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- The quick and simple answer: A genetic carrier is someone who has inherited a recessive allele for a genetic condition but doesn’t show traits or symptoms of that condition. Most genetic conditions are inherited through autosomal recessive inheritance. To clarify, everyone carries two alleles, or copies, of a given gene.
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What is a genetic carrier?
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With autosomal recessive inheritance, a genetic carrier is a person who has inherited a recessive allele of a gene that is linked to a genetic condition. However, this person doesn’t show traits or symptoms of the condition because their second allele for that gene is normal.
- Overview
- The molecular structure of DNA
- DNA molecules are polymers
- DNA monomers are called nucleotides
- There are four nucleotide monomers
- The sugar and acid in all four monomers are the same
- The four nucleotide monomers are distinguished by their bases
- Phosphodiester bonds in DNA polymers connect the 5’ carbon of one nucleotide to the 3’ carbon of another nucleotide
- Chromosomes are made of two DNA polymers that stick together via non-covalent hydrogen bonds
- DNA polymers direct the production of other polymers called proteins
DNA is the information molecule. It stores instructions for making other large molecules, called proteins. These instructions are stored inside each of your cells, distributed among 46 long structures called chromosomes. These chromosomes are made up of thousands of shorter segments of DNA, called genes. Each gene stores the directions for making protein fragments, whole proteins, or multiple specific proteins.
DNA is well-suited to perform this biological function because of its molecular structure, and because of the development of a series of high performance enzymes that are fine-tuned to interact with this molecular structure in specific ways. The match between DNA structure and the activities of these enzymes is so effective and well-refined that DNA has become, over evolutionary time, the universal information-storage molecule for all forms of life. Nature has yet to find a better solution than DNA for storing, expressing, and passing along instructions for making proteins.
The molecular structure of DNA
In order to understand the biological function of DNA, you first need to understand its molecular structure. This requires learning the vocabulary for talking about the building blocks of DNA, and how these building blocks are assembled to make DNA molecules.
In order to understand the biological function of DNA, you first need to understand its molecular structure. This requires learning the vocabulary for talking about the building blocks of DNA, and how these building blocks are assembled to make DNA molecules.
Polymers are large molecules that are built up by repeatedly linking together smaller molecules, called monomers. Think of how a freight train is built by linking lots of individual boxcars together, or how this sentence is built by sticking together a specific sequence of individual letters (plus spaces and punctuation). In all three cases, the la...
Just like a sentence “polymer” is composed of letter “monomers,” a DNA polymer is composed of monomers called nucleotides. A molecule of DNA is a bunch of nucleotide monomers, joined one after another into a very long chain.
The English language has a 26 letter alphabet. In contrast, the DNA “alphabet” has only four “letters,” the four nucleotide monomers. They have short and easy to remember names: A, C, T, G. Each nucleotide monomer is built from three simple molecular parts: a sugar, a phosphate group, and a nucleobase. (Don’t confuse this use of “base” with the oth...
All four nucleotides (A, T, G and C) are made by sticking a phosphate group and a nucleobase to a sugar. The sugar in all four nucleotides is called deoxyribose. It’s a cyclical molecule—most of its atoms are arranged in a ring-structure. The ring contains one oxygen and four carbons. A fifth carbon atom is attached to the fourth carbon of the ring. Deoxyribose also contains a hydroxyl group (-OH) attached to the third carbon in the ring.
The phosphate group is a phosphorous atom with four oxygen atoms bonded to it. The phosphorous atom in phosphate has a marked tendency to bond to other oxygen atoms (for instance, the oxygen atom sticking off the deoxyribose sugar of another nucleotide).
Each type of nucleotide has a different nucleobase stuck to its deoxyribose sugar.
•A nucleotide contains adenine
•T nucleotide contains thymine
•G nucleotide contains guanine
•C nucleotide contains cytosine
All four of these nucleobases are relatively complex molecules, with the unifying feature that they all tend to have multiple nitrogen atoms in their structures. For this reason, nucleobases are often also called nitrogenous bases.
The nucleotide monomers in a DNA polymer are connected by strong electromagnetic attractions called phosphodiester bonds. Phosphodiester bonds are part of a larger class of electromagnetic attractions between atoms that chemists refer to as covalent bonds.
In order to keep things organized, biochemists have developed a numbering system for talking about the molecular structure of nucleotides. These numbers are applied to the carbon atoms in the sugar, starting at the carbon immediately to the right of the oxygen in the deoxyribose ring, and continuing in a clockwise fashion: the numbers range from 1’ (“one prime”), identifying the carbon immediately to the right of the oxygen) all the way to 5’ (“five prime”), identifying the carbon that sticks off the fourth and final carbon in the deoxyribose ring.
Chromosomal DNA consists of two DNA polymers that make up a 3-dimensional (3D) structure called a double helix. In a double helix structure, the strands of DNA run antiparallel, meaning the 5’ end of one DNA strand is parallel with the 3’ end of the other DNA strand.
The nucleotides forming each DNA strand are connected by noncovalent bonds, called hydrogen bonds. Considered individually, hydrogen bonds are much weaker than a single covalent bond, such as a phosphodiester bond. But, there are so many of them that the two DNA polymers are very strongly connected to each other.
The hydrogen bonds that join DNA polymers happen between certain hydrogen atoms on one base (called hydrogen bond donors) and certain oxygen or nitrogen atoms on the base across from it (called hydrogen bond acceptors). Adenine (“A”) and Thymine (“T”) each have one donor and one acceptor, whereas Cytosine (“C”) has one donor and two acceptors, and Guanine (“G”) has one acceptor and two donors.
The A nucleotides are always hydrogen bonded to T nucleotides, and C nucleotides are always hydrogen bonded to G nucleotides. This selective binding is called complementary base pairing, and creates consistency in the nucleotide sequences of the two DNA polymers that join together to make a chromosome. This was first observed by Erwin Chargaff, who developed methods for counting nucleotides in DNA samples, and found that the percent of A nucleotides always equaled the percent of T nucleotides, and the percent of G nucleotides always equaled the percent of C nucleotides (within a margin of error). Now, we know that complementary base pairing can be explained by reference to hydrogen bonding between the donors and acceptors on the bases of each nucleotide: A nucleotides and T nucleotides have a match (one donor and one acceptor each), and C nucleotides and G nucleotides have a match (the former has one donor and two acceptors, while the latter has one acceptor and two donors).
A protein is one or more polymers of monomers called amino acids. Proteins are the workhorse molecules in your cells. They act as enzymes, structural support, hormones, and a whole host of other functional molecules. All traits derive from the interactions of proteins with each other and the surrounding environments.
The other crucial advance made in the 1940s was the identification of deoxyribonucleic acid (DNA) as the likely carrier of genetic information. But the mechanism whereby the hereditary information is copied for transmission from cell to cell, and how proteins are specified by the instructions in the DNA, remained completely mysterious.
- Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter
- 2002
- 2002
Deoxyribonucleic acid ( / diːˈɒksɪˌraɪboʊnjuːˌkliːɪk, - ˌkleɪ -/ ⓘ; [1] DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses.
A section of the DNA molecule is labeled as a gene. Chromosomes are found in the nucleus of the cell. Each chromosome is made up of a single DNA molecule that contains many genes.
- Around 50,000 genes. There are also 24 chromosomes (groups of genes) in every cell.
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Nucleic acids, macromolecules made out of units called nucleotides, come in two naturally occurring varieties: deoxyribonucleic acid ( DNA) and ribonucleic acid ( RNA ). DNA is the genetic material found in living organisms, all the way from single-celled bacteria to multicellular mammals like you and me.
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