Unit Overview – pages 250-251

1. What are the 3 components of this DNA nucleotide?2. What is the function of DNA in the cell?

Genetics

DNA and Genes

DNA: The Molecule of Heredity

• The environment influences how an organism develops.

• DNA ultimately determines an organism’s traits.

• DNA molecules do this through the genetic information they hold.

What is DNA?What is DNA?

• Within the structure of DNA is the information for life—the complete instructions for manufacturing all the proteins for an organism.

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

– Your body is made up of proteins.

– Your body’s functions depend on proteins called enzymes.

What is DNA?What is DNA?

• In 1952 Alfred Hershey and Martha Chase performed an experiment using radioactively labeled viruses that infect bacteria.

• These viruses were made of only protein and DNA.

DNA as the genetic materialDNA as the genetic material

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

Phosphate group

Sugar (deoxyribose)

Nitrogenous base

The structure of nucleotidesThe structure of nucleotides

• The phosphate group is composed of one atom of phosphorus surrounded by four oxygen atoms.

• Deoxyribose is the simple sugar in DNA


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

• 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) Thymine (T)Cytosine (C)


• Thus, in DNA there are four possible nucleotides, each containing one of these four bases.


• Nucleotides join together to form long chains.

• The phosphate groups and deoxyribose molecules form the backbone of the chain.

• The nitrogenous bases stick out like the teeth of a zipper.


• In DNA, adenine always pairs with thymine, and guanine always pairs with cytosine.


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

The structure of DNAThe structure of DNA

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

The structure of DNAThe structure of DNA

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

The importance of nucleotide sequencesThe importance of nucleotide sequences

Chromosome

The sequence of nucleotides forms the unique genetic information of an organism. The closer the relationship is between two organisms, the more similar their DNA nucleotide sequences will be.

Replication of DNAReplication of DNA

• Before a cell can divide by mitosis or meiosis, it must first make a copy of its chromosomes.

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

• Without DNA replication, new cells would have only half the DNA of their parents.

Replication of DNA

Replication of DNA

DNA

Replication

Replication

• DNA is copied during interphase prior to mitosis and meiosis.

• It is important that the new copies are exactly like the original molecules.

Copying DNACopying DNA

Copying DNACopying DNA

Original DNA

Original DNA

Strand

Original DNA

Strand

Free Nucleotides New DNA

moleculeNew DNA

Strand

New DNA molecule

DNAi• How DNA is packaged

•DNA unzip

•Replicating the helix

•Mechanism of replication

Book of Life• DNA Basics

Question 1

What importance did the experiment performed by Alfred Hershey and Martha Chase have in determining what genetic material was?

Many scientists believed protein was the genetic material. However, an experiment using radioactively labeled viruses allowed Hershey and Chase to provide convincing evidence that DNA is the genetic material.

Answer

Question 2

Which of the following is NOT a component of DNA?

D. proteins

C. nitrogenous bases

B. phosphate groups

A. simple sugars

The answer is D.

Question 3

Which of the following correctly comprises a complimentary base pair?

D. cytosine – thymine

C. guanine – adenine

B. thymine – guanine

A. adenine – thymine

The answer is A.

1. What parts does RNA have that DNA does not have?

• The sequence of nucleotides in DNA contain information.

Genes and ProteinsGenes and Proteins

• This information is put to work through the production of proteins.

• Proteins fold into complex, three- dimensional shapes to become key cell

structures and regulators of cell functions.

• You learned earlier that proteins are polymers of amino acids.

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

Genes and ProteinsGenes and Proteins

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

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

RNARNA

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

Ribose

RNARNA

• Both DNA and RNA contain four nitrogenous bases.

• Rather than thymine, RNA contains a similar base called uracil (U). • Uracil forms a

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

Uracil

Hydrogen bonds Adenine

RNARNA

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

• The workers for protein synthesis are RNA molecules.

RNARNA

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

• 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.

RNARNA

• Ribosomal RNA (rRNA) binds to the mRNA and uses the instructions to assemble the amino acids in the correct order.

• The ribosome is made of ribosomal RNA.

RNARNA

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

RNARNA

TranscriptionTranscription

• Transcription is a process in which enzymes make an RNA copy of a portion of a DNA strand in the cell’s nucleus.

• Transcription results in the formation of one single-stranded RNA molecule.

