Where are we going with this? The information on this page should increase understanding related to this standard: Demonstrate how DNA sequence information is decoded through transcriptional and translational processes within the cell in order to synthesize proteins. Examine the relationship of structure and function of various types of RNA and the importance of this relationship in these processes.
Article includes ideas, images, and content from Troy Smigielski (2022-01)
DNA Structure and Replication
(Ooo! Fancy!)
Recall that…
All life is made of cells and all cells are made of biomolecules and all biomolecules are made of monomers.
Isn't monomer such a fun word to say? I mean, not as fun as kaboodle, but still…
As our exploration of life looked at reproduction, we learned a little about DNA and genes:
- Specific sequences of DNA are called genes.
- Genes are kept side by side on structures called chromosomes.
- Genes also code for specific proteins, which can contribute to a certain trait.
When we move to the next part of this packet, we will learn how sequences of DNA create these proteins.
Nucleotides
The monomers of DNA are a nucleotides.
Nucleotides are made up of…
- a sugar
- a phosphate
- a nitrogen base
The nitrogen bases in DNA can be broken down into two categories: Purines and Pyrimidines.
Purines have 2 rings. Examples are Adenine (A) and Guanine (G).Pyrimidines have 1 ring. Examples are Cytosine (C), Thymine (T), and Uracil (U).
The nitrogen base is what differs between nucleotides.
In DNA, the nucleotides combine in very specific ways to create a code.
DNA Structure
Watson and Crick won the Nobel Prize in 1962 for discovering the structure of DNA. Like we said before, DNA is built out of nucleotides. Nucleotides have a sugar in them.
The sugar in DNA is deoxyribose while the sugar in RNA is ribose.
Nucleotides also have a nitrogen base in them.
The nitrogen bases in DNA are are Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
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| Unraveling a chromosome would reveal a long, long chain of DNA which is made up of a sequence of nucleotides. |
There is another nucleotide… Uracil (U)… but it is in RNA, not DNA… It takes the place of the Thymine (T).
Thus…
The bases in DNA are A, T, C, and G while the bases in RNA are A, U, C, and G.
Back to that purine and pyrimidine information…
The nitrogen base will either be a purine or a pyrimidine.
Purines have 2 rings. Examples of purines are Adenine (A) and Guanine (G).
Pyrimidines have 1 ring. Examples of purines are Cytosine (C) and Thymine (T), for DNA and Uracil (U) in RNA.
In nucleic acids, a purine will always bond with a pyrimidine. In DNA:
Adenine (A) bonds with Thymine (T) in a double bond.Cytosine (C) bonds with Guanine (G) in a triple bond.
In RNA, Adenine bonds with Uracil because there is no Thymine in RNA.
So much bonding! I need a hug!
So, the DNA double-helix structure is relatively specific. Though the order can vary, any strand of DNA will find specific nucleotides bond to each other.
The strands on each side of the helix are complementary to each other.
Chargaff's Rules
The way that the nucleotides bond in DNA follows a simple system. To repeat,
Adenine (A) bonds with Thymine (T) in a double bond.
Cytosine (C) bonds with Guanine (G) in a triple bond.
Thus…
The number of A must be the same as the number of T.The number of C must be the same as the number of G.
HOW DID HE DID HE COME UP WITH ALL THAT!
Chargaff collected data on DNA nitrogenous bases in five organisms. He compared the percentages of each type of nitrogenous base. What he found looked something like the information in this table:
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| (Source, 2022-02) |
From this, he was able to conclude that the pairs combined in specific combinations. From this data, the rules were developed.
Holding the DNA together is structure called a double-helix that is held together by a sugar-phosphate backbone. So you could say that DNA has a double-helix shape with a sugar-phosphate backbone.
Also, DNA is antiparallel meaning its two strands move in opposite directions.
DNA Replication
DNA replicates during S phase of Interphase during the cell cycle. The goal of DNA replication is for a cell to create an exact copy of its DNA, which would result in two identical strands of DNA.
Okay… new term coming! Think…
What does the word “semi” mean? What is a semi-circle?What does the word “conserve” mean? What are forest conservation efforts?So…
What would semi-conservative mean?
DNA replication is semiconservative, which means that each resulting strand has one original strand and one new strand. (You save half of the original).
Before DNA can replicate, it must first separate. DNA separation happens at a region called a replication fork (see below).
STEP 1
DNA helicase (recall that enzymes end in "ase") “unzips” the molecule and separates it into two strands. Each original strand now serves as a template for the new strands.
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| DNA replicates in three stages. Helicase acts in step 1, polymerase acts in step 2, and ligase acts in step 3. |
STEP 2DNA polymerase adds nucleotides to each of the template strands following the rules of base pairing. (A to T and C to G) This produces two new complementary strands.
Then DNA polymerase comes in and adds many (poly) nucleotides to the template strands. Once done, the original strands are now matched according to Chargaff's Rules. The result is two exact duplicates of the original DNA.
STEP 3
DNA ligase ties off the new molecule and finishes the process.
It's like stitches!
In closing…
DNA replication allows S phase to finish, which lets mitosis happen. When mitosis happens, your body is able to heal, grow, and maximize cell transport.
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| 2022-02, artist S. Clark c/o 2025 |
2022-02, created by A. Chinn, A. Pollard, A Proctor, P. Pfeiffer, and K. Coulter c/o 2025
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