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Chapter 19: Eukaryotic Genomes and Gene Expression Ms. Springstroh Adapted from Ms. Gaynor AP Biology
General Characteristics of Eukaryotic Genomes • Much larger than prokaryotic genome • Have longer genes • In eukaryotes, the DNA-protein complex is called chromatin • ~25,000 genes in human genome • 98% consists of non-coding regions
Most of eukaryotic genomes • Consists of noncoding DNA sequences, often described in the past as “junk DNA” • However, much evidence is accumulating • That noncoding “junk” DNA plays important roles in the cell
Both prokaryotes and eukaryotes • Must alter their patterns of gene expression in response to changes in environmental conditions • How do prokaryotes do this??? • Now, let’s learn how eukaryotes do it!!!
Regulation of enzyme production Regulation of enzyme activity Precursor Feedback inhibition Enzyme 1 Gene 1 LE 18-20 Gene 2 Enzyme 2 Regulation of gene expression Gene 3 Enzyme 3 Enzyme 4 Gene 4 Gene 5 Enzyme 5 Tryptophan
Chromatin structure is based on successive levels of DNA packing • Eukaryotic DNA • Is precisely combined with a large amount of protein (called histones) • Eukaryotic chromosomes • Contain an enormous amount of DNA relative to their condensed length due to supercoiling
2 nm DNA double helix Histone tails His- tones 10 nm Histone H1 Linker DNA (“string”) Nucleosome (“bead”) (a) Nucleosomes (10-nm fiber) • In electron micrographs • Unfolded chromatin has the appearance of beads on a string • Each “bead” is a nucleosome • The basic unit of DNA packing consists of 8 histones • Allows for super coiling to occur Figure 19.2 a
30 nm Nucleosome (b) 30-nm fiber Higher Levels of DNA Packing • The next level of packing uses the H1 histone proteins • 10-nm fiber folds & coils to form the 30-nm chromatin fiber • 30 nm = diameter
Protein scaffold in nuclear membrane (lamina) Loops Scaffold 300 nm (c) Looped domains (300-nm fiber) • The 30-nm fiber, in turn • Forms looped domains, making up a 300-nm fiber • Looped domains are attached to a protein scaffold, which helps organize areas of ACTIVE transcription Figure 19.2 c
700 nm 1,400 nm (d) Metaphase chromosome • In a mitotic chromosome • The looped domains themselves coil and fold forming the characteristic metaphase chromosome • REVIEW: Figure 19.2 d
What is heterochromatin? • Remains HIGHLY condensed during interphase • Non-coding chromatin • NOT actively transcribed • So why do we have it?...helps with gene expression and chromosome structure • What is euchromatin? • Remains LESS condensed during interphase • Becomes HIGHLY condensed during mitosis • Coding chromatin • ACTIVELY transcribed