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Author Topic:   Introduction to Genetics
Taq
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Posts: 9973
Joined: 03-06-2009
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Message 235 of 236 (833235)
05-18-2018 4:08 PM
Reply to: Message 234 by Faith
05-18-2018 11:01 AM


Re: Picturing DNA as related to phenotypes and diseases
I may quote your previous post out of order so that my explanations will make more sense.
Faith writes:
Does "two copies" refer to the two sides of the DNA strand?
No. The "two copies" refers to two separate chromosomes in a chromosome pair. Complementary DNA is the DNA on the other strand while alleles are copies of a gene found on separate chromosomes. You get one chromosome of each pair from each parent so that you have two alleles for each gene. This is what the chromosomes look like in the nucleus of human cells:
An allele is the copy of a specific gene found on each of those paired chromosomes.
Is there a "sidedness" to the strand?
Yes. The "direction" of each strand is determined by the carbons in its sugar backbone. DNA is copied and transcribed in what is called the 5' to 3' direction. DNA sequences are often written with the 5' and 3' designations, especially when showing double stranded DNA. For example:
5'--ATATTCGCATT--3'
3'--TATAAGCGTAA--5'
If you write a DNA sequence without the 5' and 3' designations then it is assumed the first letter is on the 5' side.
There are youtube videos on the subject if you are interested:
https://www.youtube.com/watch?v=IV53GZGr11g
It is also important to note that the two strands go in opposite direction, and genes can be found on either strand throughout a chromosome. The strands are arbitrarily labeled the + or - strand.
For instance reading about genetic diseases I found out that a mutation in a "single gene" or "single copy of a gene" usually doesn't cause disease, that disease usually (though not always) manifests only when two copies of a gene have the same mutation (meaning one copy got inherited from each parent.)
In general that is true. If one allele produces a non-functional protein while the other allele produces a functional protein the functioning allele can usually make up for the non-functional one. If you have two copies of the recessive non-functional allele then you may have problems.
However, a mutation in one allele can produce a protein that causes disease all on its own which a non-disease allele can not make up for. This is the case with achondroplasia (aka dwarfism) where having just one allele can cause dwarfism and having two disease alleles results in a nonviable embryo. You can also have alleles that are equally functional, as in the case of blood types.
Is there a way to recognize a recessive versus a dominant gene by the appearance of the DNA double helix itself?
Not to any great extent. It is very difficult to know if a mutation is going to produce non-functional protein that is also non-detrimental, a protein with the same function, a protein with a different function that combines with the other allele in an equal fashion, or a mutation that results in a protein that causes disease all on its own.
And this raises a question about recessive versus dominant. You can have either a BB for brown eyes or a bb for blue eyes or the heterozygous Bb that also produces brown eyes. So are we talking about the two DNA sides here and does it make sense to say "homozygous recessive" and "homozygous dominant" or what?
If you have two copies of the same allele then you are homozygous. If you have two copies of different alleles then you are heterozygous. BB and bb are homozygous while Bb is heterozygous.
I also found it interesting that the site said mutations can cause either missing or deformed proteins.
Mutations can also result in proteins with different functions or the same function. For example, if you compare the cytochrome C gene found in humans with the same gene in yeast you will find that they differ by 40% yet they still function in the same way.

This message is a reply to:
 Message 234 by Faith, posted 05-18-2018 11:01 AM Faith has replied

Replies to this message:
 Message 236 by Faith, posted 05-19-2018 10:47 AM Taq has not replied

  
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