Having genes in common accounts for the resemblance of a mother and her daughters. Genes must be expressed to exert an effect, and proteins regulate such expression. One such regulatory protein, a zinc-finger protein (zinc ion is blue, protein is red), (more...)
DNA and RNA are long linear polymers, called nucleic acids, that carry information in a form that can be passed from one generation to the next. These macromolecules consist of a large number of linked nucleotides, each composed of a sugar, a phosphate, and a base. Sugars linked by phosphates form a common backbone, whereas the bases vary among four kinds. Genetic information is stored in the sequence of bases along a nucleic acid chain. The bases have an additional special property: they form specific pairs with one another that are stabilized by hydrogen bonds. The base pairing results in the formation of a double helix, a helical structure consisting of two strands. These base pairs provide a mechanism for copying the genetic information in an existing nucleic acid chain to form a new chain. Although RNA probably functioned as the genetic material very early in evolutionary history, the genes of all modern cells and many viruses are made of DNA. DNA is replicated by the action of DNA polymerase enzymes. These exquisitely specific enzymes copy sequences from nucleic acid templates with an error rate of less than 1 in 100 million nucleotides.
Genes specify the kinds of proteins that are made by cells, but DNA is not the direct template for protein synthesis. Rather, the templates for protein synthesis are RNA (ribonucleic acid) molecules. In particular, a class of RNA molecules called messenger RNA (mRNA) are the information-carrying intermediates in protein synthesis. Other RNA molecules, such as transfer RNA (tRNA) and ribosomal RNA (rRNA), are part of the protein-synthesizing machinery. All forms of cellular RNA are synthesized by RNA polymerases that take instructions from DNA templates. This process of transcription is followed by translation, the synthesis of proteins according to instructions given by mRNA templates. Thus, the flow of genetic information, or gene expression, in normal cells is:
This flow of information is dependent on the genetic code, which defines the relation between the sequence of bases in DNA (or its mRNA transcript) and the sequence of amino acids in a protein. The code is nearly the same in all organisms: a sequence of three bases, called a codon, specifies an amino acid. Codons in mRNA are read sequentially by tRNA molecules, which serve as adaptors in protein synthesis. Protein synthesis takes place on ribosomes, which are complex assemblies of rRNAs and more than 50 kinds of proteins.
The last theme to be considered is the interrupted character of most eukaryotic genes, which are mosaics of nucleic acid sequences called introns and exons. Both are transcribed, but introns are cut out of newly synthesized RNA molecules, leaving mature RNA molecules with continuous exons. The existence of introns and exons has crucial implications for the evolution of proteins.
5.1 A Nucleic Acid Consists of Four Kinds of Bases Linked to a Sugar-Phosphate Backbone
5.2 A Pair of Nucleic Acid Chains with Complementary Sequences Can Form a Double-Helical Structure
5.3 DNA Is Replicated by Polymerases that Take Instructions from Templates
5.4 Gene Expression Is the Transformation of DNA Information Into Functional Molecules
5.5 Amino Acids Are Encoded by Groups of Three Bases Starting from a Fixed Point
5.6 Most Eukaryotic Genes Are Mosaics of Introns and Exons
For many research questions, I use the genetic information that is available on my research species. But to understand why this information is useful I need to explain a bit about genetics first. Every organism consists of cells, all multicellular organisms have a cell and a cell nucleus. This nucleus contains the DNA, the hereditary material. But this DNA does not float around randomly through the nucleus, it is neatly packed into something we call chromosomes (see the figure below). We people have 46 chromosomes, fruit flies have 8 and the flour beetle I work on has 20. Almost all animals are diploid, which means that you have 2 copies of each chromosome. So of the 46 chromosomes you have, 23 originate from your mother and 23 originate from your father. These chromosomes contain all the hereditary information in the form of double stranded DNA.
DNA is short for Deoxyribonucleic acid. The DNA in every nucleus of an organism is exactly the same in all cells. The only exceptions are the sperm cells and the eggs, they only contain half of the DNA that a normal cell contains (sperm and eggs in humans contain only 23 of the 46 chromosomes).
