Protein Synthesis ppt

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10.6 Genes control phenotypic traits

through the expression of proteins

•

DNA specifies traits by dictating protein synthesis.

•

Proteins are the links between genotype and phenotype.

•

The molecular chain of command is from DNA in the nucleus to RNA

(4)

10.6 Genes control phenotypic traits

•

Transcription

is the synthesis of RNA under the direction of DNA.

(5)

Figure 10.6a-1

DNA

NUCLEUS

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Figure 10.6a-2

DNA

NUCLEUS

CYTOPLASM

RNA

(7)

Figure 10.6a-3

DNA

NUCLEUS

CYTOPLASM

RNA

Transcription

Translation

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10.6 Genes control phenotypic traits

•

Genes provide the instructions for making specific proteins.

• _{The initial one gene–one enzyme hypothesis was based on studies of}

inherited metabolic diseases.

(9)

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10.6 Genes control phenotypic traits

•

Most recently, the one gene–one polypeptide hypothesis recognizes

that some proteins are composed of multiple polypeptides.

•

Even this description is not entirely accurate, in that the RNA

transcribed from some genes is not translated but nonetheless has

important functions.

•

In addition, many eukaryotic genes code for a set of polypeptides

(11)

10.7 Genetic information written in

codons is translated into amino

acid sequences

•

The sequence of nucleotides in DNA provides a code for constructing

a protein.

• _{Protein construction requires a conversion of a nucleotide sequence to an}

amino acid sequence.

• _{Transcription rewrites the DNA code into RNA, using the same nucleotide}

(12)

10.7 Genetic information written in

codons is translated into amino

acid sequences

•

The flow of information from gene to protein is based on a

triplet code

.

•

The genetic instructions for the amino acid sequence of a polypeptide

chain are written in DNA and RNA as a series of nonoverlapping three-base

“words” called

codons

.

•

Translation involves switching from the nucleotide “language” to the amino

acid “language.”

•

Each amino acid is specified by a codon.

• _{64 codons are possible.}

(13)

Figure 10.7-0

DNA molecule

Gene 2

Gene 1

Gene 3 DNA

RNA

Amino acid Codon Transcription

Translation

Polypeptide

A A A C C G G C A A A A

(14)

Figure 10.7-1

DNA

RNA

Amino acid Codon Transcription

Translation

Polypeptide

A A A C C G G C A A A A

(15)

10.8 The genetic code dictates how

codons are translated into amino

acids

•

The

genetic code

is the amino acid translations of each of the

nucleotide triplets.

• _{Three nucleotides specify one amino acid.} • _{Sixty-one codons correspond to amino acids.}

(16)

10.8 The genetic code dictates how

codons are translated into amino

acids

•

The genetic code is

• _redundant_{, with more than one codon for some amino acids,}

• _unambiguous_{, in that any codon for one amino acid does not code for any}

other amino acid, and

• _nearly _universal_{, in that the genetic code is shared by organisms from the}

(17)

Figure 10.8a

Second base of RNA codon

T h ir d b as e o f R N A c o d o n F ir st b as e o f R N A c o d o n

U C A G

U C A G U C A G U C A G U C A G UUU UUC UUA UUG UCU UCC UCA UCG UAU UAC UGU UGC UGG Phe Leu Leu lle Val Ala Thr Pro Ser Tyr Cys Trp UGA Stop UAA Stop

UAG Stop

(18)

Figure 10.8b-1

Strand to be transcribed

DNA T A C T T C A A A A T C

(19)

Figure 10.8b-2

Strand to be transcribed

DNA T A C T T C A A A A T C

A T G A A G T T T T A G

RNA

Transcription

(20)

Figure 10.8b-3

Strand to be transcribed

DNA T A C T T C A A A A T C

A T G A A G T T T T A G

RNA

Transcription

A U G A A G U U U U A G

Translation

Polypeptide Met Lys Phe

Stop codon Start

(21)

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10.9 VISUALIZING THE CONCEPT: Transcription

produces genetic messages in the form of RNA

•

Transcription of a gene occurs in three main steps:

1. initiation, involving the attachment of RNA

polymerase to the promoter and the start of RNA synthesis,

2. elongation, as the newly formed RNA strand

grows, and

3. termination, when RNA polymerase reaches the

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Figure 10.9-1

Initiation

RNA synthesis begins after RNA polymerase attaches to the promoter.

RNA polymerase

DNA of gene

Promoter

Terminator DNA

Newly formed

RNA Template strandof DNA Unused strand of DNA

(25)

Figure 10.9-2

Initiation

RNA polymerase DNA of gene Promoter Terminator DNA Newly formed

Direction of transcription

Elongation

Using the DNA as a template, RNA polymerase adds free RNA nucleotides one at a time.

Newly made RNA

DNA strands reunite

Free RNA nucleotide

DNA strands separate

U

T C C A A

T

A G G T T

C A

T

G

AG A U C C A

A U

A A

(26)

Figure 10.9-3

Initiation

RNA polymerase DNA of gene Promoter Terminator DNA Newly formed

Elongation

Using the DNA as a template, RNA polymerase adds free RNA nucleotides one at a time.

