbell and Reece 8th edition Cell Biology

[Stephen1993]

hello

sorry but i got an urgent desperate question i want to ask because this is very important!

About the regulation of gene expression in bacteria
What does it mean that in a repressible operons that the default state is ON?
and the opposite for inducible operons
what does default state mean? i tried looking that up in the dictionary

thank you

 

[mishrashubham]

hello

sorry but i got an urgent desperate question i want to ask because this is very important!

About the regulation of gene expression in bacteria
What does it mean that in a repressible operons that the default state is ON?
and the opposite for inducible operons
what does default state mean? i tried looking that up in the dictionary

thank you

An operon is a group of genes that is under the control of a single promoter. Their expression is regulated by proteins called repressors which bind to the DNA and prevent transcription which is decided based upon environmental factors. In repressible operons, the repressors are normally not binded to the DNA in normal conditions; however and inducer can interact with the repressor and cause it to bind with the DNA and hence stop exression of the genes.

It is just the opposite in inducible operons where under natural conditions the repressor prevents expression and an inducer is need to remove it.

Thus default state is the condition in which it is present under normal conditions.

 

[Stephen1993]

thank you for that

so it this something to do with the repressor protein made constitutively?

thank you

 

[mishrashubham]

thank you for that

so it this something to do with the repressor protein made constitutively?

thank you

Sorry I don't get you. What do you mean by "made constitutively"?

 

[Stephen1993]

sorry about that

the repressor protein is made constitutively means that it is produced ALL of the time.

 

[mishrashubham]

sorry about that

the repressor protein is made constitutively means that it is produced ALL of the time.

I really don't know. But it definitely depends on the type of gene that the transcriptional repressor acts upon. I think proteins required for the expression of genes in response to a regular environmental stimulus, for example those maintaining the circadian rythm, would be constantly in need. However I cannot say for sure. We will need experts on the topic.

 

[mishrashubham]

i thought so
well thank you for that

next question:
"During anaphase, the region of overlap is reduced as motor proteins attached to the mircotubules walk them away from one another, using energy from ATP."

i really have trouble with this as i am not sure what they originally mean by the "walking" bit?

thank you

Motor proteins such as kinesins, that move along microtubules, have a motion similar to the way humans walk. Wikipedia has a nice animation. http://en.wikipedia.org/wiki/Kinesin

One more question, are you sure you are reading the textbook in an ordered and systematic manner? Because in many of your questions, you seem to miss some prior information.

 

[Stephen1993]

thank you for that

well i have studied the stuff before but now i would like to ask some questions that i don't get before

Steps in endosymbiotic theory
"would be the engulfing of a photosynthetic prokaryote (cyanobacterium), the precursor of the plastids including the chloroplasts."
what does the word "precursor" used in this context?

another one
"A mutation can occur either within the protein-encoding region of a gene, or in a non-coding sequence. If a mutation occurs within a gene, it can change the gene from one form to a new form. However it remains the same gene."

firstly which is slightly out of this topic: i thought the coding part of the DNA are genes and the non-coding parts are not genes. so how come in the paragraph above that there is coding and non-coding part of the gene?

secondly what does the last sentence mean?

thank you

 

[mishrashubham]

Steps in endosymbiotic theory
"would be the engulfing of a photosynthetic prokaryote (cyanobacterium), the precursor of the plastids including the chloroplasts."
what does the word "precursor" used in this context?

It is theorized that the early eukaryotic cells engulfed cyanobacteria as a symbiont which then became plastids. Therefore precursor simply means that these prokaryotes would eventually become plastids.

"A mutation can occur either within the protein-encoding region of a gene, or in a non-coding sequence. If a mutation occurs within a gene, it can change the gene from one form to a new form. However it remains the same gene."

firstly which is slightly out of this topic: i thought the coding part of the DNA are genes and the non-coding parts are not genes. so how come in the paragraph above that there is coding and non-coding part of the gene?

Again missing some basic info.
There are stretches of DNA within genes which have no apparent function and do not code for any proteins, called introns.

secondly what does the last sentence mean?

That is how different alleles of genes are formed.

