Eukaryotes

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Many

Common

Extensive Yes

Common Yes

Common Many

► Eukaryotic genomes are larger. The genomes of eukaryotes (in terms of haploid DNA content) are larger than those of prokaryotes. This difference is not surprising, given that in multicellular organisms there are many cell types, many jobs to do, and many proteins—all encoded by DNA—needed to do those jobs. A typical virus contains enough DNA to code for only a few proteins—about 10,000 base pairs (bp). The most thoroughly studied prokaryote, E. coli, has sufficient DNA (about 4.5 million bp) to make several thousand different proteins and regulate their synthesis. Humans have considerably more genes and regulators: Nearly 6 billion bp (2 meters of DNA) are crammed into each diploid human cell. However, the idea of a more complex organism needing more DNA seems to break down with some plants. For example, the lily (which produces beautiful flowers each spring, but produces fewer proteins than a human does) has 18 times more DNA than a human.

► Eukaryotic genomes have more regulatory sequences. Eukaryotic genomes have many more regulatory sequences— and many more regulatory proteins that bind to them— than prokaryotic genomes do. The great complexity of eukaryotes requires a great deal of regulation, and this fact is evident in the many processes and points of control associated with the expression of the eukaryotic genome.

► Much of eukaryotic DNA is noncoding. Interspersed throughout the eukaryotic genome are various kinds of repeated DNA sequences that are not transcribed into proteins. Even the coding regions of genes contain sequences that do not appear in the mRNA that is translated at the ribosome.

► Eukaryotes have multiple chromosomes. The genomic encyclopedia of a eukaryote is separated into multiple volumes. This separation requires that each chromosome have, at a minimum, three defining DNA sequences that we have described in previous chapters: an origin of replication recognized by the DNA replication machinery; a centromere region that holds the replicated chromosomes together before mitosis; and a telomeric sequence at each end of the chromosome.

► In eukaryotes, transcription and translation are physically separated. The nuclear envelope separates DNA and its transcription (inside the nucleus) from the sites where mRNA is translated into protein (in the cytoplasm). This separation allows for many points of regulation before translation begins: in the synthesis of a pre-mRNA transcript, in its processing into mature mRNA, and in its transport to the cytoplasm for translation (Figure 14.1).

The yeast genome adds some eukaryotic functions to a prokaryotic model

In comparison with E. coli, whose genome has about 4,500,000 bp on a single chromosome (one circular DNA molecule), the genome of budding yeast (Saccharomyces cerevisiae), a single-celled eukaryote, has 16 linear chromosomes and a haploid content of more than 12,068,000 bp. More than 600 scientists around the world collaborated in mapping and sequencing the yeast genome. When they began, they knew of about 1,000 yeast genes coding for RNAs or proteins. The final sequence revealed 5,900 genes, and sequence analyses have assigned probable roles to about 70 percent of them. Some of these genes are homologous to genes found in prokaryotes, but many are not. The functions of the other 30 percent are being investigated by gene inactivation studies similar to those performed on prokaryotes (see Figure 13.22). This process of discovering the protein product and function of a known gene sequence is called annotation. These accomplishments have made yeast an important model for eu-

DNA in the nucleus contains genes that encode proteins.

2| The genes are transcribed to make messenger RNA (see Chapter 12).

Pre-mRNA transcript is produced.

I Pre-mRNA is processed—parts are removed, ends are added— and the resulting mRNA is exported to the cytoplasm.

In the cytoplasm, ribosomes translate the mRNA to make the protein (polypeptide) coded for by the gene.

DNA in the nucleus contains genes that encode proteins.

2| The genes are transcribed to make messenger RNA (see Chapter 12).

Pre-mRNA transcript is produced.

I Pre-mRNA is processed—parts are removed, ends are added— and the resulting mRNA is exported to the cytoplasm.

Processing Rna Eukaryotes

The nematode genome adds developmental complexity

14.1 Eukaryotic mRNA Is Transcribed in the Nucleus but Translated in the Cytoplasm Compare this "road map"to the prokaryotic one shown in Figure 12.3.

In the cytoplasm, ribosomes translate the mRNA to make the protein (polypeptide) coded for by the gene.

Ribosome nit""":

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  • tyko kes
    Why does a lily plant have 18 times more dna than a human?
    8 years ago

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