HUMAN BIOLOGY

DNa, GeNes, aND BioteChNoloGy 413

hoW is mrNa traNslateD iNto proteiN?

• Translation begins when a small ribosome unit and an initiator
  tRNA both arrive at an mRNA’s start codon and a large ribosome
  subunit binds to them.


• tRNAs deliver amino acids to the ribosome in the order dictated
  by the sequence of mRNA codons. A chain of amino acids
  (a polypeptide chain) grows as peptide bonds form between
  the amino acids.


• Translation ends when a stop codon triggers events that cause
  the chain and the mRNA to detach from the ribosome.

taKe-home messaGe

F i g u r e 21.12 Animated! Translation takes place in a cell’s cytoplasm. A Translation begins while transcription is still going on in the
cell’s nucleus. B The micrograph below shows clusters of ribosomes (polysomes) all translating DNA in a cell. (© Cengage Learning)

polysomes

Transcription

ribosome

subunits

polysomes

tRNA

Translation

Convergence of RNAs

polypeptide

mRNA

Many newly forming polypeptide chains carry out

their functions in the cytoplasm. Others have a “shipping

label,” a special sequence of amino acids. The label allows

them to enter the rough ER of the endomembrane system

(Section 3.7). There they are modi fied into their final form

before being shipped to their ultimate destinations inside

or outside the cell.

A

B

Biology Pics/Science Source

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).