Newly made RNA

DNA strands reunite

Free RNA nucleotide DNA strands separate U Termination

RNA synthesis ends when RNA polymerase reaches the

terminator DNA sequence.

Terminator DNA

RNA polymerase detaches

Completed RNA

T C C A A

T

A G G T T

C A

T

G

AG A U C C A

A U

A A

(27)

10.10 Eukaryotic RNA is processed

before leaving the nucleus as mRNA

•

Messenger RNA

(

mRNA

)

• _{encodes amino acid sequences and}

• _{conveys genetic messages from DNA to the translation machinery of the cell.}

• In prokaryotes, this occurs in the same place that mRNA is made.

• But in eukaryotes, mRNA must exit the nucleus via nuclear pores to enter the cytoplasm.

•

Eukaryotic mRNA has

introns

, interrupting sequences that separate

(28)

10.10 Eukaryotic RNA is processed

before leaving the nucleus as mRNA

•

Eukaryotic mRNA undergoes processing before leaving the nucleus.

• _{RNA splicing}_{removes introns (intervening sequences) and joins exons}

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10.10 Eukaryotic RNA is processed

before leaving the nucleus as mRNA

•

A cap and tail of extra nucleotides are added to the ends of the mRNA

to

• _{facilitate the export of the mRNA from the nucleus,}

• _{protect the mRNA from degradation by cellular enzymes, and} • _{help ribosomes bind to the mRNA.}

(30)

Figure 10.10

Exon Exon Exon

DNA

Intron

Intron Transcription

Addition of cap and tail

Tail Introns removed

Exons spliced together

NUCLEUS

CYTOPLASM

Cap

Coding sequence RNA

transcript with cap and tail

(31)

10.11 Transfer RNA molecules serve

as interpreters during translation

•

Transfer RNA

(

tRNA

) molecules function as an interpreter, converting

the genetic message of mRNA into the language of proteins.

•

Transfer RNA molecules perform this interpreter task by

• _{picking up the appropriate amino acid and}

• _{using a special triplet of bases, called an}_anticodon_{, to recognize the}

(32)

Figure 10.11a

Amino acid

attachment site

Hydrogen bond

RNA polynucleotide chain

A simplified

representation of a tRNA A tRNA molecule, showing

its polynucleotide strand and hydrogen bonding

(33)

Figure 10.11b

tRNA

Enzyme

(34)

10.12 Ribosomes build polypeptides

•

Translation occurs on the surface of the

ribosome

.

• _{Ribosomes coordinate the functioning of mRNA and tRNA and, ultimately, the}

synthesis of polypeptides.

• _{Ribosomes have two subunits: small and large.}

• _{Each subunit is composed of}_{ribosomal RNAs}_{and proteins.} • _{Ribosomal subunits come together during translation.}

(35)

Figure 10.12-0 tRNA molecules Growing polypeptide Ribosome Large subunit

tRNA binding sites

Small subunit

mRNA binding site P

site siteA

Growing polypeptide

mRNA

tRNA

The next amino acid to be added to the polypeptide

(36)

Figure 10.12-1

tRNA

molecules

Large subunit

Small subunit

(37)

Figure 10.12-2

Large subunit

tRNA binding sites

Small subunit

mRNA binding site P site

(38)

Figure 10.12-3

mRNA

tRNA

The next amino acid to be added to the polypeptide

(39)

10.12 Ribosomes build polypeptides

•

The ribosomes of bacteria and eukaryotes are very similar in function.

•

Those of eukaryotes are slightly larger and different in composition.

•

The differences are medically significant.

• _{Certain antibiotic drugs can inactivate bacterial ribosomes while leaving}

eukaryotic ribosomes unaffected.

• _{These drugs, such as tetracycline and streptomycin, are used to combat}

(40)

10.13 An initiation codon marks the

start of an mRNA message

•

Translation can be divided into the same three phases as

transcription:

1. initiation,

2. elongation, and 3. termination.

•

Initiation brings together

• _mRNA,

• _{a tRNA bearing the first amino acid, and} • _{the two subunits of a ribosome.}

(41)

10.13 An initiation codon marks the

start of an mRNA message

•

Initiation establishes where translation will begin.

•

Initiation occurs in two steps.

1. An mRNA molecule binds to a small ribosomal subunit, and a special initiator tRNA binds to mRNA at the start codon.

• The start codon reads AUG and codes for methionine.

• The first tRNA has the anticodon UAC.

(42)

10.13 An initiation codon marks the

start of an mRNA message

•

Initiation establishes where translation will begin.

•

Initiation occurs in two steps.

2. A large ribosomal subunit joins the small subunit, allowing the ribosome to function.

• The first tRNA occupies the P site, which will hold the growing polypeptide.

• The A site is available to receive the next amino-acid-bearing tRNA.

