What You’ll Learn

You will relate the structure of DNA to its function.

You will explain the role of DNA in protein production.

You will distinguish among different types of mutations.

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The genetic information that is held in the The genetic information that is held in the molecules of DNA ultimately determines an molecules of DNA ultimately determines an organism’s traits.organism’s traits.

DNA achieves its control by determining the DNA achieves its control by determining the structure of proteins.structure of proteins.

What is DNA?What is DNA?

DNA is a polymer made of repeating subunits DNA is a polymer made of repeating subunits called nucleotides.called nucleotides.

Nucleotides have three parts: a simple sugar, Nucleotides have three parts: a simple sugar, a phosphate group, and a nitrogenous base.a phosphate group, and a nitrogenous base.

Phosphate group

Sugar (deoxyribose)

Nitrogenous base

The structure of nucleotidesThe structure of nucleotides

The simple sugar in DNA, called The simple sugar in DNA, called deoxyribose deoxyribose (dee ahk sih RI bos)(dee ahk sih RI bos), gives DNA its name—, gives DNA its name—deoxyribonucleic acid.deoxyribonucleic acid.

The structure of nucleotidesThe structure of nucleotides

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

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

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

The structure of nucleotidesThe structure of nucleotides

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

The structure of DNAThe structure of DNA

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

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

The importance of nucleotide sequencesThe importance of nucleotide sequences

Chromosome

The sequence of nucleotides forms the unique genetic information of an organism.

Replication of DNAReplication of DNA

The DNA in the chromosomes is copied in a The DNA in the chromosomes is copied in a process called process called DNA replicationDNA replication..

Replication of DNA

Replication of DNA

DNA

Replication

Replication

Copying DNACopying DNA

Original DNA

Original DNA

Strand

Original DNA

Strand

Free Nucleotides New DNA

moleculeNew DNA

Strand

New DNA molecule

RNA like DNA, is a RNA like DNA, is a nucleic acid. RNA nucleic acid. RNA structure differs structure differs from DNA structure from DNA structure in three ways.in three ways.

First, RNA is single strandedFirst, RNA is single stranded——it looks like it looks like one-half of a zipperone-half of a zipper——whereas DNA is whereas DNA is double stranded.double stranded.

RNARNA

The sugar in The sugar in RNA is ribose; RNA is ribose; DNA’s sugar is DNA’s sugar is deoxyribose.deoxyribose.

Ribose

RNARNA

Both DNA and RNA contain four nitrogenous Both DNA and RNA contain four nitrogenous bases, but rather than thymine, RNA contains a bases, but rather than thymine, RNA contains a similar base called uracil (U).similar base called uracil (U).

Uracil forms a Uracil forms a base pair with base pair with adenine in adenine in RNA, just as RNA, just as thymine does in thymine does in DNA.DNA.

Uracil

Hydrogen bonds Adenine

RNARNA

DNA provides workers with the instructions for DNA provides workers with the instructions for making the proteins, and workers build the making the proteins, and workers build the proteins.proteins.

The workers for protein synthesis are RNA The workers for protein synthesis are RNA molecules.molecules.

RNARNA

There are three types of RNA that help build There are three types of RNA that help build proteins.proteins.

Messenger RNAMessenger RNA (mRNA), brings instructions (mRNA), brings instructions from DNA in the nucleus to the cell’s ribosome.from DNA in the nucleus to the cell’s ribosome.

RNARNA

•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 (tRNA) is the supplier. Transfer RNA delivers amino acids to the ribosome to RNA delivers amino acids to the ribosome to be assembled into a protein.be assembled into a protein.

TranscriptionTranscription In the nucleus, enzymes make an RNA copy In the nucleus, enzymes make an RNA copy

of a portion of a DNA strand in a process of a portion of a DNA strand in a process called called transcriptiontranscription..

RNA strand

DNA strand

DNA strand

RNA strand

TranscriptionTranscription

A

B

C

RNA ProcessingRNA Processing

Regions that contain information are called Regions that contain information are called exons because they are exons because they are exexpressed.pressed.

When mRNA is transcribed from DNA, both When mRNA is transcribed from DNA, both introns and exons are copied.introns and exons are copied.

The introns must be removed from the mRNA The introns must be removed from the mRNA before it can function to make a protein.before it can function to make a protein.

Enzymes in the nucleus cut out the intron Enzymes in the nucleus cut out the intron segments and paste the mRNA back together.segments and paste the mRNA back together.

The mRNA then leaves the nucleus and The mRNA then leaves the nucleus and travels to the ribosome.travels to the ribosome.

RNA ProcessingRNA Processing

Biochemists discovered that a group of three Biochemists discovered that a group of three nitrogenous bases in mRNA code for one nitrogenous bases in mRNA code for one amino acid. Each group is known as a amino acid. Each group is known as a codoncodon..

The Genetic CodeThe Genetic Code

Sixty-four combinations are possible when a Sixty-four combinations are possible when a sequence of three bases is used; thus, 64 sequence of three bases is used; thus, 64 different mRNA codons are in the genetic different mRNA codons are in the genetic code.code.

