SNS - Biology - Genetics Flashcards
Monohybrid Cross
Hallmarks
- F1 generation: 100% Xx
- F2 generation: 25% XX, 50% Xx, 25% xx
- Disappearance of recessive phenotype in the F1 generation
- Reappearance of recessive phenotype in 25% of the F2 generation
Test Cross
- 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
Dihybrid Cross
Hallmarks
- F1 generation - 100% TtPp
- F2 generation has 9:3:3:1 ratio dominant/dominant, dominant/recessive, recessive/dominant, recessive/recessive
Non Mendalian Inheritance
- Incomplete dominance
- Codominance
Non-Mendalian Inheritance
Incomplete Dominance
- 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
Non-Mendalian Inheritance
Codominance
- 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
Number:
- Autosomes
- Sex chromosomes
- 22 pairs
- 1 pair
Sex Linkage
- 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
Environmental Factors
- 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
Genetic Problems
Nondisjunction
- 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
Genetic Problems
Chromosomal Breakage
- 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
Genetic Problems
Mutations
- 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
Genetic Problems
Mutations
Mutagenic Agents
- For example, cosmic rays, X-rays, radioactivity, chemical compounds
- Generally also carcinogenic
Genetic Problems
Mutations
Mutation Types
- 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
Genetic Problems
Mutations
Genetic Disorders
Phenylketonuria
- Caused by inability to produce the enzyme for the metabolism of the AA phenylalanine
- Degradation product - phenylpyruvic acid - accumulates with deleterious consequences
Genetic Problems
Mutations
Genetic Disorders
Sickle-Cell Anaemia
- 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
DNA
Structure
Basic structural unit is the nucleotide, composed of:
- Deoxyribose
- Phosphate group
- Nitrogenous base
G is triple bonded to C, A double bonded to T
DNA
Structure
Purines
- Adenine
- Guanine
DNA
Structure
Pyrimidines
- Cytosine
- Thymine
DNA Replication
- DNA unwound by helicase and gyrase
- RNA primer added to the 5’ end of the new strand
- 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
- Okazaki fragments of the lagging strang are joined by DNA ligase
Genetic Code
Redundancy
- 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
RNA
Structure
Polynucleotide similar to DNA except sugar is ribose and contains uracil instead of thymine
Found in both the nucleus and the cytoplasm
RNA
mRNA
- 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
RNA
tRNA
- 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
RNA
rRNA
- Structural component of ribosomes
- Most abundant of all RNA types
- Synthesised in the nucleolus
Protein Synthesis
Transcription
Initiation
- RNA polymerase recognises the promoter region on the sense DNA strand - most commonly called the TATA box sequence
- DNA helix is unwound by helicase
Protein Synthesis
Transcription
Elongation
- 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
- Other molecules - transcription factors - required for this process
Protein Synthesis
Transcription
Termination
- Transcription terminates when the RNA polymerase recognizes particular sequence on the new transcript and releases from the transcribing complex
- For example the GC-rich hairpin loop in prokaryotes
Protein Synthesis
Transcription
Post-translational Modification
- 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
Protein Synthesis
Translation
tRNA
- 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
Protein Synthesis
Translation
Ribosomes
- 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
Protein Synthesis
Translation
Polypeptide Synthesis
Initiation
- Ribosome binds to mRNA near its 5’ end
- Ribosome ‘scans’ the mRNA until binds to start codon (AUG)
- Initiator aminoacyl tRNA complex, methionine tRNA (with the anticodon 3’-UAC-5’) base pairs with the start codon
Protein Synthesis
Translation
Polypeptide Synthesis
Elongation
- Hydrogen bonds form between the mRNA codon in the A site and its complementary anticodon on the incoming aminoacyl-tRNA complex.
- 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
- Following this, ribosome carries uncharges tRNA (AA no longer attached) in the P site and peptidyl-tRNA in the A site
- Translocation - ribosome advances three nucleotides along the mRNA in the 5’ to 3’ direction
- The uncharged tRNA from the P site is expelled and the peptidyl-tRNA from the A site moves into the P site
- The ribosome now comtains an empty A site ready for entry of the aminoacyl-tRNA corresponding to the next codon
Protein Synthesis
Translation
Polypeptide Synthesis
Termination
- 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
Protein Synthesis
Translation
Ribosomes
Polyribosomes
Frequently many ribosomes simultaneously translate a single mRNA molecule forming complexes called polyribosomes
Cytoplasmic Inheritance
- 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
Bacterial Genetics
Bacterial Genome
- 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
Bacterial Genetics
Episome
Plasmid capable of integrating into the genome
Bacterial Genetics
Replication
Begins at unique origin of replication (ORI) and proceeds in both directions simultaneously
DNA synthesised in the 5’ to 3’ direction as in eukaryotes
Bacterial Genetics
Genetic Variance
Although binary fission is an assexual process, bacteria have three mechanisms for increasing genetic variance:
- Transformation
- Transduction
- Conjugation
Bacterial Genetics
Genetic Variance
Transformation
Foreign plasmid incorporated into bacterial chromosome via recombination to create new, heritable genetic combinations
Bacterial Genetics
Genetic Variance
Transduction
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
Bacterial Genetics
Genetic Variance
Conjugation
- 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
Bacterial Genetics
Hfr Cell
- Bacterial cell with the sex factor integrated into the genome
- Means have a high frequency of recombination
Bacterial Genetics
Gene Regulation
- 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
Bacterial Genetics
Gene Regulation
Structrual Genes
Contain DNA sequences that code for proteins
Bacterial Genetics
Gene Regulation
Operator Gene
Sequence of non-transcribable DNA that is the repressor binding site
Bacterial Genetics
Gene Regulation
Promoter Gene
Noncoding sequence of DNA that serves as initial binding site for RNA polymerase
Bacterial Genetics
Gene Regulation
Inducible Systems
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
Bacterial Genetics
Gene Regulation
Repressible Systems
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
Bacterial Genetics
Gene Regulation
Constitutive Operons
Operations in repressible systems containing mutations or whose regulator genes code for defective repressors which are incapable of being turned off
Bacteriophage
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
Bacteriophage
Lytic Cycle
- 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
Bacteriophage
Virulent
Bacteriophage that replicate by the lytic cycle, killing its host cells
Bacteriophage
Lysogenic Cycle
- 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
Southern Blotting
- 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
Northern Blotting
- Utilises similar techniques to Southern blotting
- Used for detection of RNA sequences
Western Blotting
- Similar technique to Southern blotting
- Used for detection of specific proteins
DNA Amplification
- PCR - denaturation, primer annealing primer extension
- DNA cloning - DNA sequence of interest is joined to vectors via DNA ligase. Can then be inserted into bacterial strain via transformation
Analysis of DNA
Chain Termination Method
- Method of DNA sequencing
- Uses ddNTPs modified to prevent addition of further nucleotides
- Separated electrophoresis
Analysis of DNA
Restriction Fragment Length Polymorphisms
- Restrictions enzymes cut DNA at specific sequences
- Variation in fragment length will result when is a difference in that specific sequence within an individual
