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DAT > SNS - Biology - Genetics > Flashcards

SNS - Biology - Genetics Flashcards

1
Q

Monohybrid Cross

Hallmarks

A
  1. F1 generation: 100% Xx
  2. F2 generation: 25% XX, 50% Xx, 25% xx
  3. Disappearance of recessive phenotype in the F1 generation
  4. Reappearance of recessive phenotype in 25% of the F2 generation
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2
Q

Test Cross

A
  • The genotype of only homozygous recessive can be predicted will complete accuracy. Individuals expressing the dominant phenotype could either be XX or Xx
  • A test cross can be used to determine the unknown genotype
  • An organism with a dominant phenotype is crossed with a phenotypically recessive organism
  • If the dominant parent is homozygous, all offspring will express the dominant phenotype
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3
Q

Dihybrid Cross

Hallmarks

A
  • F1 generation - 100% TtPp
  • F2 generation has 9:3:3:1 ratio dominant/dominant, dominant/recessive, recessive/dominant, recessive/recessive
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4
Q

Non Mendalian Inheritance

A
  1. Incomplete dominance
  2. Codominance
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5
Q

Non-Mendalian Inheritance

Incomplete Dominance

A
  • Some progeny phenotypes appear to be blends of parental phenotypes
  • For example homozygous dominant red snapdragons crossed with honozygous recessive white snapdragons produce 100% pink progeny in the F1 generation. F1 x F1 produces red, pink and white progeny in a 1:2:1 ratio. This effect is the result of the combined effects of the red and white genes in heterozygotes
  • An allele is incompletely dominant if the phenotype of the heterozygote is an intermediate of the phenotypes of the two homozygotes
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6
Q

Non-Mendalian Inheritance

Codominance

A
  • Occurs when multiple alleles exist for a given genes and more than one of them is dominant
  • Each dominant allele is fully dominant when combined with a recessive allele but when two dominant alleles are present, the phenotype is a result of the expression of both dominant alleles simultaneously
  • For example, the ABO blood groups - blood type determined by the alleles IA, IB and i
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7
Q

Number:

  1. Autosomes
  2. Sex chromosomes
A
  1. 22 pairs
  2. 1 pair
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8
Q

Sex Linkage

A
  • For a recessive gene carried on the X chromosome, males express the recessive phenotype whenever it occurs as no dominant allele present to mask it
  • Since males pass on their X chromosome to their female offspring only, will pass it to female but not male offspring
  • Unless the female also receives the affected gene from her mother too, she will be a phenotypically normal carrier of the trait.
  • Since all of the daughter’s male children will receive her affected X chromosome, potentially half of her sons will receive the recessive allele
  • Thus recessive sex-linked alleles generally affect only males and can’t be passed from father to son but can be passed from father to grandson via a daughter who is a carrier - thereby skipping a generation
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9
Q

Environmental Factors

A
  • Can often affect the expression of a gene - interaction between the environment and the genotype produces the phenotype
  • For example, temp influences the coat colour of the Himalayan hair
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10
Q

Genetic Problems

Nondisjunction

A
  • Either failure of homologous chromosomes to separate proerly during meiosis I or failure of sister chromatids to separate properly during meiosis II
  • Resulting zygote might have either three copies of that chromosome (trisomy) or a simgle copy (monosomy)
  • For example Down’s syndrome - trisomy of chromosome 21
  • Most are lethal causing the embryo to abort spontaneously early in the pregnancy
  • Nondisjunction of sex chromosomes can also occur, resulting in individuals with missing or extra copies of X and/or Y chromosomes
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11
Q

Genetic Problems

Chromosomal Breakage

A
  • Can occur spontaneously or be induced by environmental factors
    such as X-rays or mutagenic agents
  • Chromosome which loses a fragment said to have a deficiency
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12
Q

Genetic Problems

Mutations

A
  • Mutations that occur in somatic cells can lead to tumours
  • Mutations that occur in gametes can be transmitted to offspring
  • Most are silent - don’t occur in regions of DNA that code for proteins
  • Mutations that do alter the AA sequence in proteins are most often recessive and deleterious
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13
Q

Genetic Problems

Mutations

Mutagenic Agents

A
  • For example, cosmic rays, X-rays, radioactivity, chemical compounds
  • Generally also carcinogenic
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14
Q

