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majör rna

8/1/2008 · Kategori: dna

Major RNAs

There are three major classes of RNA in the cell:

mRNA

messenger RNA carries the genetic information that will be expressed ultimately as proteins. In prokaryotes, it is short-lived and is turned over rapidly. Eukaryotic mRNA is more stable as a result of the modifications that are made to it after it is transcribed.

tRNA

transfer RNA is the adapter molecule. It recognizes the codons of the mRNA on the one hand, and it can be covalently bonded to the appropriate amino acid, on the other.

rRNA

ribosomal RNA is found in the ribosomes.

Prokaryotic ribosomes have 3 rRNA molecules: 23S, 16S and 5S.

Eukaryotic ribosomes have 4 rRNA molecules: 28S, 18S, 5.8S and 5S.

rRNA was once thought to be an inert scaffold for the ribosomal proteins. We now know that the 23S (and 28S) rRNA is the catalytic agent in protein synthesis.

Non-coding RNAs (ncRNA)

It has recently become clear that botyh prokaryotic and eukaryotic cells contain a large number of small RNA molecules with a diversity of ********s in processes such as regulation of tran******ion, repliation of eukaryotic chromsomomes, RNA processing, RNA modification, RNA editing, mRNA stability and degradatrion, regulation of translation, and protein translocation.

The following table from a review article in Science lists processes affected by ncRNAs and gives some examples.

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DNA is usually a double-helix and has two strands running in opposite directions. (There are some examples of viral DNA which are single-stranded). Each chain is a polymer of subunits called nucleotides (hence the name polynucleotide).

Each strand has a backbone made up of (deoxy-ribose) sugar molecules linked together by phosphate groups. The 3' C of a sugar molecule is connected through a phosphate group to the 5' C of the next sugar. This linkage is also called 3'-5' phosphodiester linkage. All DNA strands are read from the 5' to the 3' end where the 5' end terminates in a phosphate group and the 3' end terminates in a sugar molecule.

Each sugar molecule is covalently linked to one of 4 possible bases (Adenine, Guanine, Cytosine and Thymine). A and G are double-ringed larger molecules (called purines); C and T are single-ringed smaller molecules (called pyrimidines).

In the double-stranded DNA, the two strands run in opposite directions and the bases pair up such that A always pairs with T and G always pairs with C. The A-T base-pair has 2 hydrogen bonds and the G-C base-pair has 3 hydrogen bonds. The G-C interaction is therefore stronger (by about 30%) than A-T, and A-T rich regions of DNA are more prone to thermal fluctuations.

The bases are oriented perpendicular to the helix axis. They are hydrophobic in the direction perpendicular to the plane of the bases (cannot form hydrogen bonds with water). The interaction energy between two bases in a double-helical structure is therefore a combination of hydrogen-bonding between complementary bases, and hydrophobic interactions between the neighboring stacks of base-pairs.

Even in the single-stranded state, the bases prefer to be stacked (like the steps of a spiral staircase if the bases are identical) and a single-stranded chain can also have regions of helical conformation.

The backbone of polynucleotides are highly charged (1 unit negative charge for each phosphate group; 2 negative charges per base-pair). If there is no salt in the surrounding medium, there is a strong repulsion between the two strands and they will fall apart. Therefore counter-ions are essential for the double-helical structure. Counter-ions shield the charges on the sugar-phosphate backbone. They may also contribute to an attractive interaction from fluctuating counter-ions around the backbone, similar to the Van der Waals interactions for fluctuating induced dipoles.

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DNA - DEOXYRIBONUCLEIC ACID

1/12/2007 · Kategori: dna

DNA - DEOXYRIBONUCLEIC ACID

DNA is sometimes called "the blueprint of life" because it contains the code, or instructions for building and organism and ensuring that organism ********s correctly. Just like a builder uses a blueprint to build a house, DNA is used as the blueprint, or plans, for the entire organism.

It is the chemical component of chromosomes, which are located in the nucleus of every cell. Stretches of DNA (or stretches of chromosomes) code for genes.

Gene - a segment of DNA that codes for a protein, which in turn codes for a trait (skin tone, eye color..etc), a gene is a stretch of DNA.

The structure of DNA was established by James Watson and Francis Crick.

The shape of the DNA molecule is a double-helix (like a twisted ladder). The sides of the ladder are composed of alternating sugars (deoxyribose) and phosphates. The rungs of the ladder are composed of nucleotides.

Nucleotides (also called Bases)

Adenine, Thymine, , Guanine, Cytosine or A, T, G, C

Nucleotides pair in a specific way - called the Base-Pair Rule

Adenine pairs to Thymine

Guanine pairs to Cytosine

Memory helper - think "A T Granite City") - which is where you live

The rungs of the ladder can occur in any order (as long as the base-pair rule is followed)

For instance, a stretch of DNA could be AATGACCAT - which would code for a different gene than a stretch that read: GGGCCATAG. All in all, there are billions of bases (nucleotides) in cells, which code for all the things an organism needs to ********.

DNA REPLICATION

Replication is the process where DNA makes a copy of itself. Why does DNA need to copy? Simple: Cells divide for an organism to grow or reproduce, every new cell needs a copy of the DNA or instructions to know how to be a cell. DNA replicates right before a cell divides.

DNA replication is semi-conservative. That means that when it makes a copy, one half of the old strand is always kept in the new strand. This helps reduce the number of copy errors.

RNA

DNA remains in the nucleus, but in order for it to get its instructions translated into proteins, it must send its message to the ribosomes, where proteins are made. The chemical used to carry this message is Messenger RNA

RNA = ribonucleic acid.