Back To CourseCollege Biology: Help and Review
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Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.
Sometimes, when we talk about people's appearances, special skills or behaviors, we say, 'It's in your genes.' It's true: the genetic information carried in your DNA sequence determines many things about you, such as the color of your hair, the shape of your eyes, your blood type, and even your susceptibility to certain diseases.
But, did you ever wonder how a particular sequence of the nucleotides, called A, C, G and T, can lead to noticeable physical characteristics? The central dogma of biology describes just that. It provides the basic framework for how genetic information flows from a DNA sequence to a protein product inside cells. This process of genetic information flowing from DNA to RNA to protein is called gene expression.
To start off, let's look at an analogy to make the central dogma more tangible. Let's say that you want to learn how to make a wooden dresser, but you don't know anyone who knows how to do that. Luckily, in your town, there is a really good library that has a section of books about woodworking. So you grab a pen and notebook and head to the library to look up a how-to guide.
When you get to the library, you search through the shelves until you find a book that has a good set of instructions for a dresser that you like. You whip out your pen and notebook and copy down (or 'transcribe') the instructions. Satisfied, you take your notes and head back home.
At home in your workshop, you have all the wood and tools you need to make the dresser. You follow the instructions and make a good-looking dresser, effectively 'translating' the written words into a physical object that you can use in your home. Tada!
This analogy has set the stage for us to understand the central dogma on a molecular level. The library represents the nucleus, and your workshop at home represents the cytoplasm. All of the books in the library represent the DNA, the genetic material of the cell, stored in chromosomes.
The instructions for the wooden dresser you chose represent a single gene. As you may know, each gene contains the instructions needed to make a single type of protein. The finished dresser, then, represents the protein product. The wood you used to make the dresser represents amino acids, which are the building blocks of proteins.
And, what about the notebook? It represents RNA, which is a smaller molecule that can move out of the nucleus into the cytoplasm, where proteins are made. Simply put, the central dogma states that DNA leads to RNA which, in turn, leads to protein.
DNA sequence information is converted to RNA in a process called transcription, which is the first step of gene expression, where a segment of DNA is copied into RNA. This is analogous to copying down notes out of a library book. It's helpful to remember that transcribe and transcription come from words like scribe and script, which have to do with writing things down.
First, the double-stranded DNA molecule is partially 'unzipped' and an enzyme called RNA polymerase literally copies the gene's nucleotides one by one into an RNA molecule. Like DNA, RNA is made of a particular sequence of nucleotides. Unlike DNA, RNA has only a single strand, and is a more fragile and temporary molecule inside the cell. Very importantly, RNA is small and can easily exit the nucleus and go to the cytoplasm, where proteins are made.
Back to our analogy: it's very easy to carry your notes home from the library. You wouldn't want to have to build your dresser there in the library, and you certainly wouldn't want to take the library's entire permanent collection of books to your house!
The sequence encoded in the RNA molecule is decoded and converted into an amino acid sequence in a process called translation. In our analogy, this part is equivalent to building the dresser out of wood according to the written notes you brought home from the library. You can remember this word by remembering that translate means 'to convert things from one language to another.' In our analogy, we are translating between written words and a physical object. In the cell, we are translating between a nucleotide sequence and an amino acid sequence.
In the cytoplasm, the ribosome translates RNA sequence information into an amino acid sequence. Groups of three nucleic acids in the RNA, called codons, instruct the ribosome to place certain amino acids into the chain. For example, the RNA sequence AAG codes for the amino acid lysine, and the sequence GCG codes for alanine.
Cells use 20 amino acids in their proteins, and before you start arguing that the math doesn't work out, it's important to note that codons can be redundant. For instance, there are four different codons (GGT, GGC, GGA, GGG) that all instruct the ribosome to place a glycine amino acid in the chain. However, each codon only codes for one amino acid. AAA always codes for lysine, never anything else.
Codon for codon, a growing chain of amino acids in a particular sequence is made. This amino acid chain then folds into a three-dimensional structure called a protein. Due to their structures, proteins can carry out many important functions in the cell, just as the objects you make out of wood can carry out important functions in your home. Aren't cells amazing?
At the beginning of the lesson, you were promised that you would understand how DNA sequence material can lead to noticeable physical characteristics, or phenotypes. As you might expect, this is a complicated issue. But, in the case of enzymes, it's fairly easy to understand.
For example, let's say there's an enzyme (a protein) that produces a blue pigment in flowers. The structure of the protein is important for its enzymatic function, and certain structures would lead to less efficient pigment production.
Variations in the DNA sequence among different plants, as we've seen, would lead to different RNA sequences and potentially different amino acid sequences. This could lead to different enzyme structures with varying levels of pigment production, which would lead to flowers that have lighter and darker blue petals.
Did you ever wonder how a particular sequence of nucleotides, called A, C, G and T, can lead to noticeable physical characteristics? The central dogma of biology describes just that. It provides the basic framework for how genetic information flows from a DNA sequence to a protein product inside cells. This process of genetic information flowing from DNA to RNA to protein is called gene expression.
Transcription is the first step of gene expression, where a segment of DNA is copied into RNA. RNA polymerase copies the gene's nucleotides one by one into an RNA molecule. The sequence encoded in the RNA molecule is decoded and converted into an amino acid sequence in a process called translation.
In the cell, we are translating between a nucleotide sequence and an amino acid sequence. In the cytoplasm, the ribosome translates RNA sequence information into an amino acid sequence. Groups of three nucleic acids in the RNA, called codons, instruct the ribosome to place certain amino acids into the chain.
|Gene expression||process of genetic information flowing from DNA to RNA to protein|
|Transcription||DNA sequence information is converted to RNA|
|RNA polymerase||enzyme that copies the gene's nucleotides one by one into an RNA molecule|
|Translation||sequence encoded in the RNA molecule is decoded and converted into an amino acid sequence|
|Ribosome||translates RNA sequence information into an amino acid sequence|
|Codons||instructs the ribosome to place certain amino acids into the chain|
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Back To CourseCollege Biology: Help and Review
24 chapters | 433 lessons