Introduction to Biology Study Guide – Quiz 2: Genetics and Medicine
1. Nucleotide = Sugar (deoxyribose – “deoxy” because there is 1 less O in deoxyribose than
ribose; ribose has an –OH group where deoxyribose just has an -H), Phosphate group (negatively
charged), Nitrogenous base (A, T, C, G)
2. The 3 major functions of DNA:
• Ability to replicate so cells can divide
• Generates variation by mechanism/mechanisms that changed chemical info in genes
• Carries the info in genes that enables expression of organism’s phenotype via some
unknown code and a method of translating the code into amino acids
3. Protein Synthesis:
• RNA Polymerase: large molecular complex that links together the growing chain of RNA
nucleotides during transcription, using DNA strand as template
• mRNA: RNA strand that goes out of nucleus to be translated
• tRNA: RNA that interprets during translation; has an anticodon, picks up specific amino
acid and carries amino acid to appropriate codon on mRNA
• Codon: Set of 3 nucleotides that specifies a particular amino acid
• Anticodon: 3 nucleotides on tRNA that are complementary to an mRNA codon
• Amino acids: Monomers of proteins
• Ribosomes: Where the proteins are made; large and small subunit; P and A site on large
subunit are sites for 2 tRNAs; small subunit is where mRNA docks
• Transcription: DNA RNA
1) Initiation – Attachment of RNA polymerase to promoter (“start transcribing”
nucleotide sequence) on DNA template and start of RNA synthesis
2) Elongation – RNA grows longer, and strand starts peeling away from DNA template,
allowing 2 DNA strands to come back together
3) Termination – RNA polymerase reaches sequence of bases in DNA template called
terminator, signaling end of gene, causing polymerase molecule to detach from RNA
molecule and gene, allowing completed RNA to peel away completely
4) “Cap” (single G nucleotide) and “Tail” (chain of A nucleotides) added to mRNA to
make export from nucleus easier; cap and tail not translated
5) RNA Splicing – Most genes contain regions that aren’t supposed to be coded, called
introns, that are first transcribed to the mRNA but eventually removed, allowing the
exons (regions of genes that are supposed to be coded) to come together, leaving the
final mRNA product with only regions of the gene that are supposed to be coded and
a cap and tail
• Translation: RNA Protein
1) Amino acid attachment – Each amino acid attaches to its proper tRNA w/help of
specific enzyme and ATP
2) Initiation of Polypeptide Synthesis (Codon Recognition) – mRNA, first tRNA and
ribosomal subunits come together
3) Elongation (Peptide bond formation/Translocation of tRNA in A site to P site) –
tRNA goes to P site, now there’s 1 amino acid; another tRNA goes to A site; amino
acid from P site tRNA goes to tRNA of A site; P site RNA leaves; mRNA codon
attached to anticodon of tRNA at A site moves such that tRNA at A site is moved to P
site along with corresponding mRNA codon, and the cycle repeats
4) Termination – Ribosome recognizes stop codon; polypeptide terminated and
released)
4. “Central Dogma of Biology”: DNA makes RNA, which makes protein
5. DNA Replication:
• 5’ and 3’ end: At 1 end of DNA strand, sugar’s 3’ carbon atom is attached to an –OH
group, and the sugar’s 5’ carbon is attached to a phosphate group; opposite strands run in
opposite directions
• Okazaki Fragments: Fragments of DNA that are created and eventually linked in order to
form lagging strand
• DNA Ligase: Enzyme that links okazaki fragments together
• Primase: Enzyme that synthesizes short RNA segment (primer) complementary to DNA
template, after which DNA polymerase can initiate synthesis of DNA strand
• DNA Polymerase: Enzymes that link DNA nucleotides to a growing daughter strand; also
replaces the temporary RNA primers created by primase with DNA
• Leading strand: Works towards forking point in 5’ to 3’ direction
• Lagging strand: Must work in opposite direction of forking point to go in 5’ to 3’
direction, and thus, strand is constructed in fragments in 5’ to 3’ direction such that the
fragments are constructed towards forking point
• Helicase: Enzyme that unwinds double helix
• Single Strand Binding Protein: Protein that binds to single stranded regions of DNA to
prevent premature annealing (binding of strands)
6. How DNA is “packaged:”
• Histones (small proteins – about half the mass of eukaryotic chromosomes) attach to
DNA double helix
• DNA-histone complex looks like beads on a string
• Each “bead” is called a nucleosome (DNA wound around protein core of 8 histones)
• “String” (DNA) linking the beads called “linkers”
• Beaded string wrapped into tight helical fiber
• Fiber coils further into a thick supercoil
• Looping and folding further compacts the DNA into chromatin
7. Types of Mutations in DNA:
• Substitution of nucleotide
• Insertion of nucleotide
• Deletion of nucleotide
• Silent mutation = no change (ex: substitution results in new codon that codes for same
amino acid, changing nothing essentially)
• Missense mutation = change
• Nonsense mutation = change in amino acid codon to stop codon
***Insertions and deletions are generally worse than substitutions since all codons
downstream of the insertion/deletion change, whereas with a substitution only 1 codon