(43)

Figure 10.13a

Cap

Start of genetic message

End

(44)

Figure 10.13b-1

Small ribosomal subunit

U

Start codon A C mRNA

Initiator tRNA

A U G

Met

(45)

Figure 10.13b-2

Small ribosomal subunit

U

Start codon A C mRNA

Initiator tRNA

A U G

Met

1 2

Met

Large ribosomal subunit

P

site siteA U A C

(46)

10.14 Elongation adds amino acids to the polypeptide

chain until a stop codon terminates translation

•

Once initiation is complete, amino acids are added one by one to the

first amino acid.

(47)

10.14 Elongation adds amino acids to the polypeptide

•

Each cycle of elongation has three steps.

1. The anticodon of an incoming tRNA molecule, carrying its amino acid, pairs with the mRNA codon in the A site of the ribosome.

2. The polypeptide separates from the tRNA in the P site and attaches by a new peptide bond to the amino acid carried by the tRNA in the A site.

(48)

Figure 10.14-1

mRNA

Polypeptide Aminoacid

Anticodon A site

P site

Codons

Codon

recognition

(49)

Figure 10.14-2

mRNA

Polypeptide Aminoacid

P site

Codons

Codon

recognition

1

Peptide bond formation

(50)

Figure 10.14-3

mRNA

P site

Codons

Codon

recognition

1

Peptide bond formation

2

New peptide bond

Translocation

(51)

Figure 10.14-4

mRNA

(52)

10.14 Elongation adds amino acids to the polypeptide

•

Elongation continues until the termination stage of translation, when

• _{the ribosome reaches a}_{stop codon}_,

(53)

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10.15 Review: The flow of genetic

information in the cell is DNA



RNA



protein

•

The flow of genetic information is from DNA to RNA to protein.

• _{In transcription (DNA → RNA), the mRNA is synthesized on a DNA template.} • _{In eukaryotic cells, transcription occurs in the nucleus, and the messenger}

RNA is processed before it travels to the cytoplasm.

(55)

Figure 10.15-1

DNA

mRNA

RNA

polymerase

NUCLEUS

Transcription

Transcription

(56)

2

Amino acid attachment

Figure 10.15-2

DNA

mRNA

RNA

polymerase

NUCLEUS

Transcription

1

Translation

Amino acid Enzyme ATP tRNA

CYTOPLASM

(57)

10.15 Review: The flow of genetic

information in the cell is DNA



RNA



protein

•

Translation can be divided into four steps, all of which occur in the

cytoplasm:

1. amino acid attachment,

2. initiation of polypeptide synthesis, 3. elongation, and

(58)

2 Amino acid attachment Figure 10.15-3 DNA mRNA RNA polymerase Transcription 1 Translation Amino acid Enzyme ATP tRNA Anticodon Initiation of polypeptide synthesis 3 Large ribosomal subunit Small ribosomal subunit Start codon mRNA Initiator tRNA

AUG

U C_A

CYTOPLASM

Transcription

(59)

Amino acid attachment 2 Figure 10.15-4 DNA mRNA RNA polymerase Transcription 1 Translation Amino acid Enzyme ATP tRNA Anticodon 3 Large ribosomal subunit Small ribosomal subunit Start codon mRNA Initiator tRNA

AUG

U C_A

(60)

Figure 10.15-5 Amino acid attachment 2 DNA mRNA RNA polymerase Transcription 1 Translation Amino acid Enzyme ATP tRNA Anticodon 3 Large ribosomal subunit Small ribosomal subunit Start codon mRNA Initiator tRNA

AUG

U C_A

(61)

10.16 Mutations can affect genes

•

A

mutation

is any change in the nucleotide sequence of DNA.

•

Mutations can involve

(62)

10.16 Mutations can affect genes

•

Mutations within a gene can be divided into two general categories.

1. Nucleotide substitutions involve the replacement of one nucleotide and its base-pairing partner with another pair of nucleotides. Base substitutions may

• have no effect at all, producing a silent mutation,

• change the amino acid coding, producing a missense mutation, which produces

a different amino acid,

• lead to a base substitution that produces an improved protein that enhances the

success of the mutant organism and its descendants, or

(63)

10.16 Mutations can affect genes

2. Nucleotide insertions or deletions of one or more nucleotides in a gene may

• cause a frameshift mutation, which alters the reading frame (triplet grouping) of the genetic message,

• lead to significant changes in amino acid sequence, and

(64)

10.16 Mutations can affect genes

•

Mutagenesis

is the production of mutations.

•

Mutations can be caused

• _{by spontaneous errors that occur during DNA replication or recombination or} • _by_mutagens_{, which include}

• high-energy radiation such as X-rays and ultraviolet light and

(65)

Figure 10.16a

Normal hemoglobin DNA Mutant hemoglobin DNA

Sickle-cell hemoglobin Normal hemoglobin

mRNA mRNA

C T T C A T

G U A G A A

(66)

Figure 10.16b-0 Normal gene Nucleotide substitution mRNA Protein Nucleotide deletion Nucleotide insertion Inserted Deleted Met Met Met Met Lys Lys Lys Lys Phe Phe Leu Leu Gly Trp Ala Ser Ala Ala Arg