Chapter 3, Section 6

DNA is the master blueprint for protein synthesis

Gene: Segment of DNA with blueprint for one polypeptide

Triplets of nucleotide bases form genetic library

Each triplet specifies coding for an amino acid

Copyright © 2010 Pearson Education, Inc. Figure 3.34

Nuclearpores

mRNA

Pre-mRNARNA Processing

Transcription

Translation

DNA

Nuclearenvelope

Ribosome

Polypeptide

Messenger RNA (mRNA)◦ Carries instructions for building a polypeptide,

from gene in DNA to ribosomes in cytoplasm

Ribosomal RNA (rRNA)◦ A structural component of ribosomes that, along

with tRNA, helps translate message from mRNA

Transfer RNAs (tRNAs)◦ Bind to amino acids and pair with bases of codons

of mRNA at ribosome to begin process of protein synthesis

Transfers DNA gene base sequence to a complementary base sequence of an mRNA

Transcription factor◦ Loosens histones from DNA in area to be

transcribed◦ Binds to promoter, a DNA sequence specifying

start site of gene to be transcribed◦ Mediates the binding of RNA polymerase to

promoter

RNA polymerase◦ Enzyme that oversees synthesis of mRNA◦ Unwinds DNA template◦ Adds complementary RNA nucleotides on DNA

template and joins them together◦ Stops when it reaches termination signal◦ mRNA pulls off the DNA template, is further

processed by enzymes, and enters cytosol

Copyright © 2010 Pearson Education, Inc. Figure 3.35

RNA polymerase

RNA polymerase

RNApolymerase

DNA

Coding strand

Template strandPromoterregion

Terminationsignal

mRNA

mRNA

Template strand

mRNA transcript

Completed mRNA transcript

Rewindingof DNA

Coding strand of DNA

DNA-RNA hybrid region

The DNA-RNA hybrid: At any given moment, 16–18 base pairs ofDNA are unwound and the most recently made RNA is still bound toDNA. This small region is called the DNA-RNA hybrid.

Templatestrand

Unwindingof DNA

RNA nucleotides

Direction oftranscription

Initiation: With the help of transcription factors, RNApolymerase binds to the promoter, pries apart the two DNA strands,and initiates mRNA synthesis at the start point on the template strand.

Termination: mRNA synthesis ends when the termination signalis reached. RNA polymerase and the completed mRNA transcript arereleased.

Elongation: As the RNA polymerase moves along the templatestrand, elongating the mRNA transcript one base at a time, it unwindsthe DNA double helix before it and rewinds the double helix behind it.

1

2

3

Converts base sequence of nucleic acids into the amino acid sequence of proteins

Involves mRNAs, tRNAs, and rRNAs

Each three-base sequence on DNA is represented by a codon ◦ Codon—complementary three-base sequence on

mRNA

Copyright © 2010 Pearson Education, Inc. Figure 3.36

SECOND BASE

UUG

UUA

UUC

UUUPhe

Leu

CUG

CUA

CUC

CUU

Leu

AUA

AUC

AUU

Ile

GUG

GUA

GUC

GUU

Val

UCG

UCA

UCC

UCU

Ser

CCG

CCA

CCC

CCU

Pro

ACG

ACA

ACC

ACU

Thr

GCG

GCA

GCC

GCU

Ala

UAC

UAUTyr

CAG

CAA

CAC

CAUHis

Gln

AAG

AAA

AAC

AAUAsn

Lys

GAG

GAA

GAC

GAUAsp

Glu

UGC

UGUCys

Trp

CGG

CGA

CGC

CGU

Arg

AGG

AGA

AGC

AGUSer

Arg

GGG

GGA

GGC

GGU

Gly

UAA Stop UGA Stop

AUGMet orStart

UAG Stop UGG

U C A G

G

A

C

U

G

A

C

U

G

A

C

U

G

A

C

U

U

C

A

G

mRNA attaches to a small ribosomal subunit that moves along the mRNA to the start codon

Large ribosomal unit attaches, forming a functional ribosome

Anticodon of a tRNA binds to its complementary codon and adds its amino acid to the forming protein chain

New amino acids are added by other tRNAs as ribosome moves along rRNA, until stop codon is reached

Copyright © 2010 Pearson Education, Inc. Figure 3.37

1

2

3

4

Leu

Leu

Energized by ATP, the correct aminoacid is attached to each species oftRNA by aminoacyl-tRNA synthetaseenzyme.

Amino acid

tRNA

Aminoacyl-tRNAsynthetase

G A A

tRNA “head”bearinganticodon

Psite A

siteE

site

Ile

Pro

A AU U UC C C

CG G

G

Largeribosomalsubunit

Smallribosomalsubunit

Direction ofribosome advancePortion of mRNA

already translated

Codon15

Codon16

Codon17

Nucleus

mRNA

Released mRNA

Nuclearmembrane

Nuclear pore

RNA polymerase

Templatestrand ofDNA

After mRNA synthesis in thenucleus, mRNA leaves the nucleusand attaches to a ribosome.

Translation begins as incomingaminoacyl-tRNA recognizes thecomplementary codon calling forit at the A site on the ribosome. Ithydrogen-bonds to the codon viaits anticodon.

As the ribosome moves alongthe mRNA, and each codon isread in sequence, a new aminoacid is added to the growingprotein chain and the tRNA inthe A site is translocated to theP site.

Once its amino acid is releasedfrom the P site, tRNA is ratchetedto the E site and then released toreenter the cytoplasmic pool,ready to be recharged with a newamino acid. The polypeptide isreleased when the stop codon isread.

GA A

U

UA

mRNA–ribosome complex is directed to rough ER by a signal-recognition particle (SRP)

Forming protein enters the ER Sugar groups may be added to the protein,

and its shape may be altered Protein is enclosed in a vesicle for transport

to Golgi apparatus

All cells of the body contain the same DNA but are not identical

Chemical signals in the embryo channel cells into specific developmental pathways by turning some genes off

Development of specific and distinctive features in cells is called cell differentiation

Elimination of excess, injured, or aged cells occurs through programmed rapid cell death (apoptosis) followed by phagocytosis

Wear and tear theory: Little chemical insults and free radicals have cumulative effects

Immune system disorders: Autoimmune responses and progressive weakening of the immune response

Genetic theory: Cessation of mitosis and cell aging are programmed into genes. Telomeres (strings of nucleotides on the ends of chromosomes) may determine the number of times a cell can divide.