Human Genetics Exam 2

What are single-gene traits?

TERM

Traits caused by mutations in one gene that follow predictable Mendelian inheritance, such as galactosemia.

Define polygenic traits.

Traits that involve multiple genes contributing to a phenotype, often with additive effects, like eye color and height.

What are multifactorial traits?

Traits influenced by multiple genes and environmental factors, such as blood pressure and BMI.

How do dichotomous traits differ from continuous traits?

Dichotomous traits are binary (affected/unaffected) like cleft lip, while continuous traits vary along a spectrum and are measurable, like height.

How can the environment influence the correlation between genotype and phenotype?

Factors like diet and exercise can affect how genes are expressed, leading to similar appearances or behaviors in individuals with different genotypes.

What are major gene effects?

Effects from major genes that have a large impact on phenotype, such as the HERC2 gene influencing eye color.

What are additive gene effects?

Effects from multiple genes that each contribute small effects to shape a phenotype, like kernel color in corn.

How do continuous traits reflect the inheritance of genes and the environment?

Continuous traits like height result from the combined influence of many genes and environmental inputs, producing a bell-shaped distribution.

How can dichotomous traits be multifactorial?

Dichotomous traits like cleft lip may appear binary but are influenced by multiple genes and environmental triggers, modeled as threshold traits.

What is genetic liability?

The cumulative effect of risk alleles that can lead to the expression of a trait when enough liability alleles and environmental triggers are present.

What are genome-wide association studies (GWAS)?

Studies that compare SNP frequencies between affected and unaffected individuals to identify genetic loci associated with complex traits.

How do linkage studies differ from GWAS?

Linkage analysis studies families and tracks co-segregation of alleles with traits, while GWAS studies populations and compares allele frequencies between cases and controls.

Define heritability.

A measure of how much of phenotypic variation is due to genetic differences, such as height (0.8) and BMI (0.5).

What role do twins play in studying genetic and environmental influences on traits?

Monozygotic twins share 100% of genes while dizygotic twins share about 50%, allowing researchers to compare concordance rates to estimate genetic influence.

What did Griffith's experiment demonstrate about DNA?

Griffith discovered transformation, showing that dead S-strain bacteria could transfer genetic material to live R-strain bacteria.

What was the significance of Avery, MacLeod, and McCarty's work?

They showed that DNA, not protein or RNA, was the 'transforming factor' by selectively destroying molecules and observing transformation.

What did Meselson and Stahl prove about DNA replication?

They demonstrated that DNA replicates semi-conservatively, with each new DNA molecule containing one old strand and one new strand.

What did Chargaff discover regarding base pairing in DNA?

Chargaff found that A = T and C = G, establishing the base pairing rules.

How did Franklin and Wilkins contribute to understanding DNA structure?

They used X-ray diffraction to reveal DNA's helical structure, notably through Photo 51.

What is the structure of a nucleotide?

A nucleotide consists of a sugar (deoxyribose), a phosphate group, and a nitrogenous base.

What is the difference between a nucleotide and a nucleoside?

A nucleotide includes a sugar, phosphate, and base, while a nucleoside consists only of a sugar and base.

What stabilizes the DNA double helix?

Stability is contributed by hydrogen bonds between base pairs and hydrophobic interactions between adjacent bases.

What is the significance of A=T and C=G base pairing?

A and T form 2 hydrogen bonds, while G and C form 3 hydrogen bonds, ensuring consistent spacing and stability in the DNA helix.

What is meant by semiconservative replication?

Semiconservative replication means each new DNA molecule consists of one old strand and one new strand.

What initiates DNA replication in eukaryotes?

Replication starts at multiple origins of replication (ORIs) recognized by replication machinery.

What is the structure of a replication bubble?

A replication bubble is formed with two replication forks moving in opposite directions.

What are the main differences between the leading and lagging strands during DNA replication?

The leading strand is synthesized continuously toward the fork, while the lagging strand is synthesized in fragments (Okazaki fragments) away from the fork.

List the steps involved in DNA replication.

1. Helicase unwinds DNA. 2. Topoisomerase relieves supercoiling. 3. SSB proteins prevent strand reannealing. 4. Primase lays down RNA primers. 5. DNA polymerase II synthesizes new DNA. 6. DNA polymerase I replaces RNA primers with DNA. 7. Ligase seals gaps between Okazaki fragments.

What is the role of telomeres in DNA?

Telomeres are repetitive DNA sequences at chromosome ends that form T-loops and are protected by shelterin proteins.

What is the end replication problem?

The lagging strand cannot fully replicate the end of the DNA, leading to telomere shortening.

How do certain cells avoid telomere shortening?

Telomerase adds repeats to telomeres in germ cells, some somatic cells, and most cancer cells.

Compare the structure of DNA and RNA.

DNA is double-stranded, has deoxyribose sugar, uses thymine (T), is more stable, and stores genetic information. RNA is usually single-stranded, has ribose sugar, uses uracil (U), is less stable, and transfers/expression of genetic information.

What are the major types of RNA molecules and their functions?

1. mRNA: encodes amino acid sequence of proteins. 2. rRNA: structural and catalytic part of ribosomes. 3. tRNA: brings amino acids to ribosome during translation. 4. snRNA: involved in mRNA splicing. 5. miRNA: regulates gene expression from single-stranded RNA. 6. siRNA: regulates gene expression from double-stranded RNA.

