S is for Sister Chromatids

S is for Sister Chromatids

Sister chromatids are two identical chromosomes attached to each other at the centromere. Since they are identical, they carry exactly the same genes. So what's the point of having two identical chromosomes attached to each other? This is the body's way of making more cells. When it undergoes mitosis the DNA needs to replicate itself so the new daughter cell will have the same amount of genetic material.

V is for Vesicle

V is for Vesicle 

A vesicle is something that is used to carry molecules through the body. Since the body consists of mostly water, these vesicles need to be hydrophilic on the outside, but most molecules in need of transport are hydrophobic, so the inside of the vesicle must be hydrophobic. This means that they can be made from phospholipids. Phospholipids have a hydrophobic tail and a hydrophilic body (and they look like sperm).



This is a picture of a liposome. It has a phospholipid bilayer to accommodate hydrophilic molecules.

Since there are so many molecules that need to be transported in the body there are many, many vesicles. The ones that I am most familiar with are those that carry neurotransmitters because that is what I am currently learning about in my psychopharmacology class. In our neurons we have vesicles that take our neurotransmitters to the presynaptic part of the nerve. Once the vesicles reach the destination they push the molecules through their membrane and out into the cytoplasm or cell body (wherever they are, really).

X-Linked Traits

X-Linked Traits

X-linked traits are carried on the X chromosome. Since males have one X chromosome and one Y chromosome, they are more susceptible to X-linked diseases, such as hemophilia. There are other traits found on the X chromosome that are not diseases, such as pattern baldness. Since males only have one X chromosome they only need one copy of the gene to start early balding, females need two. Since this ratio is so skewed, there are much fewer females with X-linked diseases than males.

The picture above shows the inheritance of X-linked recessive traits in families. A father cannot pass on an X-linked disease to a son, only a daughter. A daughter will only show the trait if she receives both affected alleles from both parents. A only has to receive the affected allele from the mother to get the disease.

Some X-linked traits are hemophilia, color blindness, and pattern baldness. Do you know of any others?

Y Chromosome

Y Chromosome

figured I talked about the X chromosome when I wrote about X linked traits, why not talk about the Y chromosome? Only males have a Y chromosome and it is considerably smaller than the X chromosome. This means that it contains much less genetic information (which is why females have Barr Body formation).

Scientists believe that the Y chromosome is one of the fastest evolving pieces of genetic material in humans with a 30% difference in humans and primates. It contains somewhere between 70 and 200 genes, which they are actively working on identifying. While the Y chromosome does contain genes that are only found on this one chromosome, there are areas that have pseudoautosomal genes (an autosomal chromosome is a non sex determining chromosome), which means they are found on both the X and Y chromosome.

There are diseases that can be associated with the Y chromosome, such as 46,XX where a male will have two X chromosomes and one Y, 48,XXYY where he will have double of both sex chromosomes, and 47,XYY where he will have two copies of the Y chromosome. These can have symptoms anywhere from a tall stature to infertility.

Z-DNA

Z-DNA

Believe it or not there is more than one possible structure for DNA! There are three: A-DNA, B-DNA, and Z-DNA.

Z-DNA is a left handed double helix structure, which is the opposite of the most common form, B-DNA. There are also many other differences than the common form, such as base pairs per turn. The Z form has 12 bases per turn while the B form only has 10.5. The biological significance is still largely unknown but research is going on to determine it. Right now they think that it has something to do with supercoiling during DNA transcription. This will allow the DNA to open up and allow transcription factors to bind, allowing mRNA to be made and proteins to be created.

All in all, there isn't much known about Z-DNA. Scientists are working on it though!

So this is the last post for the A to Z Challenge. I can't believe that it's already over! May 7th is the date to write a reflections post on the challenge, to talk about pros and cons, what you liked most, what you didn't like, if you met anyone cool, stuff like that. Visit the A to Z Challenge site to learn more!

Race for the Double Helix

Race for the Double Helix

The Watson-Crick model of DNA was a Nobel Prize winning structure. But what differed from this one to all of the other DNA structures that already existed? Well, Watson and Crick discovered the DNA was double-helical. Previously it was believed that DNA had a triple helix. Watson and Crick knew that the phosphates could not be located on the outside while the bases were on the inside.

The novel feature of their structure was that the two chains were held together by a purine and pyrimidine base. They are joined in pairs, a single base from one chain being hydrogen-bonded to a single base from the other chain, so that the two lie side by side with identical z-co-ordinates. Adenine (purine) is paired with thymine  (pyrimidine), and guanine (purine) is paired with cytosine (pyrimidine). Many times it will be written like: A-T, G-C.

As we already know, DNA consists of two strands, and each of those strands consist of a phosphate-sugar-phosphate backbone with bases (A, T, C or G) on the sugar molecules. The two strands run opposite of each other, with the bases pointing inwards and (as mentioned above) adenine is always paired with thymine, and guanine is always paired with cytosine. Therefore, if you know the sequence of one strand, then you know the sequence of the other strand. They are mirror images in reverse.

How You Taste Fat May Be Genetically Determined

How You Taste Fat May Be Genetically Determined

As the human genome is being explored in more detail, the genetic contribution to obesity is becoming increasingly recognized.  While we know of at least 45 genes that contribute to obesity, little is understood about how they work.  A new study has discovered a gene that affects how we sense and taste fat in our mouths, and postulates that this gene may be one more mechanism that contributes to the development of obesity in people who are genetically prone.

The study, conducted by MY Pepino and colleagues at Washington University, looked at 21 people with obesity and different variants of a gene called the CD36 gene.  They found that people who had two copies of a certain variant of the CD36 gene had an 8 fold lower threshold for sensation of fat than people who had no copies of this gene variant.  In other words, people with two copies of the gene variant were far more sensitive to the taste of fat than people without this gene variant.

Exactly how these genetic differences affect food intake is not known.  It may be, for example, that people who are less sensitive to the taste of fat need to eat more fat to feel satisfied.   Further study is needed to understand how the difference in sensitivity to the taste of fat may affect food intake and body weight.

What is increasingly clear is that genetics have a powerful role in the risk of obesity, in the context of the toxic enviroment in which we live.