Bio 201 Exam 4

Aneuploidy- organism gains or loses a chromosome or two

Mosomy: loss of a single chromosome

Trisomy: gain of the single chromosome from a diploid genome

Euploidy: complete haploid sets of chromosomes are present

Polyploidy: more than two sets of chromosomes are present

Down syndrome critical region contains dosage-sensitive genes.

Duplications = more

deletion= get rid of

Inversion - a segment of a chromosome is reversed end to end

Translocations - location of genes altered within the genome

Structural changes (deletions, duplications, inversions, and translocations)

  • Due to one or more breaks along the chromosomal axis, followed by loss or rearrangement of genetic material.
  • Breakages occur spontaneously
  • Exposure to chemicals or radiation can cause breakage

Alterations in gametes are heritable.

2 chromosomes are needed for translocations.

Deletion (deficiency)

  • Missing region of a chromosome
  • A portion of chromosome lost due to breaks
  • Deletion can occur near one end (terminal deletion) or from the interior of chromosome
  • A portion of the chromosome that repairs the centromere is maintained when a cell divides

Chri du chat

  • An eerie cry similar to a cat’s meow
  • Intellectual disability
  • Delayed movement
  • Small head size

Duplication

  • Repeated segment of chromosome
  • Pairing in heterozygotes produces a compensation loop
  • Arise through unequal crossing over between synapse chromosomes during meiosis
  • Arise through replication error before meiosis

Three aspects of duplications

  • Gene redundancy
  • Phenotype variation
  • Source of genetic variability during evolution

Ribosomal RNA rRNA

  • Multiple copies of genes that encode for rRNA
  • Single copy not sufficient
  • DNA that codes for rRNA is called rDNA
  • Gene products essential in abundance in a cell to support protein synthesis

Inversions: class of structural variation

  • Type of chromosomal aberration
  • Rearrangement of linear gene sequence
  • Segment of chromosome turned 180 within chromosome
  • No loss of genetic information

Inversions:

  • Require two breaks
  • Chromosomal loop forms before breakage
  • The inverted segment may not include the centromere
    • Paracentric inversion does not include centromere
    • Pericentric inversion includes the centromere

Linear synapse is not possible if only one member of the homologous pair has an inverted segment

Inversion heterozygotes

  • Organisms with one inverted chromosome

Translocation = pieces moving to a new location on a different chromosome or the other end of the same chromosome

Reciprocal

  • Exchange of segments (swap) between two nonhomologous chromosomes
  • Genetic information is not lost or gained just rearranged
  • Does not directly alter the viability of individual

Fragile sites

  • In studies, observations of metaphase chromosomes showed unstained regions–gaps. Susceptible to chromosome breakage when cells are cultured in the absence of certain chemicals such as folic acid

Wide range of reproductive modes and life cycles in the biological world:

  • Some organisms are entirely asexual
  • Some alternate between short periods of sexual reproduction and prolonged periods of asexual reproduction

There are several different mechanisms of sex determination. The X and Y chromosomes pair during meiosis, even though they are not homologous.

Hetermorphic chromosomes characterize one sex or the other in a wide range of species:

  • Labeled as sex chromosomes
  • Genes not chromosomes ultimately serve as the basis for sex determination

Fertilization by X-bearing sperm -> female offspring

Fertilization by X-deficient sperm -> male offspring

Protenor and Lygaeus insects:

  • Males produce unlike gametes (heterogametic sex). Their gametes determine the sex of progeny
  • Females produce like gametes (homogametic sex) - uniform gametes
  • In some organisms, females produce unlike gametes:
    • Protein or xx/xo or leagues xx/xy mode of sex determination

Monoecious: both male and female reproductive structures in the same organism

  • Hermaphroditism: both sexes in the same organism (most plants and some animals)

dioecious: either male or female reproductive structures in one organism

Y chromosome has far fewer genes than X

Pairing region critical for segregation of X and Y chromosomes during male gametogenesis.

SRY gene encodes protein TDF: present in all mammals - triggers testes formation

  • Causes undifferentiated gonadal tissue of embryo to form testes

Sex determining region Y: SRY

  • A critical gene controlling sexual development
  • Gene becomes active in XY embryos at 6-8 weeks of development

TDF is the master switch, which creates a biological male

Females don’t need a second X chromosome, just 1.

Lyon hypothesis: inactivation of the X chromosome is random; all descendant cells have the same inactivation

Some species determine gender from temperature.

  1. Low temp = female, high = male
  2. Low temp = male, high = female
  3. Low and high yield 100% females, middle temperatures give males

Aromatase is affected by temperature, which converts hormones.

