Genetics, Meiosis, and Biotechnology (Review #4) Flashcards
Meiosis Overview
-Produces 4, haploid (n = one copy of each chromosome) cells from 1, diploid (2n) cell
-Haploid cells are gametes (sperm/ ova) – come together at fertilization to create a zygote
-PRIOR to the beginning of meiosis, DNA replication occurs (copied chromosomes = sister chromatids - attach at centromeres)
What is Meiosis I?
genetic recombination and reduction division
What is Meiosis II?
resembles mitosis – separates sister chromatids
Meiosis I: Prophase I
-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)
Meiosis I: Metaphase I
Random orientation: homologous chromosomes line up randomly along middle of cell (2n possible orientations)
Meiosis I: Anaphase I
Spindle fibers pull homologous chromosomes to opposite ends of the cell (independent
assortment – genes on different chromosomes separate independently of each other)
Meiosis I: Telophase I
Reduction division (cytoplasm divides – each new cell now haploid)
Meiosis II: Prophase II
New meiotic spindle forms (eggs in females arrested in this stage)
Meiosis II: Metaphase II
Chromosomes (made of sister chromatids) line up along middle of cell
Meiosis II: Anaphase II
Centromeres break, sister chromatids separate, one copy of each pulled to opposite ends of cell
Meiosis II: Telophase II
Cytoplasm divides: 4 haploid cells that are genetically UNIQUE
Infinite genetic variation in meiosis
Crossing over in prophase I and random orientation in metaphase I (random fertilization with another individual too – takes into account THEIR crossing over and random orientation!)
Non-Disjunction Overview
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 Diagnosis
-Fetal cells obtained from amniotic fluid (amniocentesis) or
chorionic villus (placenta)
-Chromosomes arranged in pairs according to size/ structure
-23rd pair used to diagnose gender (XX = female, XY = male)
Genetics
science of heredity (passing on of genetic material from parent to offspring)
Genetics of Prokaryotes
circular, naked (no proteins) chromosome – passed directly to offspring (asexual reproduction)
Genetics in Eukaryotes
linear, with proteins (histones) – many pairs, passed to offspring through sexual reproduction
-Pairs #1-22 in humans = autosomes, Pair #23 in humans = sex chromosomes (determine gender)
More DNA does not mean……
more advanced than another organism (# of genes/ chromosomes/ size of genome is unique to each species)
Genome
The complete set of all DNA base sequences of an organism
Genes
-carried on chromosomes
-heritable factors (DNA) that determine specific traits (code for proteins)
-located on specific places of chromosomes (locus) and come in different forms (alleles)
Blood Type A (Genotype)
IA
Blood Type B (Genotype)
IB
Blood Type AB (Genotype)
IAIB
Blood Type O (Genotype)
ii
Codominant alleles and an Example
Blood types, (equal in strength – both SHOW if present)
Some traits on the _____ __ ______ are absent from the ______ ___ ________
larger X chromosome, smaller Y chromosome (Sex-linked)
Why are sex-linked traits more commonly seen in males?
Only one X chromosome (whatever inherit will show – no other chromosome to overpower etc.)
Examples of Sex-Linked Traits
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 _____ can be carriers of sex-linked traits
females
Polygenic
-Some genes have multiple alleles
-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.
-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
Mutations
Changes in genetic material (DNA): rare
Base Substitution
-One base in DNA changed; causes wrong codon in mRNA; causes wrong amino acid in translation)
-Example: Sickle cell anemia (in DNA: GAG changed to GTG; in mRNA: wrong codon
codes for valine instead of glutamic acid, in polypeptide chain): Hemoglobin misshapen (sickle shaped)
-cannot carry oxygen as well
-Provides SOME advantage (malaria resistance with sickle cell) – only beneficial if you LIVE in an environment where malaria is present though.
-The ENVIRONMENT determines if the allele is good or bad
Heterozygous Dihybrid Cross Phenotypic Ratio
9:3:3:1
Dihybrid Cross
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)
Linked genes
-Thomas Hunt Morgan and fruit flies
-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)
-MOST offspring will have grey bodies with long wings (like mom) OR black bodies with short wings (like dad)
-IF have grey body with short wings OR black body with long wings, crossing over occurred (these are recombinants = different phenotypes from mom and dad)
DNA Profiling and Gel Electrophoresis
- Amplify (copy) DNA samples using PCR then cut with restriction enzymes/ endonucleases
- Run samples through gel electrophoresis and analyze banding patterns (shows fragment lengths/ SIZES)
-Can also add probes (fluorescently labeled specific, complementary DNA sequences) to gels to identify genes of interest/ alleles that cause disease (sickle cell etc.)
Gel Electrophoresis Overview
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)
To make multiple copies of a gene:
- PCR (polymerase chain reaction) – makes MANY copies of SMALL amount of DNA (“amplifies” it) using a thermocycler
- Gene cloning (clone = genetically identical copy) using recombinant DNA
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 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 GMOs
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 GMOs
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?)
Cloning
genetically identical to original – does happen naturally
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?
Human Genome Project
-ALL base sequences (including mutations) sequenced for humans (as a species) – Sanger technique with dideoxynucleotides (ddNT’s)/ computers
–Mapping outcome: know number, location, and basic sequence of all human genes
–Screening outcome: Specific gene probes to detect genetic disorders/ carriers
–Medical outcome: Specific genes targeted to produce specific proteins for those who cannot
–Ancestry outcome: Improved insight into human origins/ evolution/ migration patterns etc.