changes, and that change may not even change the amino acid coded for
8. Why do diverse species have so many genes in common?
• Species in all major groups radiated from single ancestral species
• Genes remain same over vast amounts of time b/c species share many of the basic
cellular/bodily functions needed to live
• These functions are so basic that any major changes would have been selected against
• Another possible explanation is the idea of horizontal gene transfer, in which genes are
transferred between unrelated species
9. Mechanisms for Origin of New Genes:
• Duplication = Duplication of region of DNA containing a gene; mutations have no
deleterious effects on host organism
• Transposons = Genes that move from 1 location of genome to another
• Transcription Errors = DNA not transcribed properly
• Exon or Domain Shuffling = Exons either within a gene or b/w 2 nonallelic genes are
mixed
• Lateral Gene Transfer = Gene transfer b/w unrelated species
• De Novo Gene Origination
10. Define:
• Karyotype: # of chromosomes in nucleus of eukaryotic cell (46 in humans – 22 pairs of
autosomal, or non-sex, chromosomes, and 1 pair of sex chromosomes)
• Haploid: Half the number of chromosomes
• Diploid: # of chromosomes in somatic (non-sex) cells
• Polyploidy: Species w/2 or more sets of chromosomes, common in plants & important for
plant speciation
11. Autosomal (Somatic) Chromosomes: Non-sex chromosomes
12. Homologous Chromosome Pairs: Chromosomes identical in terms of the genes carried (1
copy of homologous pair from mom, other from dad)
13. Gene Locus: Gene’s location on chromosome
14. Allele: Different versions of a gene (ex: brown eye vs. green eye)
15. Mitosis:
• Chromosomes replicate (2N 4N)
• The new, replicated chromosomes, consist of “sister chromatids” that are attached by a
centromere at the center
• Centrioles are involved in organization of mitotic spindles
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Mitotic spindle (fibers) are tubular structures that connect centromeres (kinetochores) of
chromosomes to spindle poles
Mitotic spindles are composed of microtubules – hollow cylinders made of the protein
tubulin
Kinesins are motor proteins that can move along surface of microtubules and exert forces
that separate chromosomes
Cytokinesis follows mitosis; the cytoplasm and cell membrane are divided equally into 2
daughter cells (the division is called a cleavage furrow in animals and a cell plate in
plants)
16. Meiosis:
• Random alignment and allocation of chromosomes: Since each pair of chromosomes
aligns independently at equator, there is an equal probability of maternal/paternal
chromosome facing a given pole and thus 223 possible combinations for chromosomes to
be packaged into gametes (for each of the 23 pairs of chromosomes in humans, there are
2 possible chromosomes)
• Division 1: 4N (after replication) 2N
• Division 2: 2N N
• Recombination: Exchange of pieces of non-sister chromatids, by breakage and reunion;
results in genetic variation
• Gametes: Sex cells (also called germ cells) – sperm and egg in humans
17. Nondisjunction – Trisomy 21:
• Nondisjunction = failure of chromosomes (meiosis I) or chromatids (meiosis II) to
separate
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Trisomy 21 = 3 copies of chromosome 21 inherited
18. Describe:
• Down Syndrome: Result of Trisomy 21; characterized by short life span, susceptibility to
disease, cognitive impairment and characteristic facial features; most common human
chromosomal abnormality
• Turner Syndrome: Female with all or part of 1 X chromosome is missing (X); Effects =
short stature, broad chest, non-functional ovaries
• Klinefelter Syndrome: Males have extra X chromosome (XXY); Effects = low
testosterone, low fertility
19. Dominant and Recessive Allele Disorders with Examples:
Dominant
• Huntington’s Disease = muscle coordination affected; eventually leads to dementia
• Alzheimer’s = mental deterioration
• Hypercholesterolemia = heart disease
• Achondroplasia = dwarfism
Recessive
• Galactosemia = inability to break down galactose fully; results in damage to liver,
kidneys, eyes and brain
• Sickle Cell Disease = sickle red blood cells
• Albinism = lack of pigment in skin/hair/eyes
• Tay Sachs = lipid accumulation in brain; mental deficiency; childhood death
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Cystic Fibrosis = Excess mucus in lungs
Phenylketonuria = too much phenylalanine in blood; lack of normal skin pigment
20. X-Linked (Recessive) Genetic Disorders and Examples:
• Hemophilia = inefficient blood clotting, leading to excessive bleeding; Queen Victoria of
England was a carrier; hemophilia spread through royal families of several nations as a
result
• Red-Green color blindness = malfunction of light-sensitive cells in eyes (normal vision =
150 colors visible; Red-green color blindness = less than 25)
• Muscular dystrophy = weakening of muscles, loss of coordination
21. Lac Operon – How it Functions; Positive and Negative Feedback:
• Lac Operon is a cluster of genes w/related functions and control sequences; consists of
Promoter, Operator and Lactose-Utilization Genes
• Operator acts as switch, which determines whether RNA polymerase can attach to the
promoter or not