Four major characteristics are needed for a molecule to serve as genetic material

  • Replicate
    • Cell cycle
  • Storage of information
    • The molecule acts as a repository of genetic information
  • Expression of information
    • Information flows within the cell
  • Allow variation by mutation
    • Change in chemical composition

The central dogma of molecular genetics is made up of two processes

  • Transcription: synthesis of RNA from information in DNA
    • Messenger RNA
    • Ribosomal RNA
    • Transfer RNA
  • Translation
    • Uses info in mRNA to synthesize proteins

Genetic material is the source of variation among organisms through the process of mutation

  • Mutation in DNA reflected in transcription and translation affecting specific protein
  • Mutation present in gametes passed to future generations
  • Genetic variation provides raw material for the process of evolution

Genetic material is physically transmitted from parent to offspring

  • Proteins and nucleic acids are major candidates for genetic material
  • Many geneticists favored proteins due to diversity and abundance in cells

1868 Miescher isolated cell nuclei

  • Derived acid substance containing DNA-nuclein
  • Found to be present in chromosomes
  • Thought to lack chemical diversity needed for genetic information

1910 Levene proposed the tetranucleotide hypothesis

  • DNA contains equal amounts of four nucleotides
    • 1940 chargeoff showed this was incorrect
  • Postulated identical groups and repeats of four components were the basis for the DNA structure
  • The lack of chemical diversity in DNA suggested it could not store extensive genetic information
  • Proteins are favored as genetic material

Avery, MacLeod, and McCarty

  • 1944 publication on the chemical nature of transforming principle in bacteria
  • Reported obtaining transforming principle
  • The demonstrated transforming principle was DNA, not protein

In 1927 Griffith provided the foundation for Avery, MacLeod, and McCarty’s research

  • Studies of different stains of diplococcus pneumonia
  • Some were virulent strains – that caused pneumonia
  • Some were avirulent– and did not cause pneumonia

Hershey and Chase

  • Used viruses and bacteria
  • Demonstrated that DNA, not protein, is the genetic material
  • Used radioisotopes
  • Demonstrated that DNA enters bacterial cells during infection and directs viral reproduction

Indirect evidence: distribution of DNA

  • Protein is abundant in the cytoplasm; DNA is Not.
  • Mitochondria and chloroplast perform genetic functions; DNA is present in these organelles
  • DNA is only found where primary genetic function is known to occur
  • Protein is found everywhere in the cell

Indirect evidence: mutagenesis

  • Uv light: most mutagenic at wavelength 260 nm
  • The action spectrum of UV can be compared to the absorption spectrum of molecules
  • DNA and RNA absorb UV at 260
  • Protein absorbs UV at 280 nm

Recombinant DNA technology

  • Splicing together DNA sequences from different organisms
  • Example: Human Hormone Insulin
    • Bacterial DNA combined with DNA encoding HHl
    • Recombinant DNA molecules introduced into bacteria and replicated

Nucleotides

  • DNA is nucleic acid
  • Nucleotides: building blocks of nucleic acid

Nucleotides consist of:

  • Nitrogenous base, purines and pyrimidines
  • Pentose sugar
  • Phosphate group

Purine (nine-member double ring), A and G

Pyrimidines (six-member double ring), CT and U

Watson and Crick

  • Complementarity of DNA strands
  • Unwound double helix serves as a template as each nucleotide along two parent strands has an affinity for its complementary nucleotide
  • Four bases, AT and CG have an affinity for the complement – each replicated strand would have an old and new strand (semiconservative replication)

Three modes of DNA replication

  • Semiconservative
    • Each replicated DNA molecule consists of one “old” and one new strand
  • Conservative
    • Two newly synthesized strands come together, the original helix is conserved
  • Dispersive
    • Parental strands are dispersed into new double helices

Meselson and Stahl

  • Showed that semiconservative replication was the mode used by bacteria for replication
  • N labeled E.Coli grown in a medium containing N
  • Each new DNA molecule consists of one old and one newly synthesized strand
  • Used sedimentation equilibrium configuration technique to distinguish between DNA

Taylor-Woods Hughes

  • Showed semiconservative replication mode in eukaryotes
  • Used root tips of Vicia faba (broad bean)

DNA replication

  • DNA replication begins at the ori (origin of replication)
  • At site of replication, helix is unwound, creating replication fork
  • Replication is bidirectional; therefore, there are two replication forks
  • Replicon: length of DNA replicated

Bacteria only have one ORI

  • Single region in E.Coli - oriC
  • Replication initiated at oriC
  • E.coli replicon consists of the entire genome (4.6 million base pairs)
  • The entire bacterial chromosome constitutes one replicon (4.6 million base pairs)

23 pairs. 46 chromosomes

DNAA

  • E.Coli oriC studies
    • Initiator protein encoded by dnaA gene
    • Binds to ORI, causing conformation change
    • Causes helix to destabilize and open up
    • Exposes single-stranded DNA (ssDNA)

DNA helicase

  • Made of DnaB polypeptides
  • Hexamer of subunits: assembles around exposed ssDNA
  • Recruits holoenzyme to bind to replication fork and initiate replication
  • Helicases require energy supplied by hydrolysis of ATP– denatures hydrogen bonds and stabilizes double helix

DNA Gyrase

  • Enzyme relieves coiled tension from the unwinding of the helix (DNA supercoiling)
  • Member of larger enzyme group: DNA topoisomerases
  • Gyrase makes single or double-stranded cuts undoing supercoiling knots and twists
  • Driven by energy released during ATP hydrolysis

Holoenzyme: active form of DNA polymerase III

Primase: RNA polymerase

  • Recruited to replication form by helicase
  • Synthesizes rna primer
  • Provides free 3- OH required by DNA Pol III for elongation