DNA is made up from 4 different bases (nucleotides), adenine (A), thymine (T), guanine (G) and cytosine (C). This is true for plants, animals, bacteria, in fact it is true all life forms on earth that contain DNA. The bases on one strand of DNA form base pairs with a second strand of DNA to form the double helix. But the base pairs that can be formed are limited; adenine (A) can only form a base pair with thymine (T) and guanine (G) can only form a base pair with cytosine (C). So when we know the sequence of bases on 1 strand of DNA, we also know the sequence of bases on the other strand of DNA. The order of bases is referred to as the sequence. An example of a short sequence of a single strand of DNA is: ATTGCTCAT
Because we know the sequence of this strand we also know which bases are on the other strand:
I will often talk about sequences; this is because the sequence of DNA codes for the type of protein that is being made and these proteins are the important in all aspects of life. The way DNA encodes a protein is something I will get back to in a bit. First it is important to know that the information of the sequence of DNA gives us the opportunity to “read” the DNA. A lot of extra information is needed to properly read DNA, but I will not go into detail here. Modern technology has provided us with the complete sequence of a couple of different organisms already. So we know the sequence of all the DNA in all the chromosomes of this organism! This complete sequence is called a genome and this genome is freely accessible through this website: http://www.ncbi.nlm.nih.gov/
I want to note here that the DNA is not identical for all individuals of a species, so the human genome that is available online is not identical to your genome. But, we can learn a lot from the genome that is available online. This is because the most important parts of the genome vary considerably less than the less important parts. Take for example eye color, it is not important for survival whether you have blue or brown eyes, so this is a less important character. The red blood cells that are able to transport oxygen on the other hand are very important; people with red blood cells that are unable to transport oxygen will not survive. So variation in a character as important as the ability to transport oxygen is sort of “not tolerated”. I will later write more about selection.
From DNA to protein
But how does DNA code for protein? (also referred to as the Central Dogma) To make protein from DNA we first need to take a different step. That is to make RNA from DNA. RNA is important for a lot of different functions but I will only talk about messenger RNA here, which is used to synthesize protein from. RNA (Ribonucleic Acid) is synthesized in the nucleus and is very similar to DNA. The synthesis of RNA also involves the use of bases, but in RNA synthesis no thymine (T) is used but uracil (U) is used instead. The sequence of RNA corresponds to the sequence of DNA from which the RNA is synthesized (see the figure below).
So now we have an RNA strand. From this strand the protein will be synthesized, this is called translation (RNA is translated into protein). A protein is made from amino acids, these form a strand. I show the protein strand as a linear line, but in reality complex interactions between amino acids lead to 3 dimensional forms that are essential for the functioning of the protein. The translation of RNA to protein is different than the synthesis of RNA from DNA (transcription). When the DNA was transcribed into RNA, one base of DNA corresponded to one base of RNA, this 1 to 1 relation is not used in the translation to protein. During this translation, 1 amino acid is added to the protein strand for every 3 bases in the RNA. So a RNA sequence of 48 bases codes for a protein strand of 16 amino acids. A certain combination of 3 bases always gives the same amino acids, so we can put the translation into a table (see below). We take the first 3 bases from the figure above as example, which are AUG. The first base is A, we look it up on the left side of the table, which shows us that we have to look in the 3rd row of the table. The second base is U, we look it up on the top of the table which shows us that we have to look in the 1st column and 3rd row. There we see our third base and our combination. We can see that the combination of AUG codes for the amino acid Methionine (Met). In this way we can translate the complete RNA sequence into the protein sequence.
But how does this work in an actual cell? And why make RNA first and then protein? Why not make protein from the DNA directly? Well the DNA is located in the nucleus of the cell, here RNA is transcribed but protein is not translated. After transcription the RNA is relocated to the cytoplasm of the cell, here it is translated into protein. So the separation of nucleus and cytoplasm prevents protein from being made directly from DNA. But there are other reasons why RNA is made. I will name a few, but not all (there are so many).
First, the DNA is well protected in the nucleus against everything that floats around in the cytoplasm, which prevents the DNA from getting damaged. The transcription of DNA to RNA prevents that the DNA has to be translated itself in the cytoplasm and thereby prevents DNA damage. Another reason is that we only have 1 copy of DNA in each cell, but sometimes we need a lot of the same protein. Therefore it would be convenient if we could make more than one copy of the same protein at the same time. When the DNA is transcribed into RNA 10x, the are 10 RNA templates to make protein from. So protein can be made 10x as fast. So making RNA prevents DNA damage and provides flexibility in the amount and speed of protein synthesis (see the figure below).
Here is a video by Nature of the same process:
- Replyon Oct 27, 2014, 7:40 am
Is it possible to send me the video? I want to check it frame by frame.
- Replyon Oct 27, 2014, 7:57 am
It is not my video. It is on youtube and there are tools to get them from there but you’ll have to google it. Search “the central dogma” on youtube to find it.
- Replyon Feb 13, 2015, 8:48 am
- Replyon Feb 13, 2015, 9:12 am
No problem, thanks for the comment.
- Replyon Feb 24, 2015, 11:22 pm
7th grader that needs help
so waht is dna job
- Replyon Feb 28, 2015, 1:03 pm
The job of DNA is to transfer all the information necessary to build and maintain an organism from one generation to the next. In other words, the DNA is sort of a manual of how to make your body and how to keep it alive.
This information was transferred to you from your parents, so from one generation to the next.
- Replyon Mar 27, 2015, 12:46 pm
Mr. Jacobs, Thank you for taking the time to write this in laymen’s terms. Very helpful. SGR
- Replyon Mar 27, 2015, 2:01 pm
It is my pleasure, I am glad it is appreciated!