Genetic Problems

Mutations

Mutation Types

A
  • Bases can be added, deletion or substituted
  • Can potentially create different genes as may alter AA sequence during translation
  • However, due to degenerate nature of genetic code, gene could still code for the same AA
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15
Q

Genetic Problems

Mutations

Genetic Disorders

Phenylketonuria

A
  • Caused by inability to produce the enzyme for the metabolism of the AA phenylalanine
  • Degradation product - phenylpyruvic acid - accumulates with deleterious consequences
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16
Q

Genetic Problems

Mutations

Genetic Disorders

Sickle-Cell Anaemia

A
  • Disease in which RBCs become crescent shaped as contain defective haemoglobin
  • Carry less oxygen
  • Caused by substitution of valine (GUA or GUG) for glutamic acid (GAA or GAG) in the gene coding for haemoglobin
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17
Q

DNA

Structure

A

Basic structural unit is the nucleotide, composed of:

  1. Deoxyribose
  2. Phosphate group
  3. Nitrogenous base

G is triple bonded to C, A double bonded to T

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18
Q

DNA

Structure

Purines

A
  1. Adenine
  2. Guanine
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19
Q

DNA

Structure

Pyrimidines

A
  1. Cytosine
  2. Thymine
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20
Q

DNA Replication

A
  1. DNA unwound by helicase and gyrase
  2. RNA primer added to the 5’ end of the new strand
  3. DNA polymerase works on both template strands simultaneously, replication occuring in the 5’ to 3’ direction resulting in the formation of a leading strand and a lagging strand
  4. Okazaki fragments of the lagging strang are joined by DNA ligase
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21
Q

Genetic Code

Redundancy

A
  • Since only 20 AAs are coded for, and 64 different combinations of the bases C, G, A and T, most AAs have more than one code specifying them - referred to as the redundancy or degeneracy of the genetic code
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22
Q

RNA

Structure

A

Polynucleotide similar to DNA except sugar is ribose and contains uracil instead of thymine

Found in both the nucleus and the cytoplasm

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23
Q

RNA

mRNA

A
  • Carries the complement of the DNA sequence (antisense, transcribe from the sense DNA strand) and transports it from nucleus to cribosomes where protein synthesis occurs
  • Has inverted, complementary codes to the original DNA
  • Each mRNA strand is monocistronic - codes for a single polypeptide - in most eukaryotes and polycistronic in most prokaryotes
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24
Q

RNA

tRNA

A
  • Small RNA found in the cytoplasm
  • Aids in the translation of mRNA into sequence of AAs
  • Is at least one type of tRNA for each AA - are approximately 40 known types
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25
Q

RNA

rRNA

A
  • Structural component of ribosomes
  • Most abundant of all RNA types
  • Synthesised in the nucleolus
26
Q

Protein Synthesis

Transcription

Initiation

A
  1. RNA polymerase recognises the promoter region on the sense DNA strand - most commonly called the TATA box sequence
  2. DNA helix is unwound by helicase
27
Q

Protein Synthesis

Transcription

Elongation

A
  1. RNA polymerase adds nucleotides to the growing RNA transcript in the 5- to 3- direction using a simgle strand of the DNA as a template
  2. Other molecules - transcription factors - required for this process
28
Q

Protein Synthesis

Transcription

Termination

A
  1. Transcription terminates when the RNA polymerase recognizes particular sequence on the new transcript and releases from the transcribing complex
  2. For example the GC-rich hairpin loop in prokaryotes
29
Q

Protein Synthesis

Transcription

Post-translational Modification

A
  • In eukaryotes, transcription produces only an intermediate RNA molecule - hnRNA
  • Introns must be removed and exons spliced together
  • 5’ cap and poly A tail added
  • Once these modifications are complete, the mRNA is ready to move out of the nucleus and into the cytoplasm
30
Q

Protein Synthesis

Translation

tRNA

A
  • Brings AAs to the ribosome in the correct sequence for polypeptide synthesis
  • Has one end containing a three nucleotide sequence - anticodon - and another end which is the site of AA attachment
  • Each AA has own aminoacyl tRNA synthetase, an enzyme with an active site that binds to both the AA and its corresponding tRNA, catalysing their attachment to form an aminoacly-tRNA complex
31
Q