• Not all the nucleotides in the DNA of eukaryotic cells carry instructions—or code—for making proteins.

• Genes usually contain many long noncoding nucleotide sequences, called introns, that are scattered among the coding sequences.

RNA ProcessingRNA Processing


• Regions that contain information are called exons because they are expressed.

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

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

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

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


DNAi

•Transcription

•mRNA splicing

1. What are the three types of RNA?

2. How do they work together to build a protein?

1. What are the three types of RNA?

2. How do they work together to build a protein?

• The nucleotide sequence transcribed from DNA to a strand of messenger RNA acts as a genetic message, the complete information for the building of a protein.

The Genetic CodeThe Genetic Code

• A code is needed to convert the language of mRNA into the language of proteins.

• Sixty-four combinations are possible when a sequence of three bases is used.

• There are 64 different mRNA codons in the genetic code.


• A codon is a group of three nitrogenous bases in mRNA that code for one amino acid.

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)

• Some codons do not code for amino acids; they provide instructions for making the protein.

• More than one codon can code for the same amino acid.

• However, for any one codon, there can be only one amino acid.


Translation: From mRNA to ProteinTranslation: From mRNA to Protein

• Translation is the process of converting the information in a sequence of nitrogenous bases in mRNA into a sequence of amino acids in protein.

• Translation takes place at the ribosomes in the cytoplasm.

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

• This is the role of transfer RNA.

The role of transfer RNAThe role of transfer RNA

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

• An anticodon is a sequence of three bases found on tRNA.

Amino acid

Chain of RNA nucleotides

Transfer RNA molecule

Anticondon



Ribosome

mRNA codon

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

• AUG signals the start of protein synthesis.

• Then the ribosome slides along the mRNA to the next codon.


tRNA anticodon

Methionine


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

Alanine


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

AlanineMethionine

Peptide bond


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

Stop codon


DNAi

•Triplet code

•Translation

http://www.pbs.org/wgbh/nova/body/rnai.html

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What are the three chemical differences between RNA and DNA?

Question 1

Answer

RNA consists of a single strand of nucleotides whereas DNA is a double strand. RNA contains ribose as its sugar and DNA contains deoxyribose as its sugar. Uracil in RNA replaces thymine in DNA as the nitrogenous base.

What is the role of rRNA in protein synthesis?

Question 2

Answer

Ribosomal RNA binds to messenger RNA and assembles the amino acids in the order needed for the protein to be synthesized.

Which regions of the mRNA travel to the ribosome; introns, exons, or both?

Question 3

Answer

Only exons, which contain coding information, travel to the ribosome. Introns, noncoding nucleotide sequences, do not travel to the ribosome.

What is an anticodon, and what does it represent?

Question 4

Answer

An anticodon is a sequence of three nucleotides on the tRNA molecule that binds to a codon of the mRNA strand.

What is the product of replication; of transcription; of translation?

Question 4

Answer

Two DNA molecules; one mRNA molecule; a protein

1. What would be the resulting sequence of amino acids in this growing protein chain, based on the sequence of bases illustrated in the mRNA? (p. 292)2. Why is this exact base sequence important?

• Organisms have evolved many ways to protect their DNA from changes.

Mutations

• In spite of these mechanisms, however, changes in the DNA occasionally do occur.

• A mutation is any change in a DNA sequence.

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

• Mutations can occur in the reproductive cells.

– This then becomes part of the genetic makeup of the offspring.

– If the change makes a protein nonfunctional, the embryo may not survive.

Mutations in reproductive cells

• What happens if powerful radiation, such as gamma radiation, hits the DNA of a nonreproductive cell, a cell of the body such as in skin, muscle, or bone?

• If the body cell’s DNA is changed, this mutation would not be passed on to offspring.

• The mutation may cause problems for the individual.

Mutations in body cells

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

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

The effects of point mutations

Frameshift mutations

THE DOG BIT THE CAT

THE DOG BIT THE CAR

• Affects only a single codon.

• Since some amino acids have many codons, this would not cause a change in the protein.

• Or the resulting protein may be nonfunctional.

The effects of point mutations

Normal

Point mutation

mRNA

ProteinStop

Stop

mRNA

Protein

Replace G with A


• This mutation would cause nearly every amino acid in the protein after the deletion to be changed.

