Chromosome Structure and Types

Chromosome Structure

• Chromosomes are thread-like structures made of DNA and protein
• All living organisms have chromosomes
• They are the vehicle of heredity

Prokaryotic vs. Eukaryotic DNA

• Prokaryotic DNA is typically a single, circular chromosome located in the cytoplasm
• Eukaryotic DNA is multiple, linear chromosomes located in the nucleus
• Prokaryotic DNA is not usually wrapped around histones, while Eukaryotic DNA is

Bacterial Chromosomes

• Bacterial chromosomes are typically circular and supercoiled
• They are folded into twisted loops, which are clumped together in the nucleoid region
• Bacterial chromosomes are typically attached to proteins

Prokaryotic DNA

• Prokaryotic DNA usually contains a single origin of replication
• This origin is where DNA replication begins

Viral Chromosomes

• Viral chromosomes can be either DNA or RNA
• Viral DNA is packaged within a protein coat called a capsid

Eukaryotic Chromosomes

• Eukaryotic chromosomes are usually linear and tightly packaged with histone proteins to create chromatin
• Unlike prokaryotic cells, eukaryotic cells have a nucleus where the chromosomes are located
• Eukaryotic chromosomes contain many origins of replication per chromosome
• Each species has its own distinct chromosome number
• Eukaryotic chromosomes exist in pairs, this is called diploid

Genome

• The entire genetic material of an organism
• In humans, the genome is made up of 23 pairs of chromosomes for a total of 46 chromosomes
• The genome size can be extremely different between species

Eukaryotic Chromosomes

• Autosomal chromosomes are the non-sex chromosomes (e.g. human chromosomes 1 to 22)
• Sex chromosomes (e.g. human X and Y chromosomes) determine an individual's sex (XX = female, XY = male)

Homologous Chromosomes

• Homologous chromosomes are pairs of chromosomes that contain the same genes, but may have different versions (alleles)

Karyotype

• A karyotype is a physical description of the chromosomes within a cell, including information about their number, type, shape, banding patterns, and other cytogenetic features
• Karyotyping can be used to identify chromosomal abnormalities

Karyotyping

• To perform karyotyping, cells are often arrested during cell division using colchicine, a tubulin inhibitor
• Chromosomes are often stained with Giemsa, which binds to phosphate groups in DNA and creates a banding pattern

Karyotype - G-banding

• Giemsa staining allows for the identification of distinct banding patterns on each chromosome
• These patterns help to distinguish the different chromosomes in a karyotype

Chromatid

• Each chromosome is made up of two identical structures called chromatids
• These chromatids are attached together at the centromere

Basic Structure of a Eukaryotic Chromosome

• Eukaryotic chromosomes are made up of a centromere, telomeres, and the arms of the chromosome

Centromere

• The centromere is a specialized region of heterochromatin where the two chromatids are joined
• The centromere plays a crucial role in the proper segregation of chromosomes during cell division
• Chromosomes can be classified based on the location of the centromere
• A metacentric chromosome has a centromere in the middle of the chromosome.
• A submetacentric chromosome has a centromere closer to one end than to the other.
• An acrocentric chromosome has a centromere very close to one end, with the result that one arm is extremely short relative to the other arm.
• A telocentric chromosome has a centromere at the end of the chromosome, only possessing one arm.

Telomeres

• Telomeres are repetitive sequences found at the ends of chromosomes
• Help to protect the ends of chromosomes from degradation and fusion

Complexity of Eukaryotic Chromosomes & Genomes

• Eukaryotic genomes are much more complex than prokaryotic genomes

Complexity in Eukaryotic Genomes

• If double-stranded DNA is denatured by heating, it separates into two single strands
• The temperature at which this happens is called the melting temperature
• Reannealing complementary strands of genomic DNA from eukaryotes shows regions with very different hybridization (reannealing) rates
• These differences in hybridization rates suggest that there are at least three types of DNA regions within the eukarytic genome:
  • Highly repetitive DNA
  • Moderately repetitive DNA
  • Unique DNA