Biology Chapter: DNA Structure and Length

Length of DNA

• Each human cell contains approximately 2 meters of DNA.
• Human DNA comprises about 3 billion base pairs, with each strand having 1.5 billion nucleotides.
• The human genome includes around 30,000 genes.

Organization of DNA

• DNA is structured into chromatin, which consists of:
  • Long double-stranded DNA molecules.
  • Histones that condense DNA into nucleosomes.
  • Non-histone proteins, including replication and transcription enzymes.
• Nucleosomes are formed when DNA wraps around a set of 8 histones.

Levels of Coiling

• The primary level involves a 4nm DNA filament (naked DNA).
• In a normal chromosome, double-stranded DNA wraps around histones, forming a 10nm filament separated by 4nm DNA filaments and Histone 1.
• Further coiling produces 30nm nucleoprotein fibers, which loop around proteins to form individual chromosomes.

Human Chromosomes

• Chromatin organizes into chromosomes, which consist of sister chromatids joined at a centromere.
• Telomeres, located at the chromatid ends, are associated with aging and cancer.
• Telomeres contain TTAGGG repeats, typically around 12,000 base pairs long, losing approximately 100 base pairs per mitosis.

Telomeres and Cellular Aging

• Replicative senescence occurs when cells lose mitotic speed as telomeres shorten.
• Newborn cells can divide almost 100 times in culture before their mitosis declines, indicating telomere length's importance in cell division.
• Certain cells, such as germline cells, some unicellular eukaryotes, and some cancer cells, maintain telomere length.
• Telomerase acts as a reverse transcriptase, synthesizing DNA from an intrinsic RNA template.

Human Gene Structure

• The human genome consists of a nuclear genome and a mitochondrial genome including vital genetic information and potential implications in genetic diseases.

Central Theme of Molecular Biology

• All cells must perform essential biochemical reactions for survival and reproduction.
• Proteins serve as the necessary tools for living processes, constructed based on information stored in DNA.
• DNA is inherited from parent cells, emphasizing the importance of accurate DNA replication for genetic stability.

Central Dogma of Molecular Biology

• The process involves three key stages: DNA replication, transcription, and translation.

DNA Replication

• DNA replication is crucial for producing more DNA, ensuring progeny inherit parent genetic information.
• Helicases unwind the double helix structure of parental DNA.
• Single-strand binding proteins stabilize separated strands to prevent re-annealing.
• Topoisomerases create nicks to facilitate unwinding and later reseal the strands.
• DNA polymerase synthesizes the leading strand continuously in a 5’ to 3’ direction; the lagging strand is synthesized discontinuously via Okazaki fragments.
• Primase synthesizes RNA primers for the lagging strand, which are replaced by DNA polymerase and linked by DNA ligase.
• The entire replication must be precise to maintain genetic integrity.

Transcription

• Transcription is the process of synthesizing RNA from DNA, resulting in mRNA, tRNA, and rRNA.
• The process initiates at a promoter and terminates at a specific terminator sequence on the DNA.
• RNA polymerase unwinds the DNA and synthesizes an RNA strand complementary to the template strand.
• In eukaryotes, pre-mRNA undergoes processing to form mature mRNA.

Translation

• Translation synthesizes polypeptides (proteins) from mRNA using ribosomes and tRNA.
• Initiation involves multiple initiation factors (IF1, IF2, IF3) and the small ribosomal subunit binding to mRNA, scanning for the start codon (AUG).
• During elongation, charged tRNA molecules enter the ribosome, adding amino acids to the growing polypeptide chain.
• Key factors (EF-Tu, EF-Ts, EF-G) facilitate tRNA binding, the transition between states, and the movement of tRNAs and mRNA.
• Prokaryotic translation can add an amino acid approximately every 0.05 seconds, leading to rapid polypeptide synthesis.
• Termination occurs when the ribosome encounters a stop codon, releasing the completed polypeptide chain.

Summary of Molecular Processes

• The central dogma highlights the flow of genetic information from DNA to RNA to protein, underscoring the interdependence of replication, transcription, and translation in cell biology.

Explore the fascinating world of DNA in this quiz, which covers its impressive length in human cells, the organization of chromatin, and the roles of histones and non-histone proteins. Test your knowledge on the structure, function, and genetic implications of DNA, including base pairs and genes.