Interest Grabber

• Information, Please

• DNA contains the information that a cell needs to carry out all of its functions. In a way, DNA is like the cell’s encyclopedia. Suppose that you go to the library to do research for a science project. You find the information in an encyclopedia. You go to the desk to sign out the book, but the librarian informs you that this book is for reference only and may not be taken out.

Section 12-3

1. Why do you think the library holds some books for reference only?2. If you can’t borrow a book, how can you take home the information in it?3. All of the parts of a cell are controlled by the information in DNA, yet DNA

does not leave the nucleus. How do you think the information in DNA might get from the nucleus to the rest of the cell?

Go to Section:

Protein Synthesis

•Occurs in 2 steps:• In nucleus- Transcription• In cytoplasm/ ribosomes-

Translation

from to to make up

Concept MapSection 12-3

also called which functions to also called also called which functions towhich functions to

can be

RNA

Messenger RNA Ribosomal RNA Transfer RNA

mRNA Carry instructions rRNACombine

with proteins tRNABring

amino acids toribosome

DNA Ribosome Ribosomes

Go to Section:

RNADNA

RNApolymerase

Figure 12–14 TranscriptionSection 12-3

Adenine (DNA and RNA)Cystosine (DNA and RNA)Guanine(DNA and RNA)Thymine (DNA only)Uracil (RNA only)

Go to Section:

Figure 12–17 The Genetic CodeSection 12-3

Go to Section:

Messenger RNA Messenger RNA is transcribed in the nucleus.

Transfer RNAThe mRNA then enters the cytoplasm and attaches to a ribosome. Translation begins at AUG, the start codon. Each transfer RNA has an anticodon whose bases are complementary to a codon on the mRNA strand. The ribosome positions the start codon to attract its anticodon, which is part of the tRNA that binds methionine. The ribosome also binds the next codon and its anticodon.

mRNA Start codon

Ribosome

Methionine

Phenylalanine tRNALysine

Nucleus

Figure 12–18 TranslationSection 12-3

mRNA

Go to Section:

The Polypeptide “Assembly Line”The ribosome joins the two amino acids—methionine and phenylalanine—and breaks the bond between methionine and its tRNA. The tRNA floats away, allowing the ribosome to bind to another tRNA. The ribosome moves along the mRNA, binding new tRNA molecules and amino acids.

mRNARibosome

Translation direction

Lysine tRNA

tRNA

Ribosome

Growing polypeptide chain

mRNA

Completing the PolypeptideThe process continues until the ribosome reaches one of the three stop codons. The result is a growing polypeptide chain.

Figure 12–18 Translation (continued)Section 12-3

Go to Section: