Can you help me understand basic biology, please?

Question

What are the junk genes and why we have junk genes?

What is genes? What are the junk genes?

What are the junk DNA? What are the noncoding and coding DNA?

How we all they different DNA code, even tough T and A, C and G always pair together? How we have different DNA? Can you help me understand this process? I am bilingual, so do not use hard English words. Thank you so much.

Answer 1

"Junk" genes are genes that are no longer used to make proteins in the cell.

A gene is a stretch of DNA that codes for a particular protein, and that is usually turned "on" or "off" (transcribed or not transcribed) in response to the binding or coming off of certain molecules called "transcription factors".

Here is a diagram of a gene:

http://www.ncbi.nlm.nih.gov/Class/MLACou...

The "promoter" region is where a molecule binds to make transcription of DNA (copying it to RNA) start or stop.

The "start" sequence is 200+ base pairs long and has more C/G pairs than usual. That is where transcription of RNA starts.

When DNA mutates - which can happen by mistake when it is copied during cell division, or from exposure to DNA-attacking chemicals, or from exposure to UV light, or from a virus splicing its DNA into the gene - the promoter or start sequence can get messed up, so the gene stops responding and becomes "junk". It accumulates lots of mutations, since it's out of use; harmful mistakes can no longer hurt the cell.

A lot of junk DNA got deactivated a long time ago, so it's just stretches of DNA containing what used to be genes that we no longer need to survive. It's just there.

Noncoding DNA is just DNA that doesn't have instructions for making proteins. Coding DNA is DNA that does have instructions for making proteins.

When DNA is copied into messenger RNA (mRNA), and the mRNA leaves the nucleus and goes out into the cytoplasm (the rest of the cell), the mRNA is further edited. "Introns", which are noncoding regions of the mRNA, are taken out, and "exons" (which are "EXpressed" as proteins) are spliced together.

Then, the edited mRNA joins with a ribosome, which is an organelle (cell "organ") that makes proteins, and the ribosome starts attracting amino acids - which make up proteins - to it, in the order specified on the mRNA.

Each amino acid has a 3 nucleotide code (A,U,C and G are all nucleotides - T only exists in DNA, U is its RNA replacement). For example, methionine is AUG.

Since each amino acid has a 3 nucleotide code, you could technically read mRNA 3 ways. This is called "open reading frame", since you don't really know how to read it.

Say the RNA is

CAAGAUGCACGCUACG...

You could read it:

CAA GAU GCA CGC UAC G..

or

..CA AGA UGC ACG CUA CG..

or

..C AAG AUG CAC GCU ACG

But you need a frame of reference to know which is right, otherwise you would have a completely wrong string of amino acids and a completely wrong protein!

So, the ribosome scans along the mRNA until it comes to an AUG, and that's when it starts reading the code and "translating" it into an amino acid sequence, which then becomes a protein.

There are 20 standard amino acids, and each one has 1-4 ways of being written in the 3-nucleotide code. So that's how the various possibilities of the 3 nucleotide code are all covered by amino acids :)

3 of them are "stop" codons, which just make the ribosome stop translating and leave the mRNA.

http://www.cbs.dtu.dk/staff/dave/roanoke...

And since proteins are often hundreds of amino acids in length, and the lengths differ widely, there's almost infinite possibilities for different proteins.