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DNA Structure

DNA Structure

Description:

This lesson will examine the structure of DNA as being in the form of a double helix composed of nucleotides.

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Tutorial

What's Covered


Today you are going to be examining the structure and function of DNA. Specifically you will focus on:

  1. DNA Structure

1. DNA STRUCTURE

You're going to start at the smallest level with DNA and then work your way up to see how DNA composes chromosomes.

DNA is actually composed of nucleotides. Nucleotides are the subunits of nucleic acids. A nucleotide is actually composed of a five-carbon sugar. In DNA, this five-carbon sugar is deoxyribose, and in RNA this five-carbon sugar is ribose.

Term to Know


Nucleotide

Organic molecules that consist of a 5 carbon sugar (ribose in the case of RNA, and deoxyribose in the case of DNA), a phosphate group and a nitrogenous base; nucleotides are the building blocks of nucleic acids (DNA & RNA).

This five-carbon sugar that composes DNA is a deoxyribose sugar. These nucleotides also contain a phosphate group and a nitrogen base. These nitrogen bases can be either adenine, thymine, cytosine, or guanine

Terms to Know


Adenine (A)
A nucleotide building block of DNA and RNA, adenine is classified as a purine and complements thymine (T) in DNA and uracil (U) in RNA.

Thymine (T)
A nucleotide building block of DNA, thymine is classified as a pyrimidine and complements adenine (A) in DNA; thymine is not found in RNA.

Guanine (G)
A nucleotide building block of DNA and RNA, guanine is classified as a purine and complements cytosine (C) in DNA and RNA.

Cytosine (C)
A nucleotide building block of DNA and RNA, it is classified as a pyrimidine and complements guanine (G) in DNA and RNA.

You can see that here is the five-carbon sugar, it's a deoxyribose sugar.

The deoxyribose sugar and the phosphate groups compose this backbone of DNA. DNA is said to have a phosphate sugar backbone because that's what the outside part of the DNA is composed of here.

Then these nucleotides are held together in the double helix by covalent bonds. Covalent bonds play a role in holding these nucleotides together in this DNA structure. Inside the DNA are the base pairs. The base pairs are composed of your nitrogen bases.

Terms to Know

Double Helix
The shape of the DNA molecule, often times is referred to as the “twisted ladder” and is the title to the book about Watson & Crick's discovery of DNA's structure.

Base Pair

The way that nucleotides interact with one another, A bonds with T and C bonds with G in DNA, while C bonds with G and A bonds with U (uracil) in RNA. The sequence of base pairs creates the genetic code that is transcribed and translated into proteins.

The nitrogen bases pair up in specific ways. You have two base pairs in DNA, the base pair of adenine and thymine and the base pair of cytosine and guanine. Adenine always pairs with thymine, and cytosine always pairs with guanine.

Adenine and thymine will always pair together, and then cytosine and guanine will always pair together. These are our two base pairs.

The reason that these always pair together-- adenine with thymine and cytosine with guanine-- is because of the shape and the site for the hydrogen bond. They're held together by a hydrogen bond. The shape and the site of that hydrogen bond is what allows adenine to always pair with thymine and cytosine to always pair with guanine.

They almost fit together without hydrogen bond like a little puzzle. Adenine wouldn't properly fit with guanine, and cytosine wouldn't properly fit with thymine. The way that that bond is allows them to fit together nicely. These are what make up the chemical structure of DNA.

DNA is described as being a double helix structure. It's almost like a ladder that's been twisted, and the backbone of the ladder are the phosphate sugar and the rungs of the ladder are your base pairs.

DNA will mix with proteins and will become condensed and wind around itself in order to form chromosomes. Chromosomes DNA that's been twisted around and condensed to form a chromosome structure. Chromosomes are condensed DNA that contain genes.

Term to Know


Gene

A segment of DNA that codes for a specific protein, genes are a sequence of nucleotides.

Genes are units of heredity that are made up of sequences of nucleotides. The nucleotide sequence of a gene codes for a specific polypeptide chain. Polypeptide chains are the structural units of proteins and determine our traits.

The base pairs that you have pair up in certain ways to produce nucleotide sequences.Those nucleotide sequences code for specific polypeptide chains, which then produce specific proteins. The order in which the DNA is composed basically determines different types of genes.

Term to Know

Nucleotide Sequence
The arrangement of nucleotides that form genes in strands of DNA.

Summary

You have examined the structure of DNA. Specifically, you learned about how DNA is composed of nucleotides, that these nucleotides are held together in the double helix, and that chromosomes are condensed DNA that contain genes.

Good luck.

Source: This work adapted from Sophia Author Amanda Soderlind.

TERMS TO KNOW
  • Double Helix

    The shape of the DNA molecule, often times is referred to as the “twisted ladder” and is the title to the book about Watson & Crick's discovery of DNA's structure.

  • Nucleotide Sequence

    The arrangement of nucleotides that form genes in strands of DNA.

  • Gene

    A segment of DNA that codes for a specific protein, genes are a sequence of nucleotides.

  • Base Pair

    The way that nucleotides interact with one another, A bonds with T and C bonds with G in DNA, while C bonds with G and A bonds with U (uracil) in RNA. The sequence of base pairs creates the genetic code that is transcribed and translated into proteins.

  • Cytosine (C)

    A nucleotide building block of DNA and RNA, it is classified as a pyrimidine and complements guanine (G) in DNA and RNA.

  • Guanine (G)

    A nucleotide building block of DNA and RNA, guanine is classified as a purine and complements cytosine (C) in DNA and RNA.

  • Thymine (T)

    A nucleotide building block of DNA, thymine is classified as a pyrimidine and complements adenine (A) in DNA; thymine is not found in RNA.

  • Adenine (A)

    A nucleotide building block of DNA and RNA, adenine is classified as a purine and complements thymine (T) in DNA and uracil (U) in RNA.

  • Nucleotide

    Organic molecules that consist of a 5 carbon sugar (ribose in the case of RNA, and deoxyribose in the case of DNA), a phosphate group and a nitrogenous base; nucleotides are the building blocks of nucleic acids (DNA & RNA).