Biology (Holt)

2 The Evolution of Cellular Life

First eukaryotes

JURASSIC PERIOD

•••••1,500

Analysis

1. Defendthe theory of
   endosymbiosis using
   these data.


2. Inferwhat these data indicate
   about the evolution of plant cells.


3. Describe how these data can
   be used to support the idea that
   more than one type of cell evolved
   early in the history of life.

Analyzing Signs of Endosymbiosis

Background

You may recall that mitochondria have their own DNA and produce

their own proteins. The data below were collected by scientists

studying the proteins produced by mitochondrial DNA. The scien-

tists found that the three-nucleotide sequences (codons) in the

nucleus of an organism’s cells can code for different amino acids

than those coded for in the cell’s mitochondria. Examine the data

below, and answer the questions that follow.

0100010110

011101010

0010010001001

1100100100010

0000101001001

1101010100100

0101010010010

Magnification: 6930x

Amino Acids Made in the Nucleus and Mitochondria

Amino acids or other Amino acids or other

instructions coded for instructions coded for

in the nucleus in mitochondria

Codon Plants and mammals Plants Mammals

UGA Stop Stop Tryptophan

AGA Arginine Arginine Stop

AUA Isoleucine Isoleucine Methionine

AUU Isoleucine Isoleucine Methionine

CUA Leucine Leucine Leucine

The Evolution of Eukaryotes

About 1.5 billion years ago, the first eukaryotes appeared. A eukary-

otic cell is much larger than a prokaryote is. Eukaryotic cells have

a complex system of internal membranes. Eukaryotic DNA is

enclosed within a nucleus. Almost all eukaryotes have mitochon-

dria. Chloroplasts, which carry out photosynthesis, are found only

in protists and plants. Mitochondria and chloroplasts are the size of

prokaryotes, and they contain their own DNA.

The Origins of Mitochondria and Chloroplasts

Most biologists think that mitochondria and chloroplasts originated

as described by the theory of that was proposed in

1966 by the American biologist Lynn Margulis. This theory proposes

that mitochondria are the descendants of symbiotic, aerobic (oxy-

gen-requiring) eubacteria and cholorplasts are the descendants of

symbiotic, photosynthetic eubacteria.

endosymbiosis

2C 3A 6A 7A 9A