What are the four stages of transcription?

1. Promoter Recognition: RNA polymerase and transcription factors bind to promoter sequences. 2. Initiation: RNA polymerase unwinds DNA and begins RNA synthesis at the +1 site. 3. Elongation: RNA polymerase synthesizes RNA while moving along the template strand. 4. Termination: A poly-A signal triggers cleavage of the RNA transcript.

What is the significance of the TATA box in gene structure?

The TATA box is a consensus sequence located in the promoter region that helps initiate transcription.

What is the coding region of a gene?

The coding region contains the sequence where transcription ends.

What is the +1 site in transcription?

The +1 site is the transcription start site.

What is the role of SSB proteins during DNA replication?

SSB proteins prevent the reannealing of DNA strands after they have been unwound.

What is the function of primase in DNA replication?

Primase lays down RNA primers necessary for DNA polymerase to initiate synthesis.

What is the role of ligase in DNA replication?

Ligase seals gaps between Okazaki fragments on the lagging strand.

What does RNA polymerase do during transcription?

RNA polymerase unwinds DNA and synthesizes RNA from the template strand.

What is the function of topoisomerase in DNA replication?

Topoisomerase relieves supercoiling that occurs ahead of the replication fork.

What are Okazaki fragments?

Okazaki fragments are short segments of DNA synthesized on the lagging strand during DNA replication.

What is the function of micro-RNA (miRNA)?

miRNA regulates gene expression from single-stranded RNA.

What is the significance of the poly-A signal in transcription?

The poly-A signal triggers the cleavage of the RNA transcript, marking the end of transcription.

What is the role of RNase in RNA processing?

RNase degrades leftover RNA and causes RNA polymerase to detach.

What are Cis Regulatory Elements (CREs)?

Short DNA sequences in the promoter that serve as binding sites for transcription factors.

What are Transcription Factors (TFs)?

Proteins that bind to CREs and recruit RNA polymerase.

What is the function of TFIIH in transcription?

TFIIH unwinds DNA and activates RNA polymerase via phosphorylation.

When does mRNA processing occur in gene expression?

After transcription and before translation, in the nucleus.

What structural differences exist between pre-mRNA and mature mRNA?

Mature mRNA has a 5' cap and a poly-A tail, while pre-mRNA does not; introns are removed in mature mRNA.

What is the structure of the 5' cap?

A modified guanine nucleotide (7-methylguanosine) added via a 5'-to-5' triphosphate linkage.

What are the functions of the 5' cap?

Protects mRNA from degradation, facilitates nuclear export, enhances translation efficiency, and assists in splicing.

Describe the steps of Polyadenylation.

The poly-A signal (AAUAAA) is recognized, pre-mRNA is cleaved at the 3' end, and polyadenylate polymerase adds 100-250 adenine nucleotides.

What is the role of the spliceosome in mRNA splicing?

The spliceosome, a complex of snRNPs and snRNA, facilitates the removal of introns and ligation of exons.

What are the important sequences in pre-mRNA for splicing?

5' splice site (GU), branch point (adenine), and 3' splice site (AG).

How does RNA polymerase II facilitate mRNA processing?

It has a carboxyl-terminal domain (CTD) that recruits processing machinery during transcription.

What are the four levels of protein structure?

Primary (amino acid sequence), Secondary (a-helices and B-sheets), Tertiary (3D shape), Quaternary (assembly of polypeptides).

What are the three sites of a ribosome and their functions?

A site (accepts incoming tRNA), P site (holds tRNA with growing polypeptide), E site (exit site for uncharged tRNA).

What initiates the translation process?

The small ribosomal subunit binds mRNA near the Kozak sequence, and the initiator tRNA binds the start codon (AUG).

What is the genetic code?

A set of 64 codons (triplets of mRNA bases) that correspond to specific amino acids.

What are synonymous and non-synonymous codons?

Synonymous codons code for the same amino acid, while non-synonymous codons code for different amino acids.

What is the wobble position in codons?

The third base of the codon allows flexible pairing, enabling one tRNA to recognize multiple codons.

What are the steps of PCR?

Denaturation (DNA strands separate), Annealing (primers bind), Extension (DNA polymerase synthesizes new strands).

What distinguishes Sanger sequencing from Next Generation Sequencing (NGS)?

Sanger sequencing is cheaper with longer read lengths but processes one sequence at a time, while NGS is more expensive, has shorter read lengths, and can process millions of sequences simultaneously.

What is the principle behind Southern Blotting?

It detects specific DNA fragments using hybridization with labeled probes.

What is the role of antibodies in protein detection?

Antibodies bind specifically to antigens (proteins) and are used in techniques like Western blotting and immunohistochemistry.

What is polyploidy?

The presence of extra sets of all chromosomes, which is often incompatible with human life.

What is aneuploidy?

The gain or loss of individual chromosomes, leading to conditions like trisomy or monosomy.

What is a deletion in chromosome structure?

The loss of a segment of a chromosome.

What is a translocation in chromosome structure?

A segment of a chromosome is moved to another chromosome, which can be balanced or unbalanced.

What is the significance of balanced translocations?

They involve an exchange between non-homologous chromosomes without a net gain or loss of genes, often resulting in no phenotype.

How can atypical chromosome structures show no phenotype?

If balanced and gene expression isn't disrupted, the phenotype may be silent, though it may still cause infertility.