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The Many Levels of DNA

Technically, if you have a few nucleotides joined together, you have some DNA.  The purpose of DNA is to store information to make proteins, and no protein can be coded for by just a few nucleotides.  No cell or organism is made of just one protein, so you need more DNA to code for your 25,000 different proteins.  So we have different words for just how much DNA we are talking about.

  • Nucleotides are the monomers of DNA, they join with their complementary nucleotides to make base pairs.

  • Genes are long stretches of base pairs that code for a polypeptide/protein/trait.

  • Chromosomes are pieces of DNA, humans have 46 in each cell.

  • A genome is all of the genes it takes to make an organism.

Nucleotide to Genome

All of the following are terms that describe how much DNA you are talking about.

  • Nucleotides - monomers (repeating units) that make up all DNA

    • made of a phosphate, sugar, and one of four bases (A,C,G,T)​

  • Base Pair - two nucleotides joined at their bases by a weak hydrogen bond​

    • A always pairs with T

    • G always pairs with C

    • base pairs are anti-parallel - they are oriented in opposite directions

    • The length of DNA is often measured in base pairs (bp)

  • Gene - section of DNA that codes for a trait​

    • They are usually 100s to 1000s of bp long

    • Genes are used to make mRNA in protein synthesis​

    • Technically, genes code for polypeptides, which combine to make proteins, which determine your traits; so it is accurate to say that genes code for any of the three.

  • Chromosome - a piece of DNA, usually with a beginning and an end (bacteria have circular chromosomes).

    • They contain many genes; they also have DNA in between genes that doesn't code for any proteins​

    • Every species has a specific number of chromosomes in each cell​

      • Humans have 46 chromosomes (23 pairs)​

      • A cow has 60 chromosomes (30 pairs)

    • The DNA of chromosomes are wrapped around proteins called histones​, and also coils up in different ways

      • Chromatin is loosely wrapped DNA, it is easier to read for transcription​

      • Chromatids are tightly coiled up so that they don't get tangled during cell division

  • Genome - all of the genes in a cell/organism​​​​​​

    • ​Humans have 23 distinct chromosomes and 20-25,000 genes, so roughly 1,000 genes per chromosome.​

    • A karyotype is a picture of all of the chromosomes from a cell

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