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Tag Archives: prokaryotic

A quick followup on that lac operon post

Last week I posted a quick link about operons for my micro class to check out before taking their quiz on bacterial gene regulation This post is intended to complement that one. To go back to that post, click here. If there’s one thing to remember about operons it is that bacteria, lacking a nuclear membrane, regulate their genes differently than Eukaryotes. Having a nuclear membrane separates transcription and translation into two distinct compartments allowing for more subtle tweaking of Eukaryotic mRNAs before they are exported for translation.

Image

Click on this figure to go to a good description of how polycistronic genes work

One thing this does is it makes it very beneficial to package genes with related function closely on the genome and use a single regulatory region to control them all together. They wind up getting packed so closely together that they are actually expressed as a single messenger RNA – known as a polycistronic (meaning ‘many gene’) message.

Upstream of this polycistronic cassette are regulatory elements. One element common to all regulatory elements is the promoter. The promoter consists of several elements which ‘promote’ the binding of an RNA polymerase to the DNA. Additional regulatory elements exist to ensure that this polymerase only transcribes the genes if they are needed. In doing so, the cell conserves energy and components (e.g. Amino Acids) for only necessary processes.

In the case of the paradigm lac operon, lactose is a fuel source, but not as good as glucose. Therefore, enzymes to digest lactose are only needed when lactose is present, but glucose is not. In order to interrogate both conditions, two additional regulatory elements are present.

First, the operator sequence. This sequence binds a repressor protein that physically blocks the polymerase’s path in the absence of lactose. However, if lactose is present, the sugar binds to the repressor, causing a conformational (shape) change that causes the protein to release its grip on the operator sequence.

Second, a catabolite activator protein (CAP) will only bind to the DNA behind the RNA polymerase if cAMP is present. Let’s not get too distracted, other than to say that cAMP levels are high in the ABSENCE of glucose, and low when that sugar is present. When cAMP binds to the CAP protein it can now bind the DNA and do it’s other job: making a nice binding site for the RNA polymerase. Without CAP, the polymerase binds very inefficiently.

Together, the production of lactase enzymes (those that digest lactose) is exquisitely controlled in a way that conserves the most energy.

ImagePs – take a look at this graph and tell me why (not mechanistically, but rationally) the cell does not make lactase enzymes when both glucose and lactose are present.

 
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Posted by on March 15, 2014 in Uncategorized

 

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Aside

Cell Division is an essential part of life. However, this means different things to different cells. For single-celled organisms cell division is reproduction – one mother cell divides into two daughter cells. If these single-celled organisms are prokaryotic, then this division happens by the relatively simple process of binary fission. If the organism is eukaryotic, division occurs by a more structured process of mitosis.

Regardless of the differences between these organisms, cell division consists of three core components:

  1. Duplication of DNA
  2. Segregation of DNA (into areas that will become the new cells)
  3. Cytokinesis

In Prokaryotes, this describes the cellular events perfectly. The circular DNA (arguably a chromosome) is duplicated, the two copies of the DNA goes to opposite sides of the ce

ll and cytokinesis divides the cell in half along the midline.

An illustration of a number of bacterial cells undergoing binary fission:

The entire process is outlined below:

In eukaryotes, the same basic process occurs, but because of the differences in how DNA is organized and localized (in a nucleus), differences emerge.

The basic form of eukaryotic cell division is called mitosis. In single celled organisms this process functions as an asexual form of replication, while in multicellular eukaryotes, this form of division serves to add to the cell number (i.e. growth / healing).

Mitosis is actually the division of the nucleus and DNA, this process is accompanied by the division of the cell itself, cytokinesis.

There are four phases of mitosis:

  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase
  5. Cytokinesis overlaps somewhat with Telophase, but continues on afterwards to complete cell division.

Each stage of Mitosis is defined by a number of events describing the organization of the DNA, the state of the nuclear membrane and the development of mitotic spindles that maneuver the chromosomes within the cell. I won’t describe these here as this can be found in any number of other sources.

What I will discuss is the pattern of chromosome number, DNA copy number and the chromosome number.

The chromosome number is referred to as ‘n’.

The DNA copy number is referred to as ‘c’.

 Humans have an n number of 23. What this number tells you is how many pairs of homologous chromosomes an organism has.* This immediately begs the question, ‘what’s a homologous chromosome?”

Homologous chromosomes are those that have the same genes – that is, they code for the same traits even if they do not have exactly the same versions of these genes on each chromosome. For each of the 23 different kinds of chromosomes humans have, each person gets one copy from their mom and one from their dad. So these 23 pairs of chromosomes are 23 pairs of homologous chromosomes.

This 23 number we keep talking about is the n number in humans.  Each species n number may be different, but all the members of that species have the same n number. Further, the n number does not change at any time during the cell’s life

Because we have one chromosome from each of our parents in every one of the cells of our body we are diploid organisms (from the Latin, di – meaning and two ploid – well, ‘ploid’ doesn’t really come from Latin)

All the cells in our body are diploid – except the gametes (sex cells). These cells are formed by a special kind of cell division called meiosis. This type of division is very similar to mitosis, except that it consists of two separate rounds of division and the resulting sex cells have only one of each chromosome type and are therefore called haploid (think ‘half’)

Cells of the body are 2n = diploid.

Sex cells are n = haploid.

This leaves the c number…

The easiest way to determine the c number is to simply count the number of chromatids for each type of chromosome.  Consider the cells below. On the left is a cell in G1 phase of the cell cycle. Then the cell goes through S phase where DNA replication occurs and looks like the cell on the right.

Note that the n number does not change, but the c number doubles when the number of chromatids doubles.

* It’s important to note here that not all organisms have similar patterns of chromosome arrangements as we humans do. In this essay I am referring to humans and organisms that handle their DNA as we do.

Cell Division i…

 
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Posted by on October 10, 2012 in Uncategorized

 

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