 

[bobze]

sorry about that

the repressor protein is made constitutively means that it is produced ALL of the time.

Yes constitutively is the correct word. And yes, they can be made constitutively.

 

[bobze]

Some of this stuff, you'll get a better explanation for and more in depth understanding of the further along in your studies you get. For instance with the walking, you'll learn later in high level biochemistry or cellular biology classes about the structure of http://en.wikipedia.org/wiki/Dynein" and other ATP-dependent motor proteins (like myosin) and how they "cock", "release" and "lever" forward.

This is an incredibly in depth subject, so I think it would be best (for your own sake) to learn it when the time is appropriate in your formal education and not over a forum (which is somewhat of a daunting task having people trying to explain something so complex to you over the internet).

Aside from that, to supplement your understanding and preview the [STRIKE]horrifying[/STRIKE] complex depth of knowledge to come, I'd honestly take a peek at some wiki pages on the subject. While it certainly isn't an academic standard, there are good articles, well written by people in the field nevertheless. Of course, use that as a spring board and just a way to help augment your formal studies (your textbooks, syllabus, class notes etc) and not the primary source of your information.

well i have studied the stuff before but now i would like to ask some questions that i don't get before

Steps in endosymbiotic theory
"would be the engulfing of a photosynthetic prokaryote (cyanobacterium), the precursor of the plastids including the chloroplasts."
what does the word "precursor" used in this context?

Like mitochondria, chloroplasts were very, very likely at one time a free living organism. Who, again like mitochondria and possibly some other organelles, traded off a free living life for one of a symbiotic-dependence. "Precursor" then, is simply referring to those prokaryotic ancestors of modern chloroplasts.

another one
"A mutation can occur either within the protein-encoding region of a gene, or in a non-coding sequence. If a mutation occurs within a gene, it can change the gene from one form to a new form. However it remains the same gene."

firstly which is slightly out of this topic: i thought the coding part of the DNA are genes and the non-coding parts are not genes. so how come in the paragraph above that there is coding and non-coding part of the gene?

secondly what does the last sentence mean?

thank you

The "idea" of a gene has changed over time. It used to be thought that 1 gene=1 protein in neat little ratios. Turns out, that biology is much messier than that. Most of our DNA doesn't code for proteins, heck most of a "gene" doesn't code for a protein. Large sums of it is given over toward regulatory regions, which bind inducers and suppressors and activators and modify the structure for access via replication/transcription machinery, etc.

Like Mish points out then, this is how alleles or gene "flavors" are made. A loci is a spot on a chromosome that is occupied by a gene. A change in that gene (either coding or non-coding regions) doesn't change the genes "address" on the chromosome. Thus, the mutation can make a new "flavor" (read: allele) of gene, but it isn't moving the gene itself (don't fret though, once you wrap your head around all that--They'll tell you that some types of mutations do changes a gene's "address" , don't you love learning?).

 

[mishrashubham]

Yes constitutively is the correct word. And yes, they can be made constitutively.

Thanks Bobze for clearing that out.

The "idea" of a gene has changed over time. It used to be thought that 1 gene=1 protein in neat little ratios. Turns out, that biology is much messier than that.

Seriously. I received a rude shock when I learned about alternative splicing. I am still wondering; can biology get any messier?

 

[bobze]

Thanks Bobze for clearing that out.



Seriously. I received a rude shock when I learned about alternative splicing. I am still wondering; can biology get any messier?

That's just the overture, wait till you get to the 2230948320947190873487 ways of post-translational modifications

 

[mishrashubham]

That's just the overture, wait till you get to the 2230948320947190873487 ways of post-translational modifications

Hehe big random numbers don't scare me . Anyways I will be looking forward to it.

 

[Stephen1993]

thank you for that guys

Specificity of viruses
“For example, the HIV virus attaches to a protein, called CD4, which is found on the surface of human helper T cells. The bacterial virus lambda uses its tail to attach to an E. coli protein whose job is to transport maltose sugar into the cell.”
About the last sentence what is the point for the virus to transport maltose in the cell?

