Chromosomes

Inheriting Traits• We inherit many of our physical characteristics or traits from our

parents.

• This is known as heredity – the passing of traits from one generation to the next.

• In scientific terminology, a trait is a particular characteristic or feature of an organism.

Why are traits inherited?

• Chromosomes contain the hereditary (genetic) information in living cells.

• All living cells and viruses contain genetic information in chromosomes.

• Each unique sequence of DNA (gene) carries a particular instruction for a cell.

• Genes vary in size from about 100 to 2.5million base pairs. The length of the sequence of DNA and the precise order of the base pairs in a gene are the critical factors that determine what the gene product (usually a protein) will be like and what it will do in a cell.

chromosome structure and organization of

Prokaryotes

INTRODUCTION • The term “prokaryote” means “primitive nucleus”. Cell in prokaryotes have no nucleus.

The prokaryotic chromosome is dispersed within the cell and is not enclosed by a separate membrane.• Much of the information about the structure of DNA has come from studies of

prokaryotes, because they are less complex (genetically and biochemically) than eukaryotes.• Prokaryotes are monoploid = they have only one set of genes (one copy of the genome).• In most viruses and prokaryotes, the single set of genes is stored in a single chromosome

(single molecule either RNA or DNA).• Prokaryotic genomes are exemplified by the E. coli chromosome. • The bulk of the DNA in E. coli cells consists of a single closed-circular DNA molecule of

length 4.6 million base pairs.• The DNA is packaged into a region of the cell known as the nucleoid.

DNA domains

• Experiments in which DNA from E. coli is carefully isolated free of most of the attached proteins and observed under the electron microscope reveal one level of organization of the nucleoid.

• The DNA consists of 50–100 domains or loops, the ends of which are constrained by binding to a structure which probably consists of proteins attached to part of the cell membrane. The loops are about 50–100 kb in size.

Supercoiling of the genome

• The E. coli chromosome as a whole is negatively supercoiled, although there is some evidence that individual domains may be supercoiled independently.

• Electron micrographs indicate that some domains may not be supercoiled, perhaps because the DNA has become broken in one strand, where other domains clearly do contain supercoils.

• The attachment of the DNA to the protein–membrane scaffold may act as a barrier to rotation of the DNA, such that the domains may be topologically independent.

DNA-binding proteins• The most abundant of these are protein HU, a small basic (positively charged) protein.

• It’s binds DNA nonspecifically by the wrapping of the DNA around the protein, and H-NS (formerly known as protein H1), a monomeric neutral protein, which also binds DNA nonspecifically in terms of sequence. These proteins are sometimes known as histone-like proteins, and have the effect of compacting the DNA, which is essential for the packaging of the DNA into the nucleoid, and of stabilizing and constraining the supercoiling of the chromosome.

• Half of this is constrained as permanent wrapping of DNA around proteins such as HU. Only about half the supercoiling is unconstrained.

• RNA polymerase and mRNA molecules, site-specific DNA-binding proteins such as integration host factor (IHF), a homolog of HU, which binds to specific DNA sequences and bends DNA through 140°.

Diagram of the structure of the functional state of the E. coli chromosome.

Chromosome structure and organization of

Eukaryotes

INTRODUCTION • In humans the average DNA molecule is about 6.5x107 base pairs in length.• The nucleus of a human cells is just 6mm in diameter, yet it contains 1.8m of DNA.• This can only be achieved because DNA in eukaryotes is tightly packaged into

chromosomes.

• DNA is coiled around small proteins (histones). • Where the DNA is wrapped around a core of histone proteins it forms a particle about

10nm in diameter called a nucleosome.• The nucleosomes give the DNA strand the appearance of a string of beads, and this

arrangement of DNA wrapped around histones serves to package the DNA efficiently and protected from enzymatic degradation.• When a eukaryotic cell is preparing to divide, chromosomes become very condensed and

are visible under a light microscope.

Chromatin Chromatin is isolated from interphase nuclei, the individual chromosomes are not recognizable. Instead one observes an irregular aggregate of nucleoprotein. Chemical analysis of isolated chromatin shows that it consists primarily of DNA and proteins and lesser amounts of RNA.

This proteins are two major classes:

Basic proteins-positively charged at neutral pH called histones.

A heterogeneous largely acidic (negatively charged at neutral pH) group of proteins collectively referred to as non-histone chromosomal proteins.

Histones • The major protein components of chromatin are the histones. Most of the

protein in eukaryotic chromatin consists of histones.