- Replyon Apr 16, 2015, 8:03 am
Wow, what great design behind all of this!
- Replyon May 06, 2015, 10:26 pm
How does RNA receive information from DNA?
- Replyon May 07, 2015, 5:18 pm
Basically there are proteins that interact with the DNA. When the right proteins interact, transcription will be initiated and RNA will be synthesized. See also the part: “From DNA to protein” and the movie on the bottom of the page.
- Replyon May 25, 2015, 7:38 pm
Good afternoon Mr.Jacobs. I’m a freshman in high school studying for my week of final exams. I needed extra help when preparing for biology and I found your site. Really cleared up some matters for me pertaining to transcribing DNA into a protein. Thank you so much.
- Replyon May 25, 2015, 8:35 pm
Good to hear, good luck with your exam!
- Replyon May 25, 2015, 11:25 pm
- Replyon Aug 03, 2015, 3:47 am
- Replyon Sep 17, 2015, 2:39 pm
how does protein synthesis takes place???????
- Replyon Sep 21, 2015, 9:19 am
Have you read the part about how RNA is translated into Protein? And watch the video on the bottom of the page, there you can see how protein synthesis takes place.
- Replyon Sep 21, 2015, 4:48 pm
during the actual production of proteins in a cell, what might happen to a strand of RNA before it leaves the nucleus?
- Replyon Sep 22, 2015, 3:57 pm
So the RNA leaves the nucleus before protein can be produced from it. There are several things that can/do happen to the RNA before it leaves the nucleus, like cutting out the introns and alternative splicing. If those are the processes you were thinking about than I would be happy to devote a separate post to it.
- Replyon Oct 11, 2015, 5:16 pm
all of the proteins made by an organism combine to make the organism __________?
- Replyon Oct 12, 2015, 7:34 am
Proteins perform a variety of functions, from structural (keratin = hair, collagen = bone) to transport of oxygen for example and tons of other functions. The organism is of course also build by different components, like salts and water. But proteins are definitely extremely important for bodily function.
- Replyon Oct 17, 2015, 2:07 am
Quick question, what does the order in which the nitrogenous bases appear in the genome mean? What does it determine about the DNA?
- Replyon Oct 17, 2015, 4:13 pm
I am not completely sure if I understand what you exactly want to know. The order of the bases determines which aminoacid is produced. So every 3 bases code for one amino acid. So which amino acids are synthesized into the protein depends on the order of the bases. There are however also many non-coding regions, meaning that they do not code for protein. Although we do not know of every piece of sequence what the exact role is, there are many regions that are so called transcription binding sites. They influence in combination with a transcription factor how active a gene is. In other words, how many RNAs are produced from a certain gene.
- Replyon Oct 19, 2015, 3:39 pm
you help me to understand this dogma thank you so much
and it is a great video !!
- Replyon Jan 06, 2016, 1:04 pm
An amino acid is a building block of proteins. Just like links in a chain. Different amino acids have different properties and when the right amino acids are combined it provides the complete protein with a certain function. For example binding oxygen to transport it to our organs.
- Replyon Jan 26, 2016, 12:22 am
Vincent Mason aka football superstar
Why is it important for a cell to be able to make RNA from DNA
- Replyon Jan 26, 2016, 2:31 pm
Please read the part “In the cell”. If you have questions afterwards please let me know.
- Replyon Feb 02, 2016, 5:11 pm
Your layman terms were extremely helpful as I am studying for my last exam before my BA. I had the biological psychology class semesters ago and needed to brush and I found your time saving site.
- Replyon Feb 02, 2016, 5:23 pm
- Replyon Feb 11, 2016, 11:36 pm
Would you mind answering my question?
Can we appreciate the order of DNA to NA to protein?
- Replyon Feb 12, 2016, 10:26 am
What exactly do you mean? If there is a reason that RNA is first made from DNA and next protein is made from the RNA? This is simply due to the fact that the machinery that produces the protein needs RNA as a template. It cannot make protein from the DNA directly so RNA needs to be made first.
- Replyon Feb 14, 2016, 4:38 am
Is mRNA and subsequently protein also formed from the blue strand of DNA? In which case for each DNA segment we would get two distinctly groups of amino acid sequences
- Replyon Feb 16, 2016, 8:33 am
Not necessarily. mRNA can be synthesized from both strains of DNA and thus lead to proteins. However, large parts of the DNA are non-coding. They fulfill other functions. For example to regulate the activity of genes. So genes can be located on both strains of DNA but 2 genes on opposites strains of DNA at the same location is unlikely.
- Replyon Feb 20, 2016, 12:00 am
How does DNA support RNA and how does RNA support protein?
- Replyon Feb 20, 2016, 9:12 am
In what way do you mean support? As in how they are linked together during the transcription and translation process? That is done by a protein complex that mediates the transcription or translation process.