Chromosomes and DNA Replication 12-2. DNA & Chromosomes Prokaryotes - Lack nuclei and cellular...
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Transcript of Chromosomes and DNA Replication 12-2. DNA & Chromosomes Prokaryotes - Lack nuclei and cellular...
Chromosomes and DNA Replication
12-2
DNA & Chromosomes
Prokaryotes - Lack nuclei and cellular organelles
Have single circular DNA moleculeContains nearly all cell’s genetic information
Referred to as cell’s chromosome
Eukaryotes -Have as much as 1000 times the amount
of DNA as prokaryotes
Not free in cytoplasm
Contained in nucleus in the form of a number of chromosomes
DNA Length
DNA molecule is very long
Ex: E. coli DNA contains 4,639,221 base pairs, or ~1.6 mm long
Like trying to pack 300 m of rope into a backpack
Chromosome Structure
Human cells contain almost 1000 times as many base pairs as bacteria
The nucleus of a human cell contains > 1m of DNA
The composition of chromosomes allows them to hold a lot of DNA
Chromosomes contain both proteins and DNAForm chromatin
Chromatin consists of DNA tightly coiled around histone proteinsForms beadlike structure - nucleosome
Nucleosomes pack with one another to form a thick fiber, shortened by loops and coils
Nucleosomes seem to make it possible to fold enormous lengths of DNA
Histones have changed very little during evolution
During most of cell cycle fibers are dispersed - can’t see chromosomes
During mitosis, fibers are drawn together, forming tightly packed chromosomes you can see with a microscope
DNA Replication
When Watson and Crick discovered the double helix, the structure explained how DNA could be copied
Each strand of DNA has all the information needed to reconstruct the other half because of base pairing
Since each strand is used to make the other half, the strands are said to be complimentary
In eukaryotes, DNA replication begins at 100s of places along the molecule, and continues until replication is completeSites known as replication forks
Duplicating DNA
During replication -DNA molecule separates into 2 strands
Produces 2 new complimentary strands following rules of base pairing
Each strand is a template for a new strand
DNA replication is carried out by a series of enzymes
Enzymes “unzip” the DNA molecule, creating replication forksBreaks hydrogen bonds between bases
A pairs of with T, and G pairs up with C, until the new strand is complete
Each strand created is composed of an old strand and a new strand
The principal enzyme involved in this reaction is DNA polymeraseJoins individual nucleotides to produce
DNA molecule
Proofreads each new molecule