Cellular DNA Structure

Organization of DNA in Eukaryotic and Prokaryotic Cells

• In prokaryotes (e.g., bacteria), DNA is typically circular with no free ends, relatively short, and not bound to histones.
• In eukaryotes (e.g., humans), DNA is linear with two ends, longer, and tightly wrapped around histone proteins.

Structure of Chromosomes

• In eukaryotes, DNA is wrapped around histones, forming complex structures called chromosomes.
• Chromosomes are not found in prokaryotes since their DNA is not bound to histones.

Chromatin and Chromatids

• Before cell division, chromosomes are copied, and the two copies remain attached at the centromere.
• The two DNA molecules are called chromatids.
• At this stage, the whole structure is referred to as a chromosome.

Homologous Chromosomes

• In human cells, there are 46 chromosomes, with 23 coming from each parent.
• Homologous chromosome pairs have the same genes, but may have different alleles (versions) of those genes.
• The position of a gene on a chromosome is called the locus for that gene.

Genes and Alleles

• A gene is a section of DNA that encodes the amino acid sequence of a polypeptide.
• Random mutations can cause genes to occur in different versions, known as alleles.
• The blood group gene has three main alleles: A, B, and O.

Organization of DNA in Eukaryotic and Prokaryotic Cells

• Prokaryotic DNA is typically circular, relatively short, and not bound to histones.
• Eukaryotic DNA is linear, longer, and tightly wrapped around histone proteins.

Structure of Chromosomes

• In eukaryotes, DNA wraps around histones, forming complex structures called chromosomes.
• Chromosomes are absent in prokaryotes since their DNA is not bound to histones.

Chromatin and Chromatids

• Before cell division, chromosomes are copied, and the two copies remain attached at the centromere, forming chromatids.
• The entire structure, including the two chromatids, is referred to as a chromosome.

Homologous Chromosomes

• Human cells contain 46 chromosomes, with 23 coming from each parent.
• Homologous chromosome pairs have the same genes, but may have different alleles (versions) of those genes.
• The location of a gene on a chromosome is called the locus for that gene.

Genes and Alleles

• A gene is a section of DNA that encodes the amino acid sequence of a polypeptide.
• Random mutations can cause genes to occur in different versions, known as alleles.
• The blood group gene has three main alleles: A, B, and O.