Genetics: DNA Structure, Chromosomes, Cell Division, and Sex Determination

Flashcards covering molecular genetics, DNA/RNA primary and secondary structures, eukaryotic chromatin, cell cycle, mitosis, meiosis, gametogenesis, sex determination, aneuploidy, polyploidy, and chromosomal rearrangements.

What three repeating components make up the basic structure of a nucleotide in DNA and RNA?

A pentose sugar, a phosphate group, and a nitrogenous base.

How do ribose and deoxyribose pentose sugars differ at the $2'$ carbon position?

Ribose has a hydroxyl group ($\text{-OH}$) attached to the $2'$ carbon atom, whereas deoxyribose has a hydrogen atom ($\text{-H}$) at the $2'$ carbon atom.

Which nitrogenous bases are classified as purines, and how many rings do they contain?

Adenine ($\text{A}$) and Guanine ($\text{G}$) are purines, and they contain two fused rings.

Which nitrogenous bases are pyrimidines, and how do they differ between DNA and RNA?

Pyrimidines contain a single ring. Cytosine ($\text{C}$) and Thymine ($\text{T}$) are present in DNA, whereas Uracil ($\text{U}$) replaces Thymine in RNA.

According to Chargaff's rules, what are the quantitative base composition relationships in double-stranded DNA?

The ratio of Adenine to Thymine equals $1$, the ratio of Guanine to Cytosine equals $1$, and the total number of purines equals total pyrimidines ($\text{A} + \text{G} = \text{T} + \text{C}$).

How many hydrogen bonds form between complementary base pairs in double-stranded DNA?

Adenine and Thymine ($\text{A-T}$) form $2$ hydrogen bonds, while Guanine and Cytosine ($\text{G-C}$) form $3$ hydrogen bonds.

What is the structural distinction between a hairpin and a stem secondary structure in single-stranded nucleic acids?

A hairpin (stem-loop) consists of a double-stranded stem with a terminal loop of unpaired bases, while a stem consists of a paired double-stranded region without a terminal loop.

What enzyme controls tertiary supercoiling in circular bacterial DNA by inducing or removing twists?

Topoisomerase.

What are the core structural components and histone composition of eukaryotic chromatin?

Chromatin consists of roughly equal proportions of DNA and protein. The repeating structural unit is the nucleosome core, which consists of double-stranded DNA wrapped around an octamer of eight positively charged histone proteins.

What is the difference between dioecious and monoecious species?

Dioecious species have separate male and female individuals, whereas monoecious species possess both male and female reproductive structures within a single individual.

How do the $\text{XX-XY}$ and $\text{ZZ-ZW}$ sex determination systems differ regarding heterogametic sex?

In the $\text{XX-XY}$ system, males are the heterogametic sex ($\text{XY}$) and females are homogametic ($\text{XX}$). In the $\text{ZZ-ZW}$ system, females are the heterogametic sex ($\text{ZW}$) and males are homogametic ($\text{ZZ}$).

What is the primary function of telomeres, and what sequence repeat is present in humans?

Telomeres protect linear chromosome ends from degradation and fusion. Human telomeres consist of tandem repeats of the sequence $5'\text{-TTAGGG-}3'$, featuring a single-stranded $3'$ G-rich overhang that forms a protective t-loop.

How are eukaryotic chromosomes classified based on centromere position?

Metacentric (centromere in middle), submetacentric (centromere off-center), acrocentric (centromere near one end with short p-arms), and telocentric (centromere at terminal end with no visible short arms).

What key event occurs during the $S$ phase of the interphase cell cycle?

DNA replication occurs, duplicating each chromosome into two identical sister chromatids attached at the centromere.

How do Meiosis I and Meiosis II differ in terms of chromosome separation and ploidy change?

Meiosis I is a reductional division ($2n \rightarrow n$) that pairs homologous chromosomes, conducts crossing over, and separates homologs. Meiosis II is an equational division ($n \rightarrow n$) that separates sister chromatids.

What are the cellular products produced by spermatogenesis versus oogenesis from one primary germ cell?

Spermatogenesis produces $4$ functional haploid ($1n$) sperm cells. Oogenesis produces $1$ functional haploid ($1n$) ovum and up to $2$ non-functional polar bodies that disintegrate.

At what stages of meiosis does cell cycle arrest occur during female oogenesis?

Primary oocytes arrest in Prophase I during embryonic development until puberty/ovulation, and secondary oocytes arrest in Metaphase II until fertilization occurs.

What is non-disjunction, and what gamete outcomes result when it occurs in Meiosis I versus Meiosis II?

Non-disjunction is failure of chromosomes or chromatids to separate properly. Meiosis I non-disjunction produces $100\%$ abnormal gametes ($2$ of $n+1$ and $2$ of $n-1$). Meiosis II non-disjunction produces $50\%$ normal ($n$) and $50\%$ abnormal gametes ($1$ of $n+1$ and $1$ of $n-1$).

What is the core distinction between aneuploidy and polyploidy?

Aneuploidy is a change in chromosome number involving one or a few individual chromosomes (e.g., trisomy $2n+1$). Polyploidy is a change involving addition of entire set(s) of chromosomes (e.g., triploid $3n$, tetraploid $4n$).

Why are odd-numbered polyploids (such as triploids, $3n$) usually sterile?

During Meiosis I, three homologous chromosomes cannot pair and segregate evenly, resulting in unbalanced, aneuploid gametes that are inviable.

What is the difference between autopolyploidy and allopolyploidy?

Autopolyploidy involves multiple chromosome sets derived from a single species. Allopolyploidy involves multiple chromosome sets derived from hybridization between two different species.

What are the four primary categories of chromosome structural rearrangements?

Duplication (doubling of a segment), Deletion (loss of a segment), Inversion (flipping a segment $180^\circ$), and Translocation (movement of a segment to a non-homologous chromosome).

What is haploinsufficiency?

A condition where a single wild-type copy of a gene in a diploid organism is insufficient to provide normal gene function, producing a mutant phenotype in deletion heterozygotes.

How do paracentric and pericentric inversions differ structurally?

A paracentric inversion occurs within one arm and does not include the centromere. A pericentric inversion includes the centromere and involves both chromosome arms.

What structural anomalies are produced during Anaphase I when crossing over occurs within a paracentric inversion loop?

A dicentric chromatid (containing two centromeres, forming a dicentric bridge that breaks randomly) and an acentric fragment (lacking a centromere, which is lost during cell division).

In reciprocal translocation heterozygotes, what are the gametic outcomes of alternate segregation versus adjacent segregation?

Alternate segregation produces $50\%$ viable, balanced gametes (containing complete genetic information). Adjacent segregation produces $50\%$ inviable, unbalanced gametes (containing duplications and deletions).

What function do Pseudoautosomal Regions (PARs) serve in mammalian sex chromosomes?

PAR1 and PAR2 are homologous DNA regions located at the tips of X and Y chromosomes that allow homologous pairing and recombination during male meiosis.

What gene acts as the primary master switch for male sex determination in mammals?

The $SRY$ gene (Sex-determining Region Y), located on the short arm of the Y chromosome, which encodes a transcription factor that directs the indifferent gonad to develop into a testis.