The Genetic CodeThe Genetic Code

The Genetic CodeThe Genetic CodeThe Messenger RNA Genetic Code

First Letter Second Letter

UU C A G

Third Letter

UCAGUCAGUCAG

UCAG

C

A

G

Phenylalanine (UUU)

Phenylalanine (UUC)

Leucine (UUA)

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)

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 (GGC)

Glycine (GGA)

Glycine (GGG)

Translation: From mRNA to ProteinTranslation: From mRNA to Protein The process of converting the information in a The process of converting the information in a

sequence of nitrogenous bases in mRNA into sequence of nitrogenous bases in mRNA into a sequence of amino acids in protein is known a sequence of amino acids in protein is known as as translationtranslation..

Translation takes place at the ribosomes in the Translation takes place at the ribosomes in the cytoplasm.cytoplasm.

For proteins to be built, the 20 different For proteins to be built, the 20 different amino acids dissolved in the cytoplasm must amino acids dissolved in the cytoplasm must be brought to the ribosomes.be brought to the ribosomes.

This is the role of transfer RNA.This is the role of transfer RNA.

The role of transfer RNAThe role of transfer RNA

Each tRNA Each tRNA molecule molecule attaches to attaches to only one type only one type of amino acid.of amino acid.

Amino acid

Chain of RNA nucleotides

Transfer RNA molecule

Anticondon

The role of transfer RNAThe role of transfer RNA

The role of transfer RNAThe role of transfer RNA

Ribosome

mRNA codon

The first codon on mRNA is AUG, which The first codon on mRNA is AUG, which codes for the amino acid methionine.codes for the amino acid methionine.

AUG signals the start of protein synthesis.AUG signals the start of protein synthesis.

When this signal is given, the ribosome When this signal is given, the ribosome slides along the mRNA to the next codon.slides along the mRNA to the next codon.

The role of transfer RNAThe role of transfer RNA

tRNA anticodon

Methionine

The role of transfer RNAThe role of transfer RNA

A new tRNA molecule carrying an amino A new tRNA molecule carrying an amino acid pairs with the second mRNA codon.acid pairs with the second mRNA codon.

Alanine

The role of transfer RNAThe role of transfer RNA

The amino acids are joined when a peptide The amino acids are joined when a peptide bond is formed between them.bond is formed between them.

AlanineMethionine

Peptide bond

The role of transfer RNAThe role of transfer RNA

A chain of amino acids is formed until the A chain of amino acids is formed until the stop codon is reached on the mRNA strand.stop codon is reached on the mRNA strand.

Stop codon

The role of transfer RNAThe role of transfer RNA

MutationsMutations

Any change in DNA sequence is called a Any change in DNA sequence is called a mutationmutation..

Mutations can be caused by errors in Mutations can be caused by errors in replication, transcription, cell division, or replication, transcription, cell division, or by external agents.by external agents.

A A point mutationpoint mutation is a change in a single is a change in a single base pair in DNA.base pair in DNA.

A change in a single nitrogenous base can A change in a single nitrogenous base can change the entire structure of a protein because change the entire structure of a protein because a change in a single amino acid can affect the a change in a single amino acid can affect the shape of the protein.shape of the protein.

The effects of point The effects of point mutationsmutations

The effects of point The effects of point mutationsmutations

Normal

Point mutation

mRNA

ProteinStop

Stop

mRNA

Protein

Replace G with A

Frameshift mutationsFrameshift mutations

A mutation in which a single base is added or A mutation in which a single base is added or deleted from DNA is called a deleted from DNA is called a frameshift mutationframeshift mutation because it shifts the reading of codons by one because it shifts the reading of codons by one base.base.

•Structural changes in chromosomes are called chromosomal mutations.

Any agent that can cause a change in DNA is Any agent that can cause a change in DNA is called a called a mutagenmutagen..

Mutagens include radiation, chemicals, and even Mutagens include radiation, chemicals, and even high temperatures.high temperatures.

Forms of radiation, such as X rays, cosmic Forms of radiation, such as X rays, cosmic rays, ultraviolet light, and nuclear radiation, rays, ultraviolet light, and nuclear radiation, are dangerous mutagens because the energy are dangerous mutagens because the energy they contain can damage or break apart DNA.they contain can damage or break apart DNA.

Causes of Causes of MutationsMutations

When a part of a chromosome is left out, a When a part of a chromosome is left out, a deletion occurs.deletion occurs.

Deletion

A B C D E F G H

A B C E F G H

Chromosomal Chromosomal AlterationsAlterations

When part of a chromatid breaks off and When part of a chromatid breaks off and attaches to its sister chromatid, an insertion attaches to its sister chromatid, an insertion occurs.occurs.

The result is a duplication of genes on the same The result is a duplication of genes on the same chromosome.chromosome.

Insertion

A B C D E F G H A B C B C D E F G H

Chromosomal Chromosomal AlterationsAlterations

When part of a chromosome breaks off and When part of a chromosome breaks off and reattaches backwards, an inversion occurs.reattaches backwards, an inversion occurs.

Inversion

A B C D E F G H A D C B E F G H

Chromosomal Chromosomal AlterationsAlterations

When part of one chromosome breaks off and When part of one chromosome breaks off and is added to a different chromosome, a is added to a different chromosome, a translocation occurs.translocation occurs.

A B E FDCBX AWC HGGE HD F

W X Y Z Y ZTranslocation

Chromosomal Chromosomal AlterationsAlterations