“Dormant proviruses can be induced, i.e. they can become active and undertake lytic development. Regression is lifted and expression of lytic genes begins.”
What do they mean by “repression is lifted”?

thank you

 

[bobze]

thank you for that guys

Specificity of viruses
“For example, the HIV virus attaches to a protein, called CD4, which is found on the surface of human helper T cells. The bacterial virus lambda uses its tail to attach to an E. coli protein whose job is to transport maltose sugar into the cell.”
About the last sentence what is the point for the virus to transport maltose in the cell?

Its not transporting maltose, its "acting" like maltose to gain entry into a cell.

“Dormant proviruses can be induced, i.e. they can become active and undertake lytic development. Regression is lifted and expression of lytic genes begins.”
What do they mean by “repression is lifted”?

thank you

Did you mean to type repression the first time above, and not regression?

Proviruses are viruses integrated into DNA. "Repression lifted" just means repression of viral proteins, which become active once it the virus moves into a lytic phase of reproduction. A good example is something like HSV-1, which lives in neurons until the lytic phase of its "life-cycle", travels down a neuron from your brain to the mucosa of your face and undergoes lytic replication, forming a blister and all.

 

[Stephen1993]

thank you for that

is there crossing over between the sex chromosomes? XX chromosomes in females and XY chromosomes in males?

thank you

 

[mishrashubham]

thank you for that

is there crossing over between the sex chromosomes? XX chromosomes in females and XY chromosomes in males?

thank you

Yes there is crossing over between sex chromosomes. XX in females cross over just like any other chromosome. However XY crossing over is different. Parts on the tips of the X and Y chromosomes are called pseudoautosomal regions . The crossing over between X and Y happens only in this region i.e. no other portion of the X or Y chromosomes is exchanged.

Check http://en.wikipedia.org/wiki/Pseudoautosomal_region

 

[Stephen1993]

thank you for that

pg 242
Loss of cell cycle controls in cancer cells

"They may make a required growth factors themselves, or they may have an abnormality in the signalling pathway that conveys the growth factor's signal to the cell cycle control system even in the absence of that factor."
What does the last part of the sentence mean?


thank you

 

[bobze]

thank you for that

pg 242
Loss of cell cycle controls in cancer cells

"They may make a required growth factors themselves, or they may have an abnormality in the signalling pathway that conveys the growth factor's signal to the cell cycle control system even in the absence of that factor."
What does the last part of the sentence mean?


thank you

Growth factors work through a number of steps which turns on or off replication machinery in the cell. This is called a http://en.wikipedia.org/wiki/Signal_transduction" , if a mutation occurs that results in one of those steps always "on" or always "off" the cascade can work, even in the absence of the initiating signal--Growth factors in this case.

 

[Stephen1993]

hello again

this is about mitosis and meiosis
what does "synapsis of homologous chromosomes" mean?
does not occur in mitosis and
occurs during prophase I along with crossing over between non-sister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion for meiosis

thank you

 

[bobze]

hello again

this is about mitosis and meiosis
what does "synapsis of homologous chromosomes" mean?
does not occur in mitosis and
occurs during prophase I along with crossing over between non-sister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion for meiosis

thank you

http://lmgtfy.com/?q=synapsis+of+homologous+chromosomes"

 

[Stephen1993]

thank you for that

Pg 297)
“As a consequence of the independent assortment of chromosomes during meiosis, each of us produces a collection of gametes differing greatly in their combinations of the chromosomes we inherited from our two parents. Figure 13.11 suggests that each individual chromosome in a gamete is exclusively maternal or paternal in origin. In fact, this is not the case, because crossing over produces recombinant chromosomes, individual chromosomes that carry genes (DNA) derived from two different parents (Figure 13.12).”
What does “Figure 13.11 suggests that each individual chromosome in a gamete is exclusively maternal or paternal in origin.” mean?

thank you

 

[atyy]

thank you for that

Pg 297)
“As a consequence of the independent assortment of chromosomes during meiosis, each of us produces a collection of gametes differing greatly in their combinations of the chromosomes we inherited from our two parents. Figure 13.11 suggests that each individual chromosome in a gamete is exclusively maternal or paternal in origin. In fact, this is not the case, because crossing over produces recombinant chromosomes, individual chromosomes that carry genes (DNA) derived from two different parents (Figure 13.12).”
What does “Figure 13.11 suggests that each individual chromosome in a gamete is exclusively maternal or paternal in origin.” mean?

thank you

A typical man's skin cells ,kidney cells, etc. have n pairs of chromosomes. In each pair of chromosomes, one is entirely from his mother, and one is entirely from his father.

