DNA

The Secret of Life

Deoxyribonucleic Acid

• DNA is the molecule responsible for controlling the activities of the cell

• It is the hereditary molecule

• DNA directs the production of protein

Structure of DNA

• In 1953, Watson and Crick proposed that DNA is made of two chains of nucleotides held together by nitrogenous bases.

• Watson and Crick also proposed that DNA is shaped like a long zipper that is twisted into a coil like a spring.

Structure of DNA

• Because DNA is composed of two strands twisted together, its shape is called double helix.

• A double helix resembles a twisted ladder.

Nucleotides

• DNA is made up of subunits called nucleotides

• Nucleotides consist of the backbone, which is made of sugar (deoxyribose) and phosphate, as well as a nitrogenous base

Nucleotide

Sugar

Phosphate Nitrogen base

How Did DNA Get its Name?

• Based on what you just learned, how do you think deoxyribonucleic acid (DNA) got its name?

Nucleotides

• A nitrogenous base is a carbon ring structure that contains one or more atoms of nitrogen.

• In DNA, there are four possible nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T).

Adenine (A) Guanine (G) Cytosine (C) Thymine (T)

Nucleotide Sequence

• The four nucleotides are represented by the first letter in their name– A – Adenine

– G – Guanine

– T – Thymine

– C – Cytosine

Nucleotide Sequence

• In DNA, Adenine always pairs with Thymine

• Cytosine always pairs with Guanine

Nucleotides

• Nucleotides stack on top of one another forming the double helix, or twisted ladder

DNA Building

• Click here to link to a DNA building activity online!

Go Get It Moment!

• When you hear “Move it!” you will have 30 seconds to come get one piece of paper with an A, G, T, or C on it.

• Then find someone else in the room whose letter correctly matches with your nucleotide letter and stand by that person.

• What are the questions?

• “Move it!”

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Go Get It Moment!

• When you hear “Double Helix”, you will have 1 minute to make a classroom DNA strand by standing next to another pair of nucleotides

• What questions are there?

• “Double Helix”

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DNA Replication

• In order for cells to divide, DNA must be able to make exact copies of itself

– This process is known as DNA Replication

DNA Replication

• DNA Replication occurs before mitosis and meiosis

• Replication results in two identical DNA daughter strands from one mother strand

Process of DNA Replication

• The DNA strand is unzipped at the hydrogen bonds by an enzyme named helicase.

• Nucleotides in the nucleus then find their corresponding nucleotides on each of the two open DNA strands and produce two new DNA double helixes.

Link to DNA Replication Animation

• Click here to see how DNA Replication works

Protein Synthesis

• The main job for DNA is to direct the production of protein

• Protein makes tissues and organs and carries out the organism’s metabolism

• Proteins are polymers (chains) of amino acids

Protein Synthesis

• The sequence of nucleotides in each gene contains information for assembling the string of amino acids that make up a single protein

• DNA sequence to make proteins!

RNA• RNA is a nucleic acid composed of

nucleotides that is crucial in making protein

• There are three differences between DNA and RNA– RNA is a single strand

– The sugar in RNA is called Ribose instead of DNA’s Deoxyribose

– Like DNA, RNA has 4 nitrogenous bases, but instead of Thymine, Uracil is the 4th base

RNA as a Single Strand

• You recall that DNA looks like a twisted ladder and is referred to as a double helix

• RNA looks like half a ladder

– There is only one side to RNA

RNA Contains Ribose

• Remember how DNA got it’s name?

– It is a nucleic acid with deoxyribose as the sugar on the backbone

– Hence the name Deoxyribonucleic acid

• Ribonucleic acid (RNA) has the sugar ribose on it’s backbone

RNA Has Uracil

• Remember the base pairs in DNA?

