Chromosome Structure week 8a

Transcription: The process of creating RNA molecules from a DNA template
- In Prokaryotes: The sigma factor binds to the promoter region to initiate transcription
- In Eukaryotes: A variety of transcription factors are involved, resulting in a more complex process
RNA Processing in Eukaryotes: Steps that modify RNA molecules after transcription
Translation: The synthesis of polypeptides based on the mRNA codon sequence
- Amino acid structure: Composed of amino and carboxyl groups with variable R-groups
- Formation of peptide bonds between amino acids
- Key processes include initiation, elongation, and termination in prokaryotic translation
- Shine-Dalgarno Sequence: A ribosomal binding site in bacterial mRNA
- Interactions among tRNA, ribosomes, and mRNA
- Identification of start and stop codons during translation
- Wobble base pairing: Allows for fewer tRNAs, increasing efficiency
Protein Structure and Function: The three-dimensional shape of a protein is crucial for its biological role

Topics to cover:
- Viral and bacterial chromosome structure
- Chloroplast and mitochondrial DNA structures
- Chromatin structure in eukaryotic cells
- Repetitive DNA in genomes

Types of viral chromosomes:
- Can be circular or linear, composed of DNA or RNA, and can be double-stranded or single-stranded
Compaction of DNA/RNA within viral structures leads to approximately 100-fold shrinkage to fit within the protein head

Typically consist of double-stranded DNA structured in a circular form
- Nucleoid: Area in the bacterial cell where chromosome is compacted approximately 100-fold
- Proteins: HU and H-NS are histone-like proteins that aid in achieving tight supercoiling of the DNA
- Bacterial genomes show a very high gene density and a lack of introns

Both mitochondria and chloroplast chromosomes are circular, double-stranded DNA similar to bacterial DNA
Lacking DNA-associated proteins like histones:
- mitochondrial DNA (mtDNA): No introns present
- chloroplast DNA (cpDNA): Contains a higher quantity of introns, non-coding regions, and instances of gene duplication compared to mtDNA

Polytene Chromosomes: Found in some cell types, these chromosomes possess a unique structure that aids in researching gene regulation
- Regions of DNA that are actively being transcribed are arranged in open “loops,” making them more accessible
Lampbrush Chromosomes: Another atypical structure with significance in gene regulation studies

A diploid human cell contains about 2 meters of DNA organized into 46 chromosomes, accounting for roughly 6 billion base pairs (6 Gbp)
The challenge arises on how such a vast amount of DNA can fit within the nucleus, which measures approximately 10 µm in width — a contraction ratio of around 10,000 times

Endonuclease Digestion: Results in 200 base pair (bp) DNA fragments, indicating that DNA is organized into repeating units
Electron Micrography Findings: Displays a "beads on a string" structure, reinforcing the concept of these repeating units
Histone Bonding: Histone proteins form octamers and interact with about 200 bp of DNA, supporting the model of chromosomal organization in units of 200 bp
Longer Nuclease Digestion: Yields 147 bp DNA fragments, revealing the presence of linker DNA between core units

Chromatin: A DNA-protein complex found in eukaryotes
Histone Proteins: Positively charged proteins that bind to negatively charged DNA
- Form octamer structures around which 147 bp of DNA is wrapped
- The wrapping of DNA around these histones reduces the DNA length by one-third
- Further coiling into chromatin fibers results in additional length reduction
Nucleosome: The combination of a histone octamer and DNA

Histone regions that interact with DNA predominantly contain amino acids with a positive charge.

Modifications to histone tails can alter interactions between histones and DNA, allowing access for other molecules
- Acetylation: Often increases gene expression through histone acetyltransferases
- Methylation and Phosphorylation: Can either increase or decrease gene expression, influenced by methyltransferases and kinases, respectively
- Functional groups involved: Acetyl groups, methyl groups, and phosphoryl groups

Euchromatin: Active regions of the chromosome that oscillate between condensed and uncondensed states, containing genes
Heterochromatin: Regions that are perpetually in a condensed state, predominantly made up of repetitive DNA sequences
Discussion Point: What other examples of this classification have been identified in previous lectures?

Human DNA Analysis: Approximately 2% of the human DNA sequence consists of protein-coding genes
- The remaining 98% corresponds to untranslated RNA or non-coding regions
- Some of these non-coding regions serve recognized functions, while others may originate from evolutionary remnants or remain functionally ambiguous

Highly Repetitive DNA:
- Examples include satellite DNA with 1-100 bp repeating units, often replicated millions of times; found in structures such as centromeres and telomeres

Tandem Repeats:
- Involve multi-copy genes that are duplicated multiple times in succession
- Includes variable number tandem repeats (VNTRs) or minisatellites, important for DNA fingerprinting applications
- Also includes short tandem repeats (STRs) or microsatellites, used for molecular markers in linkage mapping
Interspersed Repeats:
- Include transposable elements that can move around the genome
- Transposons: Function via a “cut and paste” mechanism
- Retrotransposons: Operate through a “copy and paste” process
- SINEs and LINEs (short and long interspersed elements) comprise around 34% of the human genome
- While many have transposable capabilities, others help regulate gene expression; many functions remain unclear

Future discussions may address the importance of chromosome structure in fields such as genetics, molecular biology, and evolutionary studies, elucidating how these structures influence gene regulation, expression, and cellular function.