The complete set of instructions for making any organism is called its genome. It contains the master blueprint for all cellular structur...
The complete set of instructions for making any organism is called its genome. It contains the master blueprint for all cellular structures and biological processes for the lifetime of the organism. A copy of the genome is found in each cell, and it consists of two tightly coiled threads of deoxyribonucleic acid (DNA) organized into structures called chromosomes.
To use an analogy, think of the genome as the “encyclopedia” of all instructions on how to make an organism. Chromosomes are equivalant to the different volumes of an encyclopedia. Many bacterial cells contain just one chromosome (their “encyclopedia” is only one book), however some have more than one, and humans have 24 chromosomes (i.e. equivalent to a 24 volume encyclopedia, to use the analogy).
If unwound and tied together, the strands of DNA for the genome of a human would stretch more than 5 feet but would be only 50 trillionths of an inch wide. However, through tight packaging, the DNA containing a copy of the full genome is packed into a singlecell (all life is made up of units called cells). Likewise, in bacteria all the DNA for the complete genome is packed into each – even tinier – cell.
Each strand of DNA is a linear arrangement of similar repeating units, often termed bases (think of these as the “letters” in an encyclopedia). Four different bases are present in DNA: adenine (commonly referred to as just “A”), thymine (T), cytosine (C), and guanine (you guessed it, G). The particular order of the bases is called the DNA sequence.
The sequence specifies the exact genetic instructions required to create a particular organism, including a particular bacterial species, with its own unique traits.
The two complementary strands of DNA are held together by weak bonds between the bases on each strand, forming base pairs (bp). Think of these complementary strands as “two copies” of the encyclopedia (so that if one copy is damaged, the there is another copy to refer to). Genome size is usually stated as the total number of base pairs (bp).
The human genome contains roughly 3 billion bp. Bacterial genome sequences determined by the turn of the century (year 2000) range in size from approximately half a million bp, to 8 million bp.
Note that each time a bacterial cell divides into two daughter cells, its full genome is duplicated (both strands of the DNA). Therefore, each new bacterial cell produced contains a new copy of the genome sequence (that contains the two complementary strands).
This genome sequence (encyclopedia), comprised of base-pairs (letters) is full of genes (which can be considered the “words” in an encyclopedia).
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