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Question-and-Answer style flashcards covering foundational concepts from GEN2MHG Molecular Genetics lectures 1–2, including Mendelian genetics, DNA/RNA structure, chromatin organization, cell division, DNA replication machinery, telomere biology, extra-nuclear genomes, and key terminology.
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Which scientist’s pea experiments led to the laws of segregation and independent assortment?
Gregor Mendel
According to Mendel, what happens to alleles during gamete formation?
They segregate so that each gamete carries only one allele for each gene.
In Avery, MacLeod & McCarty’s 1944 experiment, which molecule transformed avirulent bacteria into virulent forms?
DNA
What four key properties must genetic material exhibit?
Replication, information storage, expression, and ability to generate inherited variability.
What repeating units make up DNA and RNA?
Nucleotides
What three components form a nucleotide?
A pentose sugar, a phosphate group, and a nitrogenous base.
What is a nucleoside?
A base attached to a sugar (no phosphate).
Name the two purine bases in nucleic acids.
Adenine and guanine
Name the two pyrimidine bases in DNA.
Thymine and cytosine
How many hydrogen bonds connect G-C base pairs?
Three
Which base replaces thymine in RNA?
Uracil
What rule states that %A = %T and %G = % C in double-stranded DNA?
Chargaff’s rule
Why does DNA with high G-C content melt at higher temperatures?
G-C pairs have three hydrogen bonds, requiring more energy to separate.
Why is RNA usually single-stranded?
The extra 2′-OH on ribose destabilises long double helices, promoting intramolecular folding instead.
Define plasmid.
A small, extra-chromosomal, circular DNA molecule that replicates independently of the host chromosome.
What proteins package eukaryotic DNA into nucleosomes?
Histones
What term describes DNA plus its associated proteins in eukaryotes?
Chromatin
During which cell-cycle phase is DNA replicated?
S phase of interphase
List the four basic stages of mitosis in order.
Prophase, metaphase, anaphase, telophase (followed by cytokinesis).
What unique process during meiosis creates recombinant chromosomes?
Crossing-over (homologous recombination).
What enzyme unwinds the DNA double helix ahead of the replication fork?
Helicase
Which enzyme relieves torsional strain by cutting and resealing DNA?
DNA gyrase (a topoisomerase).
DNA synthesis always proceeds in which direction?
5′ → 3′
Why is an RNA primer needed for DNA replication?
DNA polymerase can only extend an existing 3′-OH end; it cannot start de novo.
What are the short DNA fragments synthesized on the lagging strand called?
Okazaki fragments
Which strand is synthesized continuously during replication?
The leading strand
What enzyme seals nicks between Okazaki fragments?
DNA ligase
Describe the proofreading function of DNA polymerase.
Its 3′→5′ exonuclease removes mismatched nucleotides, then synthesis resumes, increasing fidelity ~100-fold.
What is the "end-replication problem"?
DNA polymerase cannot fully replicate the 3′ ends of linear lagging strands, leading to progressive telomere shortening.
What sequence is repeated many times in human telomeres?
TTAGGG
Which enzyme extends telomeres in germ and stem cells?
Telomerase
What is cellular senescence?
A permanent growth arrest that occurs when telomeres become critically short after many divisions.
How is mitochondrial DNA inherited in humans?
Maternally, because sperm contribute virtually no mitochondria to the zygote.
Give one reason mitochondrial DNA accumulates mutations faster than nuclear DNA.
It is exposed to high reactive oxygen species and lacks protective histone packaging.
What theory explains the bacterial origin of mitochondria and chloroplasts?
Endosymbiotic theory
What term describes DNA segments that can move to new genomic locations?
Transposons (transposable elements)
Which chromosome region attaches to spindle fibres during cell division?
Centromere
Define nucleosome.
About 147 bp of DNA wrapped around an octamer of core histones.
Approximately what percentage of the human genome encodes proteins?
1–2 percent
What are VNTRs and STRs examples of?
Variable, short repetitive DNA sequences used in forensics and genetics.
In prokaryotes, where is the chromosome located given absence of a nucleus?
In the nucleoid region
Why are bacterial genomes transcription-ready without extensive packaging changes?
They are not tightly wrapped in chromatin like eukaryotic DNA.
What are sister chromatids?
Identical copies of a chromosome joined at the centromere after DNA replication.
What does "semi-conservative" replication mean?
Each daughter DNA molecule contains one parental strand and one newly synthesized strand.
Which proteins prevent single-stranded DNA from re-annealing during replication?
Single-stranded binding proteins (SSBPs)
What is the role of the sliding clamp (β-clamp) in replication?
It keeps DNA polymerase attached to the template, increasing processivity.
Define cytokinesis.
Division of the cytoplasm, producing two separate daughter cells after mitosis.
What percentage of bacterial genomes is coding versus non-coding?
About 85–90 % is coding.
Which nucleic acid bases are connected by two hydrogen bonds?
Adenine–thymine in DNA and adenine–uracil in RNA
What type of viral genomes can exist?
DNA or RNA, single- or double-stranded, circular or linear.
Why do plasmid vectors often carry antibiotic-resistance genes?
To allow selection of cells that have taken up the plasmid by growing them on antibiotic media.
During which mitotic stage do sister chromatids separate?
Anaphase
What is "open" chromatin?
Less condensed chromatin accessible to transcription and replication machinery.
Name two stages unique to meiosis but absent in mitosis.
Reductional division (meiosis I) and equational division (meiosis II) producing haploid gametes.
How many times can typical human somatic cells divide before senescence?
Approximately 45–65 times (Hayflick limit).
Why is telomerase reactivation considered a hallmark of cancer?
It prevents telomere shortening, giving cancer cells replicative immortality.
What is the main structural difference between ribose and deoxyribose?
Ribose has a 2′-OH group; deoxyribose lacks it (has H instead).