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

Explore the fascinating world of chromosome structures in both prokaryotic and eukaryotic cells. This quiz covers the differences in DNA organization, the nature of bacterial and viral chromosomes, and the implications for heredity. Test your understanding of these fundamental biological concepts!