DNA and Structure Flashcards
What is DNA?
• Stands for Deoxyribonucleic Acid.
• A double-stranded helical polymer of nucleotides.
• Carries genetic instructions for protein synthesis and inheritance.
• Found in the nucleus of eukaryotic cells and cytoplasm of prokaryotes.
Why is DNA considered the blueprint of life?
• Contains genes, which code for proteins.
• Controls growth, development, and function of organisms.
• Directs protein synthesis through transcription and translation.
What are the building blocks of DNA?
• Nucleotides, composed of:
o A phosphate group.
o A pentose sugar (deoxyribose).
o A nitrogenous base (A, T, C, G).
What are the four nitrogenous bases in DNA?
• Purines (double-ring): Adenine (A), Guanine (G).
• Pyrimidines (single-ring): Cytosine (C), Thymine (T).
How do bases pair in DNA?
• Adenine (A) bonds with Thymine (T) → 2 hydrogen bonds.
• Guanine (G) bonds with Cytosine (C) → 3 hydrogen bonds.
What is Chargaff’s Rule?
• The proportion of A = T and G = C in a DNA molecule.
• Ensures complementary base pairing for accurate replication.
What is the sugar-phosphate backbone?
• Structural framework of DNA.
• Consists of alternating deoxyribose sugars & phosphate groups.
What does “antiparallel” mean in DNA?
• DNA strands run in opposite directions.
• One strand runs 5’ → 3’, the other 3’ → 5’.
What do 5’ and 3’ refer to in DNA?
• 5’ (five-prime): Phosphate attached to the 5th carbon of deoxyribose.
• 3’ (three-prime): Hydroxyl (-OH) attached to the 3rd carbon of deoxyribose.
• Determines directionality in replication and transcription.
What is chromatin?
• Uncoiled, relaxed DNA in the nucleus.
• Wraps around histone proteins to form nucleosomes.
What are histones?
• Positively charged proteins that help DNA supercoil into chromosomes.
• Form nucleosomes (DNA wrapped around 8 histones).
• Helps regulate gene expression - loosely packed DNA (EUCHROMATIN) is active, while tightly packed DNA is inactive.
What is a nucleosome?
• Basic unit of DNA packaging.
• DNA wrapped around eight histone proteins.
• Looks like “beads on a string” under an electron microscope.
What are chromosomes?
• Condensed DNA structures that carry genetic information.
• Humans have 46 chromosomes (23 pairs).
What are telomeres?
• Protective caps at the ends of chromosomes.
• Prevent DNA degradation & fusion of chromosomes.
• Shorten with each cell division, contributing to aging.
What is a karyotype?
• A visual representation of an organism’s chromosomes.
• Used to detect chromosomal abnormalities.
What are sister chromatids?
• Identical copies of a chromosome post-replication.
• Joined at the centromere.
What is nuclear DNA vs. mitochondrial DNA (mtDNA)?
• Nuclear DNA: Found in the nucleus, inherited from both parents.
• Mitochondrial DNA (mtDNA): Found in mitochondria, inherited only from the mother.
How does mitochondrial DNA support the theory of endosymbiosis?
• Mitochondria have circular DNA, like bacteria.
• Suggests mitochondria evolved from free-living prokaryotic cells.
What are plasmids?
• Small, circular DNA molecules separate from chromosomal DNA.
• Found in bacteria, often carry genes for antibiotic resistance.
How does prokaryotic DNA differ from eukaryotic DNA?
• Prokaryotic DNA: Circular, in the nucleoid region, lacks histones.
• Eukaryotic DNA: Linear, stored in the nucleus, associated with histones.
What is DNA replication?
• The process of copying DNA before cell division.
• Ensures genetic continuity.
What is semi-conservative replication?
• Each new DNA molecule contains:
o One original strand (template).
o One newly synthesized strand.
What is the role of helicase in DNA replication?
• Unwinds and unzips DNA by breaking hydrogen bonds.
What is the role of DNA polymerase?
• Adds nucleotides to the growing strand in a 5’ → 3’ direction.
• Proofreads and fixes errors.
What are Okazaki Fragments?
• Short DNA segments synthesized on the lagging strand during DNA replication.
• DNA polymerase can only add nucleotides in the 5’ → 3’ direction, causing discontinuous synthesis.
• Primase lays down an RNA primer, allowing polymerase to extend fragments.
• DNA ligase joins Okazaki fragments to form a continuous strand.
What is the function of primase?
• Synthesizes RNA primers for DNA polymerase to initiate replication.
What is the difference between leading and lagging strands?
• Leading strand: Synthesized continuously in a 5’ → 3’ direction.
• Lagging strand: Synthesized discontinuously in Okazaki fragments.
What is a gene?
• A segment of DNA that codes for a specific protein or RNA molecule.
How does DNA store genetic information?
• Through the sequence of nitrogenous bases.
• Each sequence forms genes, which dictate protein production.
What is the function of non-coding DNA?
• Regulates gene expression (e.g., promoters, enhancers).
• Some code for tRNA, rRNA, and microRNA.
What are Sister Chromatids and How Do They Relate to Semi-Conservative Replication?
• Sister chromatids are identical copies of a chromosome formed after DNA replication.
• They are held together by a centromere until separated during cell division.
• Semi-conservative replication ensures each chromatid contains one original DNA strand and one newly synthesized strand.
• This guarantees genetic continuity between parent and daughter cells.
What are the Stages of DNA Replication?
- Initiation
• Helicase unwinds the DNA double helix, breaking hydrogen bonds.
• Single-stranded binding proteins (SSBs) stabilize the unwound strands.
• Primase adds RNA primers to initiate replication.
- Elongation
• DNA polymerase adds complementary nucleotides in the 5’ → 3’ direction.
• The leading strand is synthesized continuously.
• The lagging strand is synthesized discontinuously as Okazaki fragments.
- Termination
• Exonuclease removes RNA primers, replacing them with DNA.
• DNA ligase seals gaps between Okazaki fragments.
• The two identical DNA molecules coil into sister chromatids.
What are the Functions of DNA?
• Genetic Blueprint: Stores hereditary information.
• Replication: Ensures genetic continuity in cell division.
• Protein Synthesis: Provides instructions for mRNA transcription and protein translation.
• Cell Function Regulation: Controls gene expression via non-coding DNA and regulatory sequences.
How Do Different Species on Earth Differ in Their DNA?
• Chromosome Number: Varies among species (humans = 46, dogs = 78, fruit flies = 8).
• Base Sequence Differences: Unique genetic codes define species-specific traits.
• Genome Size: Some species have larger or smaller genomes (e.g., humans ≈ 3 billion base pairs, E. coli ≈ 4.6 million).
• Gene Variation: Differences in gene number and gene expression lead to diverse biological features.
What is the difference between introns and exons?
- Exons: coding sequences that remain in mRNA and are translated into proteins.
- Introns: non-coding sequences that are removed during RNA splicing.
- Non- Coding DNA: includes promotes, enhances, and introns.
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