Chromosomes and DNA Replication

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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