When he makes sperm, each sperm contains n chromosomes. However, each of the n chromosomes in the sperm are not simply one of each of the n pairs, ie. each chromosome is not entirely from the man's father or entirely from his mother. This is because of homologous recombination during meiosis, in which homologous chromosomes pair up and exchange material, so that the resulting chromosomes each contain a mix of material from the man's father and mother. (Actually it's more complicated than this, during homologous recombination, "crossing over" happens with only one chromatid of each chromosome during meiosis.)

In http://highered.mcgraw-hill.com/olc.../120074/bio16.swf::Unique Features of Meiosis , we start off with two pairs of chromosomes (one long pair, and one short pair). In each pair, one chromosome is purple and one is blue, indicating that the chromosome is entirely from one of the man's parents. During homologous recombination, one chromatid in each pair becomes a mix of blue and purple, indicating that it is now a mix of genetic material from the man's father and mother.

 

[Stephen1993]

thank you for that

“Errors in meiosis or damaging agents such as radiation can cause breakage ofa chromosome, which can lead to four types of changes in chromosome structure (Figure 15.15). A deletion occurs when a chromosomal fragment is lost. The affected chromosome is then missing certain genes. (If the centromere is deleted, the entire chromosome will be lost.)”
what does the last sentence in the brackets mean?

thank you

 

[atyy]

thank you for that

“Errors in meiosis or damaging agents such as radiation can cause breakage ofa chromosome, which can lead to four types of changes in chromosome structure (Figure 15.15). A deletion occurs when a chromosomal fragment is lost. The affected chromosome is then missing certain genes. (If the centromere is deleted, the entire chromosome will be lost.)”
what does the last sentence in the brackets mean?

thank you

The centromere is what attaches the chromosome to the spindle. The spindle pulls the chromosome to the poles. If the centromere is deleted, the chromosome cannot attach to the spindle, and will not be pulled towards the pole. One of the 4 daughter cells will have one chromosome too few, and another will have one chromosome too many.

http://highered.mcgraw-hill.com/olc...072437316/120074/bio19.swf::Stages of Meiosis

 

[Stephen1993]

thank you for that

because the centromere attaches the chromatids together then if the centromere is deleted then the spindle cord/fibre can't pull the whole replicated chromosome but only pulls one of the chromatids?
or is the spindle fibre attach to the centromere which means that if the centromere is deleted then the whole replicated chromosome cannot be pulled?

thank you

 

[mishrashubham]

thank you for that

because the centromere attaches the chromatids together then if the centromere is deleted then the spindle cord/fibre can't pull the whole replicated chromosome but only pulls one of the chromatids?
or is the spindle fibre attach to the centromere which means that if the centromere is deleted then the whole replicated chromosome cannot be pulled?

thank you

Read what he wrote carefully again.

The centromere is what attaches the chromosome to the spindle. The spindle pulls the chromosome to the poles. If the centromere is deleted, the chromosome cannot attach to the spindle, and will not be pulled towards the pole. One of the 4 daughter cells will have one chromosome too few, and another will have one chromosome too many.

http://highered.mcgraw-hill.com/olc...072437316/120074/bio19.swf::Stages of Meiosis

 

[atyy]

Actually, I never studied this part very properly, so I'm not sure how many shades of centromere deletion can occur, or what other subtleties there may be.

 

[Stephen1993]

sorry about then

“A diploid embryo that is homozygous for a large deletion (or has a single Xchromosome with a large deletion, in a male) is usually missing a number ofessential genes, a condition that is ordinarily lethal.”
What does “A diploid embryo that is homozygous for a large deletion” mean?

thank you