– Adenine pairs with Thymine

– Guanine pairs with Cytosine

RNA Has Uracil

• In RNA

– Adenine pairs with URACIL

– Guanine pairs with Cytosine

Making Protein

• Protein production starts with DNA

• DNA passes instruction to RNA

• RNA carries out the work of linking together chains of amino acids

Three Types of RNA

• There are three types of RNA involved in Protein Synthesis

– Messenger RNA (mRNA)

– Ribosomal RNA (rRNA)

– Transfer RNA (tRNA)

Messenger RNA

• Messenger RNA (mRNA), brings instructions from DNA in the nucleus to the cell’s factory floor, the cytoplasm

– On the factory floor, mRNA moves to the assembly line, a ribosome

– Remember that a ribosome is either a free-floating small dot in the cell or is attached to the endoplasmic reticulum making it “rough” ER

Ribosomal RNA

• The ribosome, made of Ribosomal RNA (rRNA) binds to the mRNA and uses the instructions to assemble the amino acids in the correct order

Transfer RNA

• Transfer RNA (tRNA) is the supplier

• Transfer RNA delivers the amino acids to the ribosome to be assembled into a protein

Transcription

• The production of messenger RNA (mRNA) is known as Transcription

• DNA acts as a template for the RNA molecule

• To View a Link to Transcription Animation Click Here.

Transcription

• The main difference between DNA Replication and transcription is that transcription results in one single strand of RNA rather than an exact duplicate of a double stranded DNA molecule

• Much of the information on the mRNA strand codes for specific amino acids to make protein

The Genetic Code

• A code is necessary to turn the language of RNA into the language of amino acids and proteins

• The four nitrogenous bases make up the code– The code letters are A, G, T, and C

• A set of three letters makes a “word” called a Codon– There are 64 possible codons

Codon

Amino Acids

• There are 20 different amino acids that make up proteins

The Genetic Code

First Letter

C

A

G

U C A G

U

Second Letter Third Letter

U C A G U

C A

G U C A GUCAG

Phenylalanine (UUU)

Leucine (UUA)

Phenylalanine (UUC)

Leucine (UUG)

Leucine (CUU)

Leucine (CUC)

Leucine (CUA)

Leucine (CUG)

Isoleucine (AUU)

Isoleucine (AUC)

Isoleucine (AUA)

Methionine;Start (AUG)

Valine (GUU)

Valine (GUC)

Valine (GUA)

Valine (GUG)

Serine (UCU)

Serine (UCC)

Serine (UCA)

Serine (UCG)

Proline (CCU)

Proline (CCC)

Proline (CCA)

Proline (CCG)

Threonine (ACU)

The Messenger RNA Genetic Code

Threonine (ACC)

Threonine (ACA)

Threonine (ACG)

Alanine (GCU)

Alanine (GCC)

Alanine (GCA)

Alanine (GCG)

Tyrosine (UAU)

Tyrosine (UAC)

Stop (UAA)

Stop (UAG)

Histadine (CAU)

Histadine (CAC)

Glutamine (CAA)

Glutamine (CAG)

Asparagine (AAU)

Asparagine (AAC)

Lysine (AAA)

Lysine (AAG)

Aspartate (GAU)

Aspartate (GAC)

Glutamate (GAA)

Glutamate (GAG)

Cysteine (UGU)

Cysteine (UGC)

Stop (UGA)

Tryptophan (UGG)

Arginine (CGU)

Arginine (CGC)

Arginine (CGA)

Arginine (CGG)

Serine (AGU)

Serine (AGC)

Arginine (AGA)

Arginine (AGG)

Glycine (GGU)

Glycine (GGC)

Glycine (GGA)

Glycine (GGG)

The Genetic Code

• As you could see on the chart of amino acids, there are more than one codon that code for certain amino acids – This results in fewer errors in protein

synthesis

The Genetic Code

• All organisms use the same genetic code

• This provides evidence that all life on earth may have evolved from the same origin

Ribosomes

• The newly produced strand of mRNA travels from the nucleus to the cytoplasm where it attaches to a ribosome

• The ribosome is the actual site for protein synthesis

Translation: From mRNA to Protein

• The process of “reading” the three letter “words” , or codons, is known as translation

• Translation takes place at the ribosomes in the cytoplasm– The amino acids are free-floating in the

cytoplasm and congregate at the ribosome during protein synthesis

The Process of Protein Synthesis

• The first codon is usually A-U-G, methionine, which starts the production of a new protein

• Following the start codon, the remaining codons call for amino acids in the order in which they appear on the mRNA strand

Transfer RNA (tRNA)

• At the ribosome, Transfer RNA (tRNA) identifies the code through translation and finds the appropriate amino acid matching the codons– Each tRNA molecule only

attaches to one specific amino acid

– The Anticodon on the bottom of the tRNA molecule corresponds with the codons on the mRNA strand

Amino acid

Chain of RNA nucleotides

Transfer RNA molecule

Anticondon

Protein Synthesis Animation

– Click Here to Link to An Animation