• Five families, or classes of histones:

H1,H2A, H2B, H3 and H4

• H2A, H2B, H3 and H4: core histones. The core histones are small proteins, with masses between 10 and 20 kDa.

• H1: little larger at around 23 kDa.

• All histone proteins have a large positive charge; between 20 and 30% of their sequences consist of the basic amino acids, lysine and arginine.

• This means that histones will bind very strongly to the negatively charged DNA in forming chromatin.

10 nm filament; nucleosomes

histones (= 1g per g DNA) DNA

proteinpurification

H1

H3H2A

H2BH4

•Basic (arg, lys);•+ charges bind to - phosphates on DNA

• Members of the same histone class are very highly conserved between relatively unrelated species, ex: between plants and animals, which testifies to their crucial role in chromatin.

• H1 histones are somewhat distinct from the other histone classes in a number of ways; in addition to their larger size, there is more variation in H1 sequences both between and within species than in the other classes.

Packaging of giant DNA molecules into chromosomes

1. DNA compacting ratio

2. Nucleosomes

I) Ultrastructure of nucleosome core protein

II) Bonding between histone core and DNA

III) Bending of DNA in a nucleosome

IV) Packing of nucleosomes into compact chromatin fiber.

DNA compaction ratio

• Human chromosome 22 contains about 48 million nucleotide pairs. Stretched out end to end, its DNA extend about 1.5 cm.

• Chromosome 22 measures only about 2µm in length, giving end-to-end compaction ration of nearly 10,000 fold.

• Compression is performed by proteins.

Nucleosomes

• The basic unit of chromatin is the nucleosome. The nucleosome is composed of approximately 146 base pairs of DNA wrapped in 1.8 helical turns around an eight-unit structure called histone protein octamer.

• This histone octamer consists of two copies each of the histones H2a, H2b, H3, and H4.

• The space in between individual nucleosomes is referred to as linker DNA, and can range in length from 8 to 114 base pairs, with 55 base pairs being the average.

• Linker DNA interacts with the linker histone, called H1.

Bonding between histone core and DNA

• About 142 hydrogen bonds are formed between DNA and the histone core in each nucleosome. Nearly half of these bonds form between the aminoacid backbone of the histones and phosphodiester bonds of DNA.

• For example: all core histones are rich in lysine and arginine.

• positive charge of these aminoacids can effectively neutralized the negatively charged DNA backbone.

Bending of DNA in a nucleosome

• Two main influences determine where nucleosomes form in the DNA:

• Difficulty of bending: bending of the DNA double helix into two tight turns around the outside of the histone octamer, a process that required firm compression of the minor groove of the DNA helix.

• A-T rich sequences in the minor groove are easier to compress than G-C rich sequences.

• The presence of certain other tightly bound proteins on the DNA also influences the position of nucleosomes on DNA molecule.

Nucleosome StructuresHistone octamer2 H2A2 H2B2 H32 H4

X-ray diffraction analyses of crystalsStructure of a nucleosome core particle

Structural Organization of the Core Histones

The bending of DNA in a nucleosome1. Flexibility of DNAs: A-T riched minor groove inside and G-C riched groove outside2. DNA bound protein can also help

Zigzag model of the 30-nm chromatin fiber

Irregularities in the 30-nm fiberFlexible linker, DNA binding proteinsStructural modulators: H1 histone, ATP-driven Chromatin remodeling machine, covalent modification of histone tails

The function of Histone H1

The function of Histone tails

Chromatin Remodeling

Cyclic Diagram for nucleosome formation and disruption

Covalent Modification of core histone tailsAcetylation of lysinesMythylation of lysinesPhosphorylation of serines

Histone acetyl transferase (HAT)Histone deacetylase (HDAC)

Eukaryotes ProkaryotesLocation of Chromosomes In the nucleus In the cytoplasm

Structure of Chromosomes Double stranded molecules of DNA, with attached protein molecules.

Single, circular chromosome composed of DNA, with very few or no attached proteins.

Number of Chromosomes Varies from species to species. Humans have 46.

One.

Reproduction Nuclear DNA is replicated prior to cell division and the chromosomes distributed evenly to daughter cells.

Single circular chromosome is replicated prior to cell division. Each daughter cell receives one copy of this chromosome.

Extra-chromosomal DNA Mitochondria contain DNA. Small circular DNA molecules known as plasmids.

• DNA, Chromosome• Centromere, telomere, replication origin• Nucleosome, Chromatin, • Histone: H1, H2A, H2B, H3, H4• Histone octamer, DNA packaging• DNA binding proteins, Histone modifications

Summary