• A frameshift mutation is a mutation in which a single base is added or deleted from DNA.

• A frameshift mutation shifts the reading of codons by one base.


• Because mRNA is read in three base sections (codons) during translation, an addition or deletion of nucleotides can alter the sequence of the bases, or reading frame, of the genetic message.

THE DOG BIT THE CAT

THE DOB ITT HEC AT


mRNA

Protein

Frameshift mutation

Deletion of U

• Changes may occur in chromosomes as well as in genes.

• Alterations to chromosomes may occur in a variety of ways.

• Chromosomal mutations are structural changes in chromosomes.

Chromosomal Alterations

• In cases where the zygote lives and develops, the mature organism is often sterile and thus incapable of producing offspring.

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

Deletion

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


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

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

Insertion

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


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

Inversion

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


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

A B E FDCBX AWC HGGE HD F

W X Y Z Y ZTranslocation


• Some mutations seem to just happen, perhaps as a mistake in base pairing during DNA replication.

• These mutations are said to be spontaneous.

• However, many mutations are caused by factors in the environment.

Causes of Mutations

• A mutagen is any agent that can cause a change in DNA.

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

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

Causes of Mutations

Causes of Mutations• The breaking and reforming of a double-

stranded DNA molecule can result in deletions.

• Chemical mutagens include dioxins, asbestos, benzene, and formaldehyde, substances that are commonly found in buildings and in the environment.

• Chemical mutagens usually cause substitution mutations.

Repairing DNA• Repair mechanisms that fix mutations in cells

have evolved.• Enzymes proofread the DNA and replace

incorrect nucleotides with correct nucleotides.

• These repair mechanisms work extremely well, but they are not perfect.

• The greater the exposure to a mutagen such as UV light, the more likely is the chance that a mistake will not be corrected.

DNAi

•DNA damage

•Sickle cell

Book of Life•Expression

Any change in DNA sequences is called a _______.

Question 1

D. translation

C. transcription

B. mutation

A. replication

The answer is B.

Which is more serious, a point mutation or a frameshift mutation? Why?

Question 2

Answer

A frameshift mutation is more serious than a point mutation because it disrupts more codons than a point mutation.

Why are chromosomal mutations rarely passed on to the next generation?

Question 3

Answer

Few chromosomal changes are passed on to the next generation because the zygote usually dies. If the zygote survives, it is often sterile and incapable of producing offspring.

Question 1

How does DNA control the structures and functions of a cell?

Answer

DNA determines the structure of proteins. Some proteins become important cell structures. Other proteins, such as enzymes, control chemical reactions that perform key life functions.

Question 2The process through which the order of bases in messenger RNA codes for the order of amino acids in a protein is:

D. point mutation

C. replication

B. translation

A. transcription

The answer is B.

Question 3

Why would scientists use nucleotide sequences to identify bodies of crime victims?

Answer

In comparing nucleotide sequences in the DNA of a crime victim with nucleotide sequences from a possible close relative of the crime victim, scientists can determine if the two are related.

Question 4

What happens when a stop codon is reached during translation?

Answer

When a stop codon is reached, translation ends and the amino acid strand is released from the ribosome.

Question 5

A ________ bond forms between adjacent amino acids during translation.

D. peptide

C. hydrogen

B. phosphate

A. nucleotide

The answer is D.

Question 6

What is the difference between a purine and a pyrimidine?

Answer

A purine is a double-ringed nitrogenous base. A pyrimidine is a single-ringed nitrogenous base.

Question 7

Why is DNA replication important to cell division?

Answer

Without DNA replication, new cells would have only half the DNA of their parents. Species could not survive and individuals could not grow or reproduce successfully.

Question 8At the beginning and end of replication, which of the following are instrumental in breaking and bonding the hydrogen bonds between bases?

D. enzymes

C. nucleotides

B. purines

A. pyrimidines

The answer is D.

Question 9

What is the role of mRNA in protein synthesis?

Answer

The messenger RNA acts as a genetic message, providing the complete information, in sequences of codons, for the building of a protein.

Question 10The DNA sequences of a parrot _________.

D. contain exactly the same nucleotides as those of a beetle

C. are exactly the same as those of a human

B. are more similar to a fern than a dog

A. are more similar to those of a clam than a robin

The answer is D.

Unit Overview – pages 250-251

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