Protein Synthesis

Translation

Ribosomes

A
  • Composed of two subunits each consisting of proteins and rRNA - one large, one small - that bind together only during initiation of protein synthesis
  • Have three binding sites - one for mRNA, two for tRNA
  • The latter are the P site (peptidyl-tRNA binding site) and the A site (amino-acyl tRNA complex binding site)
  • P site binds tRNA attached to the growing polypeptide chain, A site binds to the incoming amino-acyl tRNA complex
32
Q

Protein Synthesis

Translation

Polypeptide Synthesis

Initiation

A
  1. Ribosome binds to mRNA near its 5’ end
  2. Ribosome ‘scans’ the mRNA until binds to start codon (AUG)
  3. Initiator aminoacyl tRNA complex, methionine tRNA (with the anticodon 3’-UAC-5’) base pairs with the start codon
33
Q

Protein Synthesis

Translation

Polypeptide Synthesis

Elongation

A
  1. Hydrogen bonds form between the mRNA codon in the A site and its complementary anticodon on the incoming aminoacyl-tRNA complex.
  2. A peptide bond forms between the AA attached to the tRNA in the A site and the methionine attached to the tRNA in the P site
  3. Following this, ribosome carries uncharges tRNA (AA no longer attached) in the P site and peptidyl-tRNA in the A site
  4. Translocation - ribosome advances three nucleotides along the mRNA in the 5’ to 3’ direction
  5. The uncharged tRNA from the P site is expelled and the peptidyl-tRNA from the A site moves into the P site
  6. The ribosome now comtains an empty A site ready for entry of the aminoacyl-tRNA corresponding to the next codon
34
Q

Protein Synthesis

Translation

Polypeptide Synthesis

Termination

A
  • Terminates when one of three stop codons - UAA, UAG, UGA - arrives in A ste
  • Signals the ribosomes to terminate translation - don’t code for AAs
35
Q

Protein Synthesis

Translation

Ribosomes

Polyribosomes

A

Frequently many ribosomes simultaneously translate a single mRNA molecule forming complexes called polyribosomes

36
Q

Cytoplasmic Inheritance

A
  • Heredity systems exist outside the nucleus - for example, DNA is found in chloroplasts, mitochondria and other cytoplasmic bodies
  • May interact with nuclear DNA and are important in determining the characteristics of their organelles
37
Q

Bacterial Genetics

Bacterial Genome

A
  • Consists of single circular chromosome located in the nucleotid region of the cell (no membrane-bound nucleus)
  • Many bacteria also contain plasmids, containing accessory genes
38
Q

Bacterial Genetics

Episome

A

Plasmid capable of integrating into the genome

39
Q

Bacterial Genetics

Replication

A

Begins at unique origin of replication (ORI) and proceeds in both directions simultaneously

DNA synthesised in the 5’ to 3’ direction as in eukaryotes

40
Q

Bacterial Genetics

Genetic Variance

A

Although binary fission is an assexual process, bacteria have three mechanisms for increasing genetic variance:

  1. Transformation
  2. Transduction
  3. Conjugation
41
Q

Bacterial Genetics

Genetic Variance

Transformation

A

Foreign plasmid incorporated into bacterial chromosome via recombination to create new, heritable genetic combinations

42
Q

Bacterial Genetics

Genetic Variance

Transduction

A

Fragments of the bacterial genome packaged into viral progeny amde during viral infection. Virions may infect other bacteria and introduce new genetic rearrangements via recombination with host cell DNA. Closer two genes are to one another on a chromosome, the more likely will be transduced together

43
Q

Bacterial Genetics

Genetic Variance

Conjugation

A
  • Cytoplasmic conjugation bridge forms between two bacterial cells and genetic material transferred from the donor (male, +) to recipient (female, -) type
  • Only bacteria containing plasmids called sex factors are capable of conjugating
  • Best studied sex factor is F factor in E coli. Bacteria possessing this are called F+ cells, without, F- cells. during conjugation between a F+ and F- cell, the F+ replicates its F factor and donates to F-
  • Sometimes the sex factor becomes integrated into the bacterial genome. During conjugation, the entire bacterial genome replicates and begins to move from the donor cell into the recipient. Cytoplasmic bridge usually breaks before entire chromosome transferred but the bacterial genes that enter can recombine with the genes present to form novel genetic combinations
44
Q

