Genetics 2

Question 1

Building blocks of life?

Answer 1

-Nucleic acids:
1.) DNA: found in all organisms and many viruses
2.) RNA: found in all known forms of life
-Proteins
-Lipids
-Carbohydrates

Question 2

DNA (and RNA) structure?

Answer 2

-Each nucleotide is composed of:
1.) Nucleobases: either Cytosine, Guanine, Adenine, or Thymine
2.) Sugar: deoxyribose
3.) Phosphate
-Nucleotides are joined covalently between the sugar and the phosphate group: sugar-phosphate backbone
-Nitrogenous bases of opposite DNA strands are bound together with hydrogen bonds to make an anti-parallel double helix (RNA forms complex 3° structures).

Question 3

The base pairs?

Answer 3

-Thymine, Uracil, and Cytosine are pyrimidines (aromatic heterocyclic organic compounds)-Guanine and Adenine are purines (pyrimidine ring fused to an imidazole ring).
-Guanine pairs with Cytosine
-Thymine (and Uracil) pairs with Adenine
-G/C
-T/A
-U/A

Question 4

DNA transcription?

Answer 4

-Complex process: Transcription factors and RNA polymerase bind to the gene promoter region. Creates a transcription bubble separating the strands of DNA. RNA nucleotides (complementary to the DNA strand) are linked to form a sugar-phosphate backbone. The strand of RNA is released and may be further modified before use.

Question 5

RNA translation?

Answer 5

-Ribosomes (complex protein/ribosomal RNA structures) bind to the 5’ end of messenger RNA (mRNA).
-Transfer RNA molecules, bound to individual amino acids, interact with nucleotide triplets (codons) of the mRNA, with transfer of their amino acids to the growing peptide.
-Codon specificity to individual amino acids or ‘STOP’ is highly conserved among organisms.

Question 6

Genes?

Answer 6

-A gene comprises the sequence of DNA that encodes: Specific proteins (enzymes related to the expression of the phenotype, and factors that increase or decrease gene expression.)
-In humans, genes make up ~10% of the genome (the remainder includes sites that regulate gene expression and ‘junk’ DNA).
-Genes reside at a specific position (locus) on chromosomes.
-Different versions of the same gene= alleles

Question 7

Gene expression?

Answer 7

-Gene expression= transcription+ translation
-Many genes are conserved between species-although differences in gene positive/negative amino acid sequence can be used to distinguish between species
-The more critical the protein/ protein structure is to its function, the more conserved it is across species: canine insulin is identical to porcine insulin (so recombinant porcine insulin is used in the treatment of canine diabetes).

Question 8

Protein binding?

Answer 8

-Exposed edges of base pairs form specific structures onto which proteins bind: histones, transcription factors, polymerases
-Histones: “unstructured tails” can be modified. Histone acetylation brings about transcriptional activation. Deacetylation brings about transcriptional repression.
-Facilitate DNA folding.

Question 9

DNA methylation?

Answer 9

-The cytosine adjacent to a guanine (“CpG”) may be methylated.
-Cell differentiation: Transcription factors recruit DNA methyltransferases –> methylate –>CpGs –> repress undesirable gene expression. Methylated CpG patterns progress as cells differentiate.
-De-methylation is associated with transcription activation: only achievable by DNA repair or replication. Most methylated CpG patterens are reset in gametogenesis
-Some methylation patterns are inherited.

Question 10

Chromosomes?

Answer 10

-DNA molecule containing some/all of an organism’s genetic material
-Can typically only be visualized under a microscope during metaphase, as chromatid pairs joined by a single centromere (i.e., recently copied; condensed; centrally aligned).
-In prokaryotes, each chromosome has a specific size and shape- together their appearance defines the subject’s karyotype
-Chromosome number is typically reported as the diploid “2n” number

Question 11

Genetic material in cell division?

Answer 11

-Mitosis: cell replication during growth; usually faithfully copies DNA from one cell to the next and keeps all the genes on a given chromosome strand together
-Meiosis: production of gametes (sperm or egg); a single strand of each chromosome separates randomly-resulting in ~50:50 from paternally and maternally-derived chromosomes.
-During meiosis can get mixing up of the genetic material within a chromosome strand. This is recombination or crossing over

Question 12

Recombination?

