REVIEW 4 Flashcards
Prophase 1
Synapsis: homologous chromosomes (similar in shape/ size/ gene arrangement) line up next to each other (bivalents/ tetrads)
Crossing over: Non-identical sister chromatids exchange DNA – cross over at places called chiasmata, chromosomes break in identical locations, pieces exchanged – creates NEW combinations of genes (recombinations)
Metaphase 1
Random orientation: homologous chromosomes line up randomly along middle of cell (2n possible orientations)
Anaphase 1
Spindle fibers pull homologous chromosomes to opposite ends of the cell (independent assortment – genes on different chromosomes separate independently of each other)
Telophase 1
Reduction division (cytoplasm divides – each new cell now haploid)
Prophase 2
New meiotic spindle forms (eggs in females arrested in this stage)
Metaphase 2
Chromosomes (made of sister chromatids) line up along middle of cell
Anaphase 2
Centromeres break, sister chromatids separate, one copy of each pulled to opposite ends of cell
Telophase 2
Cytoplasm divides: 4 haploid cells that are genetically UNIQUE
What happens before meiosis?
PRIOR to the beginning of meiosis, DNA replication occurs (copied chromosomes = sister chromatids - attach at centromeres)
Non-Disjunction
Problem in Meiosis
Failure of sister chromatids to separate (anaphase II)
Cells produced missing a chromosome (monosomy –
ONE copy ONLY when fertilized) or have an extra chromosome
(trisomy – three copies when fertilized)
Non-disjunction is diagnosed using karyograms
Fetal cells obtained from amniotic fluid (amniocentesis) or
chorionic villus (placenta)
Sex-linked traits more commonly seen in males
Hemophilia and red-green color blindness on X-chromosome; Males inherit X from their mothers; IF recessive allele (for hemophilia or color blindness etc.) is on that X, the man WILL have the condition (Xn Y)
ONLY Females can be CARRIERS of sex-linked traits (two X chromosomes so can be heterozygous – a dominant allele on one X and a recessive allele on the other: XN Xn)
Polygenic Traits
Polygenic traits show CONTINUOUS (bell-shaped curve) variation (not discrete variation)
Phenotypes do NOT fit into distinct categories; phenotypes are continuous because SO many alleles influence the expression of the gene: SKIN COLOR (melanin), HEIGHT, HAIR COLOR etc.
The environment can also influence these traits (UV light, diet/ nutrition etc.)
Test cross
If dominant phenotype is showing, do a TEST CROSS – mate with homozygous
recessive – if any of offspring show recessive phenotype, parent is heterozygous,
if all offspring show dominant phenotype, parent is homozygous dominant
Mutation example
Sickle Cell Anemia
GAG changed to GTG
Glutamic Acid changed to valine
Dihybrid cross for unlinked genes
Heterozygous cross shows phenotypic ratio of 9:3:3:1 (TtRr x TtRr) – numbers in ratios are out of 16
To set up punnett square (4x4), NUMBER the alleles in the genotypes (1,2,3,4) and place the following combinations over/ next to each box for each parent (1,3), (1,4), (2,3), (2,4)
Dihybrid cross for linked genes
Linked genes do NOT follow law of independent assortment: inherited together because on SAME chromosome
Do NOT show typical ratios (9:3:3:1 or 1:2:1 etc) – VARY significantly (Chi-square test, comparing observed and expected, shows significant difference between observed and expected phenotype ratios in offspring)
Genotypes written as VERTICAL pairs with TWO horizontal lines in between them
ONLY way for recombination in linked genes is crossing over (prophase I): unlinked genes follow independent assortment to create new combinations (of chromosomes)
MOST offspring will show parental phenotypes because genes inherited TOGETHER on same chromosome (only a small percentage show NEW phenotypes, not present in parents – from crossing over- these are recombinants)
Example: Fly with grey body and long wings (GgLl) crossed with fly with black body and short wings (ggll)
PCR
(polymerase chain reaction) – makes MANY copies of SMALL amount of DNA (“amplifies” it) using a thermocycler
Gene Cloning
Produce recombinant DNA (DNA from two or more different sources/ organisms)
Cut vector (plasmid) and gene of interest with restriction enzyme (endonuclease)
Combine DNA fragments (will base pair at sticky ends)
Add DNA ligase (to seal fragments together)
Insert recombinant DNA back into host (bacteria, yeast, sheep etc)
Able to do because DNA/ genetic code is UNIVERSAL!
Allow cells to reproduce gene (and make protein)
Ex: Insulin for diabetics , Factor IX for hemophiliacs
Gene Transfer
Recombinant DNA made (donor + host) and placed into host organism
Host organism now transgenic (GMO = genetically modified organism): has had an artificial genetic change to its genome
Genes transferred to treat disease (gene therapy), for medical treatments (insulin), and for commercial use (crops/ livestock)
Pros of GMO Crops
Added nutrients (vitamin A/ beta carotene in rice)
Higher yields, longer shelf life
Resistance to herbicides, drought, cold etc.; Reduced need for pesticides (harmful to humans etc.)
Cons of GMO Crops
Introduced genes cause allergies (long term effects on human health unknown)
Introduced genes mutate (outcompete wild populations and/ or spread/ cross species) and reduce genetic variation/ biodiversity (Potato blight/ Bt corn)
Monopolies on food production (small farms out of business?)
Reproductive Cloning
(exact genetic copy of entire organism) Using adult, differentiated cells (Dolly the sheep!)
Take nucleus of differentiated cell and place in egg (remove egg’s nucleus first) – somatic nuclear transfer
“Zap” with electricity (to trick it into thinking it’s fertilized)
Mitotic divisions in “embryo”
Place “embryo” in surrogate mother and allow to develop into baby (CLONE – exact genetic copy)
Therapeutic Cloning
Use embryonic stem cells (undifferentiated) to produce new tissues for transplantation
Arguments for: can be screened for genetic abnormalities; natural process (identical twins); increased chance of offspring (for infertile couples); helps burn victims/ paralysis/ leukemia patients etc.; reduced risk of rejection (genetically identical)
Arguments against: destroys embryo (when does life begin?); higher rates of miscarriage/ developmental disorders; long term health effects unknown; suppression of patient’s immune system risky; human clones?
Gel Electrophoresis
DNA sample (from crime scene, bones, father/ baby) amplified (many copies) using PCR
Cut DNA with restriction enzymes then run through gel using electric current (separates based on SIZE and charge – fragment lengths UNIQUE to each individual due to unique sequences of DNA)
Produces banding pattern in gel (bands represent sizes of fragments – smaller fragments travel faster/ farther)
Used in DNA Profiling (typically use highly repetitive/ satellite DNA because unique to every individual):
1. Forensic Investigations (identifies crime scene suspects/ victims)
2. Paternity testing (half of baby’s bands from mom, other half MUST come from dad)