Bacterial Genetics

Hfr Cell

A
  • Bacterial cell with the sex factor integrated into the genome
  • Means have a high frequency of recombination
45
Q

Bacterial Genetics

Gene Regulation

A
  • Regulation of gene transcription in bacteria is based on accessibility of RNA polymerase to the genes transcribed
  • Regulation directed by an operon, consisting of structural genes, an operator gene and a promoter gene
  • Is also a regulator gene which codes for the synthesis of a repressor molecule which binds to the operator and blocks RNA polymerase transcribing structural genes
46
Q

Bacterial Genetics

Gene Regulation

Structrual Genes

A

Contain DNA sequences that code for proteins

47
Q

Bacterial Genetics

Gene Regulation

Operator Gene

A

Sequence of non-transcribable DNA that is the repressor binding site

48
Q

Bacterial Genetics

Gene Regulation

Promoter Gene

A

Noncoding sequence of DNA that serves as initial binding site for RNA polymerase

49
Q

Bacterial Genetics

Gene Regulation

Inducible Systems

A

Those that require the presence of a substance called an inducer for transcription to occur

Binds to repressor, preventing it from binding to the operator, permitting transcription

Most proteins regulated in this manner are enzymes and the inducer the substrate for the enzyme, or its derivative

50
Q

Bacterial Genetics

Gene Regulation

Repressible Systems

A

In a constant state of transcription unless a corepressor is present to inhibit transcription

Corepressors are often the end products of the biosynthetic pathways they control

51
Q

Bacterial Genetics

Gene Regulation

Constitutive Operons

A

Operations in repressible systems containing mutations or whose regulator genes code for defective repressors which are incapable of being turned off

52
Q

Bacteriophage

A

Virus that infects its host bacterium by attaching to it, boring a hole through the bacterial cell wall and injecting its DNA while its protein coat remains attached to the cell wall

Once inside its host, enters either a lytic cycle or lysogenic cycle

53
Q

Bacteriophage

Lytic Cycle

A
  • Phage DNA takes control of bacterium’s genetic machinery and manufactures numerous progeny
  • Bacterial cell then lyses releasing new virions
  • If the intial infection takes place on a bacterial lawn (plated culture), shortly a plaque (clearing) in the lawn seen corresponding to the area of lysed bacteria
54
Q

Bacteriophage

Virulent

A

Bacteriophage that replicate by the lytic cycle, killing its host cells

55
Q

Bacteriophage

Lysogenic Cycle

A
  • If the bacteriophage doesn’t lyse ots host cell it becomes integrated into the bacterial genome in a harmless form - prophage
  • Lies dormant for one or more generations
  • May remain integrated indefinately replicating along with the bacterial genome, however, either spontaneously or as result of environmental circumstances (eg radiation, UV light, chemicals) can reemerge and enter a lytic cycle
  • Bacteria continaing phages are resistant to further infection (superinfection) by similar phages
56
Q

Southern Blotting

A
  • Allows detection of a particular DNA sequence in a DNA sample
  • DNA cleaved into fragments by specific restriction enzymes and fragments separated by electrophoresis and transferred to an inert membrane using blotting techniques
  • Desired sequence is detected by exposing the blot to radioactively labelled probe
57
Q

Northern Blotting

A
  • Utilises similar techniques to Southern blotting
  • Used for detection of RNA sequences
58
Q

Western Blotting

A
  • Similar technique to Southern blotting
  • Used for detection of specific proteins
59
Q

DNA Amplification

A
  1. PCR - denaturation, primer annealing primer extension
  2. DNA cloning - DNA sequence of interest is joined to vectors via DNA ligase. Can then be inserted into bacterial strain via transformation
60
Q

Analysis of DNA

Chain Termination Method

A
  • Method of DNA sequencing
  • Uses ddNTPs modified to prevent addition of further nucleotides
  • Separated electrophoresis
61
Q

Analysis of DNA

Restriction Fragment Length Polymorphisms

A
  • Restrictions enzymes cut DNA at specific sequences
  • Variation in fragment length will result when is a difference in that specific sequence within an individual

DAT (53 decks)

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