Answer 12

-This has been partly the basis of how we have worked out in the past, where genes are.
-Typically, there are 2-3 recombination events per chromosome.
-Because whole segments move, genes that are close together are less likely to get separated than those further apart: This is known as linkage. This is why some characteristics appear to be inherited together (e.g., those with red hair are more likely to be fairer-skinned)
-Recombination fraction is the proportion of alleles in one parent that could only have resulted from crossing-over during meiosis

Question 13

Sex chromosomes?

Answer 13

X Chromosomes: often one of the largest chromosomes. Carries genes that are important in day-to-day metabolism. Sex-linked genetic defects tend to be carried on the X chromosome.
Y Chromosomes: Quite small. Carries few genes except those that encode for “maleness”. The presence or absence of a Y chromosome determines phenotypic sex.

Question 14

Sex Linkage?

Answer 14

-When a gene governing a trait is carried on a sex chromosome and differentially expressed between the sexes.
-Usually carried on the (larger) X chromosome
-If the carried on the X chromosome and recessive: males require only ONE recessive allele to express the trait. Remales require TWO recessive alleles to express the trait. Males exhibit these traits»_space;> females: e.g., if seen in 1 in 8 males would be 1 in 64 females (assuming the trait has no impact on breeding capacity).
-Examples include haemophilia A (factor VIII deficiency), Duchenne muscular dystrophy, and the orange coat colour in cats.

Question 15

X-inactivation and Mosaicism?

Answer 15

-The Lyon hypothesis of X-inactivation: corrects for the imbalance of X chromosomes between males and females. All but one of the X chromosomes present is inactivated in early embryogenesis (pattern is inherited by daughter cells).
-Mosaicism (aka genetic chimerism): 2+ genetically different cells within the body derived from a single zygote. Tortoiseshell female (and XXY male) cats are an example.
*NB: the tortoiseshell coat pattern may also (rarely) occur as a form of animal chimerism (following embryonic merger).

Question 16

Pseudoautosomal region?

Answer 16

-Three small regions of homology the pseudoautosomal regions (PAR1, PAR2, and PAR3) exist between X and Y chromosomes.
-These allow homologous pairing between X and Y chromosomes during meiosis.
-Pseudoautosomal genes exhibit an autosomal, rather than sex-linked, patteren of inheritance.

Question 17

DNA alterations?

Answer 17

-Damage can be physical (e.g., radiation (gamma-/X-/uv-/beta-) or chemical (endogenous and exogenous): some of these can be minimised. Others are inevitable.
-Replication during mitosis and meiosis can also go wrong
-Within somatic cells, errors are generally repaired (from the ‘correct’ stand), but when not can lead to dysplasia or neoplasia.
-Within gametes or embryonic cells, errors can lead to novel characteristics (which may be beneficial or more likely detrimental).
-Chromosomal separation during mitosis/meiosis is also not perfect.

Question 18

Clinical application-predictable pairs?

Answer 18

-The sequence of the complementary strand of DNA can be predicted from the coding sequence (and vice versa).
-Primers (short sequences of single-stranded DNA) can be designed for use in combination with DNA polymerases to amplify specific DNA fragments for use in diagnostic assays.
-Targeted DNA amplification can be used to: determine the presence/absence of the target gene. Starting quantity of target gene.

Question 19

Clinical applications-sequence data?

Answer 19

-DNA sequence data can be used to: Predict the function of a target gene. Predict the amino acid sequence of the transcribed gene. Determine species present within the sample. Identify variants (or carriers) within a species (Risk of disease within an individual, Risk of passing on disease to the next generation).

Question 20

Clinical applications-chemotherapy?

Answer 20

-Limiting cell turnover e.g., immune-mediated disease; neoplasia.
-Interference with host cell replication and transcription: Anti-metabolites;
1.) Leflunomide: inhibits the synthesis of uracil.
2.) Mycophenolate mofetil: inhibits the synthesis of guanine.
3.) Azathioprine: inhibits the synthesis of guanine, interferes with the synthesis of DNA, and induces cell death.
4.) Cytarbine: interferes with the synthesis of DNA
5.) Fluorouracil: inhibits the synthesis of thymidine from uracil.
-DNA intercalation: doxorubicin (evicts histones; inhibits enzymes required for replication).
-Cross-linking DNA: Cyclophosphamide (cross-links guanine; prevents replication). Chlorambucil (cross-links DNA).
-Binding to cell structure proteins: Vincristine (prevents chromosomal separation during replication).