• Promoter is a site where RNA polymerase attaches and initiates transcription of the 3
lactose enzyme genes
• Lactose-Utilization genes (lacZ, lacY and lacA) are genes that produce enzymes to use
lactose as an energy source
• Repressor protein binds to operator to block attachment of RNA polymerase to promoter
when there is no lactose in order to prevent lactose-utilization enzymes from being
produced unnecessarily
• Regulatory gene, located outside operon, codes for this repressor protein; small supply of
repressor molecules thus always available
• If lactose is present, a molecule called allolactose interferes w/attachment of lac repressor
to operator by binding to it and changing its shape
• Negative Feedback: Lac operon turns off due to regulatory protein (lac repressor,
produced by regulatory gene, blocks promoter since there is no lactose)
• Positive Feedback: Lac operon turns on due to presence of lactose
22. Mechanisms of Gene Regulation in Eukaryotes:
• DNA Packing: Gene expression blocked if the packing prevents RNA polymerase and
other transcription proteins from contacting DNA
• Chemical Modification: Certain enzymes add methyl group to DNA bases (usually
Cytosine), usually resulting in gene turning off; methylation patterns stay that way
through successive cell divisions
• Epigenetic Inheritance: Inheritance of traits transmitted by mechanisms not directly
involving the nucleotide sequence (ex: modifications to chromatin, such as tight packing
or methylation, which don’t affect sequence of DNA, result in modification of gene
expression and thus affect inherited traits)
• X Chromosome Inactivation: Females get 2 X chromosomes, while males get 1; 1 of the
females’ X chromosomes, called a Barr body, becomes almost completely inactive; the X
chromosome that becomes inactivated (mom’s X chromosome vs. dad’s X chromosome)
is a matter of chance, but once inactivated, all descendant cells have the same copy
turned off; Gene expression is affected by X chromosome inactivation because
phenotypic characteristic resulting from gene present on X-chromosome may depend on
which X-chromosome is inactivated, as shown below:
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Control of Transcription by Complex Assemblies of Proteins: For gene transcription to
occur, first activator proteins bind to DNA control sequences called enhancers; Next, the
DNA bends as a result; Next, bound activators can now interact w/transcription factor
proteins, which then bind as a complex at the gene’s promoter; Finally, RNA polymerase
can easily attach to the promoter on the gene and initiate transcription; Thus, it is clear
that various proteins control transcription and thus gene expression
Alternative RNA Splicing: Organism can produce more than 1 type of polypeptide from a
single gene since there can be different combination of exons as a result of splicing in
different ways
miRNA: Small, non-coding, single-stranded RNA, 20-26 nucleotides long, called micro
RNA (miRNA) associates w/protein complex, binds to complementary mRNA sequence,
and either degrades the target mRNA or blocks its translation
Breakdown of mRNA: Enzymes in cytoplasm eventually break down mRNA, regulating
the amounts of various proteins produced in the cell (long-lived mRNA translated into
many more protein molecules); mRNA in red blood cells, which produce the protein
hemoglobin, is long-lived, and thus, large quantities of hemoglobin are produced
Initiation of Translation: Many proteins involved in polypeptide synthesis, thus
regulating the start of translation and thus gene expression (ex: red blood cells have
inhibitory protein that prevents translation of hemoglobin mRNA unless cell has a supply
of heme, which is essential for hemoglobin function)
Protein Activation: Post-translation, proteins may require alteration in order to actually
function; ex: Cleavage occurs w/the protein insulin; Insulin is initially inactive, then
folds, then cut (cleaved) into two smaller chains which are active
Protein Breakdown: Proteins can be broken down, allowing cell to adjust kinds and
amounts of proteins in response to environmental change; damaged proteins usually
broken down and replaced
• Operon type feedback
***“Default” state for most genes seems to be “off”; Housekeeping genes (ones active in
virtually all cells) may be “on” by default;
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23. Transcription Factors:
• Proteins that bind to DNA sequences and control transcription via promoting or blocking
RNA polymerase
• Critical to right genes being expressed at the right time (gene expression)
• Many transcription factors involved in development of zygote
24. Homeobox (Hox) genes and control of development: Master control gene that regulates
batteries of other genes that actually determine the anatomy of parts of the body (thus affecting
development)
• Pax genes are transcription proteins important in development (regulation of genes and
control of a number of tissues and organs)
Important Names
• Hershey/Chase: Showed DNA is the genetic material, not protein
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Rosalind Franklin: Got good X-ray crystallography image of DNA, used by Watson
James Watson/Francis Crick: Showed correct model of DNA
Jacob/Monod: First proposed Lac Operon model