Genetics Chapter: Chromosome Structure Quiz

Chromosomes and DNA

• Chromosomes are thread-like structures found within the nucleus during cell division.
• Walther Fleming first observed them in 1882, while studying salamander larvae.
• The number of chromosomes varies significantly between organisms. Penicillium has 1-2 pairs; some ferns have over 500 pairs; mosquitoes have 1000+; humans have 46 chromosomes (23 pairs).
• Chromosomes contain genes that regulate development and bodily functions. Loss or damage to chromosomes can result in severe consequences, often leading to death.

Chromosome Structure

• A typical chromosome consists of chromatids (two identical copies), a centromere (primary constriction), and often a secondary constriction.
• Chromatids are joined at the centromere, which is a constricted region.
• The relative lengths of the two arms on both sides of the centromere, their staining properties, and position of constricted regions vary widely amongst chromosomes, allowing for karyotyping (detailed analysis of chromosomes).

Karyotype

• The arrangement and appearance of chromosomes in a cell, considered a unique characteristic of a species.
• Karyotypes vary between species, even individuals within the same species (e.g. variations in size, staining properties of chromosomes).
• A key analysis tool to classify chromosomes and assess genetic material.

Chromosome Types

• Telocentric: Centromere at the end of the chromosome.
• Acrocentric: Centromere close to one end, short arm.
• Submetacentric: Centromere displaced from the center, arms unequal in length.
• Metacentric: Centromere in the middle, arms equal.

Composition of Chromosomes

• DNA (deoxyribonucleic acid) makes up approximately 40% of each chromosome. A typical human chromosome has approximately 140 million nucleotides in its DNA.
• Proteins, mostly histones, account for around 60%. Histones are positively charged, attracting negatively charged DNA.
• RNA (ribonucleic acid) also plays a part. Its presence is located at sites of RNA synthesis.

Nucleosomes

• DNA and histone proteins are packaged as a chain of nucleosomes, the basic structural unit.
• Each nucleosome consists of 146 base pairs of DNA wrapped around an octamer of histone proteins.
• The nucleosome diameter is roughly 10 nm.

Condensation

• Chromatin condenses to form thicker, shorter chromosomes during cell division.
• This condenses further into a supercoiled state to fit within the nucleus.
• Histone proteins form a core for coiling.

Heterocromatin and Euchromatin

• Heterochromatin: Permanently condensed regions; rarely expressed genes.
• Euchromatin: Less condensed regions; genes are actively transcribed (expressed).

The Chromosomal Theory of Inheritance

• Genes are physically located on chromosomes.
• Karl Correns and Walter Sutton contributed to this theory.
• Evidence included reproduction, homologous chromosome pairs, and independent chromosome segregation.
• The model explains hereditary observations within cells, particularly during Meiosis.

DNA as Hereditary Material

• DNA is the primary hereditary material of cells.
• Evidence from research (e.g. Griffith's experiment and Avery, MacLeod, McCarty research):
• Experiments with mice using S (virulent) and R (non-virulent) strains of bacteria demonstrated genetic transformation and DNA's role.
• Hershey-Chase experiments marked a key event, confirming that DNA and not protein was the hereditary material in bacteriophages.

Chemical Nature of DNA

• DNA is composed of repeating units called nucleotides.
• Each nucleotide contains a phosphate group, a deoxyribose sugar, and a nitrogenous base (adenine, guanine, thymine, or cytosine).
• Erwin Chargaff's rule: equal amounts of adenine and thymine; equal amounts of guanine and cytosine.
• X-ray diffraction studies, notably those of Rosalind Franklin, provided crucial information about the DNA structure.

Double Helix Structure of DNA (Watson-Crick Model)

• DNA exists as a double helix.
• Two antiparallel strands with nucleotides hydrogen bonded, forming base pairs (A-T, G-C).
• The double helix is roughly 2 nm in diameter.
• A complete turn of the helix is approximately 3.4 nm.

DNA Replication

• DNA replicates semi-conservatively, producing two identical DNA molecules from one original molecule.
• Helicases unwind DNA; single-strand binding proteins stabilize both strands.
• DNA polymerases add nucleotides to the newly synthesized strand.

Okazaki Fragments

• Short DNA fragments synthesized discontinuously on the lagging strand during replication.
• DNA ligase connects Okazaki fragments.

DNA Polymerases

• Several DNA polymerase enzymes exist, with various functions including DNA replication and repair.
• DNA Polymerase III is vital to DNA replication.

Characteristics of Point Mutations

• Alterations of single nucleotides are called point mutations.
• May lead to a change in the amino acid sequence and potentially the function of a protein.

Types of Point Mutations

• Substitutions include transitions (purine for purine, or pyrimidine for pyrimidine) and transversions (purine for pyrimidine or vice versa).
• Insertions and deletions alter the reading frame leading to frameshift mutations.

The One-Gene One Enzyme Hypothesis

• Each gene possesses the code that produces a specific enzyme.
• Mutated genes may produce mutated proteins that don't function correctly.
• Genes interact and influence enzyme functions in numerous steps.

Genetic Code

• The genetic code consists of three-nucleotide codons that code for specific amino acids.
• The genetic code is nearly universal in all living organisms.
• The sequence of nucleotides and DNA/mRNA specify a polypeptide chain to build specific proteins.

Transcription

• DNA produces mRNA in the nucleus, using the enzyme RNA polymerase.
• Transcription is a process where DNA information is copied into mRNA.
• RNA polymerase recognizes specific regions on the DNA (promoter regions) to start transcription.

Translation (Protein Synthesis)

• mRNA directs amino acid placement in protein sequences.
• Translation occurs in ribosomes, with tRNA molecules transferring specific amino acids.
• tRNA molecules have an anticodon specific to a codon, bringing correct amino acids to the mRNA.

Post-Transcriptional Modifications

• In eukaryotes, mRNA undergoes modifications, like a 5' cap and a 3' tail, to improve stability and facilitate transport.
• These modifications help protect mRNA from degradation in the cytoplasm.

Mutations

• Mutations are changes in the DNA sequence.
• Mutations can occur due to mistakes during replication and/or DNA damage.
• Point mutations (single nucleotide change).
• Chromosomal mutations (large-scale change).
• Consequences of mutations can range from harmless to fatal, depending on the nature of the change and the affected gene.