Title: DNA and Eucaryotic Chromosomes
DNA: DeoxyriboNucleic Acid
"Backbone":
Composed of deoxyribose sugar molecules
Linked by phosphate groups via phosphodiester covalent bonds (which are polar)
DNA is classified as an acid
At pH 7:
Hydrogen ions (Hδ+) from the phosphate groups are displaced by oxygen (Oδ-) from water molecules
This results in the formation of hydronium ions (H3O+)
Consequently, most phosphate groups carry a negative charge at any given time
DNA contains four nitrogenous bases, each covalently bonded to sugar molecules:
Thymine (T)
Guanine (G)
Cytosine (C)
Adenine (A)
Result: Long polymer of deoxyribonucleotides, ranging from 50 to 300 million per mammalian chromosome
Types of deoxyribonucleotides:
Thymidine (Thymine)
Guanosine (Guanine)
Cytidine (Cytosine)
Adenosine (Adenine)
Adenine (a nitrogenous base):
When attached to a 5-carbon sugar ring, it forms a nucleoside called adenosine
Examples:
Adenosine Monophosphate (AMP)
Adenosine Diphosphate (ADP)
Adenosine Triphosphate (ATP)
These are nucleotides made of base + sugar + one or more phosphates
Nucleotides in DNA are termed deoxyribonucleoside monophosphates
GTP: Guanosine Triphosphate, a nucleotide composed of:
Guanine base
Deoxyribose sugar
Three phosphate groups
Not on the test: Terminology of nucleoside and nucleotide
DNA consists of two "backbones" connected via hydrogen bonds between bases
Base pairing is complementary and arranged anti-parallel
Chemically different ends exist for each strand
Procaryotes: Nearly 100% genome consists of mRNA, rRNA, and tRNA
Eucaryotes:
~5% of genome consists of mRNA, rRNA, tRNA genes
~80% dedicated to other RNA genes
Remaining 15% includes non-coding regulatory and non-regulatory regions
Focus on structures and functions of eucaryotic DNA
Eucaryotic DNA is separated from the cytoplasm by the nuclear envelope (nucleus)
This separation from cellular processes leads to significant implications, particularly in the organization of DNA
Eucaryotic cells possess DNA in the double helix form
Procaryotic cells generally have one circular DNA molecule lacking true chromosome architecture
Eucaryotic cells contain multiple linear DNA molecules, which are tightly packaged into chromosomes
Examples of organisms (e.g., ant vs. butterfly) show variation in chromosome numbers
Eucaryotic chromosomes are located in the nucleus
Each chromosome = one double-stranded DNA molecule during interphase
Following S-phase, chromosomes exist as two sister chromatids
Examination of eucaryotic chromosomes and their organization
DNA exists in various packaging forms:
Short region of 2 nm double helix
11 nm "beads-on-a-string" form of chromatin
30-nm chromatin fiber consisting of packed nucleosomes
300 nm condensed sections of chromosome
700 nm section containing the centromere of a chromosome
Entire mitotic chromosome measures 1400 nm
Nucleosomes consist of a complex of 8 proteins (histones)
DNA is wrapped twice around the histone complex and maintained by hydrogen bonds
Formed from folded strings of nucleosomes
Interactions between N-terminal tails of adjacent histone subunits help maintain structure
H1 and other non-histone DNA-binding proteins contribute to chromatin structure
Structure includes sequence-specific DNA-binding proteins and nucleosomes
Chromosomal structures vary across different regions:
Euchromatin: less condensed, ~90% of genes are present
Heterochromatin: highly condensed, includes centromeres and telomeres
Chromatin-remodeling complexes use ATP to drive modifications to histones, affecting gene expression
Euchromatin is more accessible for gene expression, while heterochromatin is often transcriptionally inactive and silenced
Modifications to histone tails can influence chromatin structure:
Acetylation: leads to loose packing
Methylation: causes tighter and various levels of condensation
Phosphorylation: increases condensation during cell division
Variant histones provide further diversity to chromatin structure, synthesized during interphase
Inserted into existing structures, enhancing complexity
Modifications and insertions of histone variants create a histone code, which helps determine chromatin packaging and gene expression patterns
Some variable features of chromatin structures are inherited epigenetically, based on protein structure not DNA sequence
Chromatin structures play crucial roles in maintaining chromosome identities, with features absent in procaryotic DNA.