Mastering Genetics Ch 5-7

Law of Independent Assortment

the allele you inherit from one gene does not influence the allele you inherit from another gene

Linked Genes

Genes that are close to eachother on the same chromosome

do NOT follow law of independent assortment

Complete Linkage

when genes are so close on a chromosome that they’re never separated by crossing over; when only parental gametes are produced

Linkage Group

genes on the same chromosome

Number of linkage groups = haploid number of chromosomes in an organism

What is the "recombination" mechanism in independent assortment?

Random segregation of chromosomes during metaphase I, where homologous chromosomes align with a 50% probability for each possible orientation

What is the “recombination mechanism in Complete Linkage?

Trick Question! There is no recombination in complete linkage.

True or false: Independent assortment and Linkage are BOTH forms of recombination

True! Through random segregation and crossing over, new allele combinations can be formed

True or false: The recombination frequency between linked genes is always >50%

False! Its always <50%. Parentals make up te majority

Interference

degree to which one crossover interferes with crossovers near it

→ means smth's amiss in the DNA/structure of chromatin that’s interfering w/ recomb. freq 

could be reason 2 smaller values on a genetic map don’t add up to larger one

On a genetic map, genes can appear closer together than they physically are, due to a ___________ between them

low recombination frequency

On a genetic map, genes can appear farther apart than they physically are, due to a _______________ between them

high recombination frequency

Remember when doing genetic mapping

• Recombinants are the lower observed counts

• Genes that are closer together are less likely to have recombinants

• centimorgans (cM) or map units

• Check your math before assuming interference!!!!

To Remember… Prokaryotes have a ______ chromosome. Because they lack compartmentalization, ______ and _______ occur simultaneously.

circular

transcription and translation

Define Horizontal Gene Transfer and give 3 examples

transfer of genetic material in bacteria

1. Conjugation - physical contact

2. Transduction - viruses

3. Transformation - environment

Auxotrophs

can’t synthesize a needed nutrient (can’t survive on minimal medium)

Prototrophs

can synthesize all their nutrients

Genetic exchange occurs thru infectious transfer of a ______ thru a ____.

f factor, pilus

What is an f factor?

conjugative fertility plasmid - codes for things like pilus

What’s an R factor?

resistance plasmid; contains genes conferring resistance to antibiotics

What are degradative plasmids?

carry genes allowing digestion of unusual substances

What are Col-plasmids?

contain genes that kill other bacteria

What are virulence plasmids?

Carry genes that turn bacteria into pathogenic strains

List the steps of conjugation

1. Relaxosome make cut @ origin of transfer and starts separating the DNA strands 

2. Proteins of relaxsosome are released & DNA/relaxase complex is recognized by the coupling factor (protein) & transferred to exporter (part of pilus pump on F+ donor cell side) 

3. Exporter pumps DNA/relaxase complex into F- recipient cell 

4. In donor cell, F-Factor DNA is replicated t become double stranded. 

   In recipient cell, relaxase joins ends of single-stranded DNA & its replicated to become double stranded. 

   Now the recipient has a full plasmid (after repliating) that it can donate to another bacterium 

What is Hfr?

Strains of bacteria in which there’s a high frequency of recombination to to great homology bt Hfr Strain and F-

Why doesn’t the F- recipient cell become a donor cell in Hfr?

recombination has very short window of time to happen before exonuclease chops away exogenote

AND bc nick is inside F-Factor, only part of the plasmid (which is NEEDED for further conjugation) is transferred

True or False: Double Crossover is required for all forms of conjugation.

False! Double Crossover is required for Hfr, because Hfr relies on homology bt exogenote and endogenote & crossing over to incorporate exogenote genes into F- chromosome.

→ w/o 2 crossovers, the bacterial chromosome would be linearized (unstable! → will be degraded)

BUT, not required for F+ conjugation bc just the F plasmid is donated (not incorporated into F- chromosome)

F’ Factors

Excise F plasmids from Hfr chromosomes

How might a passenger gene give selective advantage to a new host?

1. Intgration of F-lasmid to make Hfr 

2. Excision of F-plasmid from Hfr chromosome --> now F-plasmid has genetic info from og host 

3. F' factor contains all genes needed for conjugation --> promotes transfer to another bacterium 

Define and Describe the Structure of R Plasmids

F plasmids with antibiotic resistance genes

Made of: RTF (resistance transfer factor - conjugative plasmid & transfer gene) and R determinant (resistance gene, part of a transposon usually)

Transposon

can excise from palsmids and integrate into plasmids, taking genetic material with them (including drug resistance genes)

True or False: Plasmids w/ genes for antibiotic resistance usually decrease bacterial viability.

True, technically. Seems counterintuitive, but antibiotic resistance is only an advantage in the presence of an antibiotic. @ other times , its drains energy to express it

Transformation

when cell takes up isolated pieces of DNA from enviro - requires homology

Virulent Phage

bacteriophage that lyse and kill bacterial host upon infection

Temperate Phage

bacteriophage that remain w/ host bacteria before killing it 

Generalized Transduction

phage erroneously packages bacterial DNA fragment (instead of its own) and passes it on to another bacteria

Specialized Transduction

phage DNA integrates into host bacteria, then becomes excised and takes some genome w/ it --> passes on to another bacteria

What are the steps of the Phage Lytic Cycle?

1. Adsorption of free phage to host cell 

2. Entry of phage nucleic acid 

3. Phage proteins synthesized & genetic material replicated 

   • Host chromosome degraded after 

4. More Phages synthesized using cell machinery

5. Phages lyse cell (rupture its cell wall & disintegrate it) 

Homogametic Sex

produces like chromosomes

Heterogametic Sex

produces unlike hromosomes

In the case the heterogametic sex is female, we say…

ZW - female

ZZ - male

Nondisjunction

when 1+ pairs of homologous chromosomes or sister chromatids fail to separate normally during nuclear division → an abnormal distribution of chromosomes  

Examples of Nondisjunction

Klinefelter Syndrome —- 47, XXY

Turner Syndrome —- 45, X

Intersexuality

when a person has variations in their sex characteristics (like chromosomes, gonads, genitals, or hormones) that do not fit the typical definitions of male or female 

Structure of a Y chromosome (3 structures & 1 description)

• small (~50 genes vs X’s ~100)

• pseudoautosomal regions for sharing homology & crossing over w/ X

• SRY sex determining region, contains TDF

  • w/o it, individuals develop female phenotypes

• MSY male specific region, doesn’t combine, important for man things idk

TDF

testis determining factor - moves to and binds to DNA to help promote expression of multiple different genes (mult. dif protein pathways) @ a specific sequence 

Heterochromatin

region of tightly wound DNA

Euchromatin

region w/ space for DNA to dynamically wind and unwind

What non-sex-linked genes TDF (on SRY) trigger? Hints in pic.

• SOX9 = transcription factor important for cell differentiation, maintains its own feedback loop

• FGF9 = activated by SOX9, facilitates cell migration to gonad & maintains proper levels of SOX9

• AMH = causes regression of Müllerian ducts (would develop into female reproductive system)

Barr Body

highly condensed X chromosome - evidence for dosage compensation & X inactivation

Dosage Compensation

process by which organisms equalize gene expression from X chromosome between males (XY) and females (XX)

How does X-silencing work?

X-silencing is random (either mom's or dad's x can be silenced) and happens early in development (like @ blastocyst stage)  

@ molecular level: 

1. Inactive X chosen randomly 

2. Xic (=x-inactivation center in q arm) is transcribed into XIST

   • Xist = long non coding RNA (no protein), interacts w/ chromatin 

   • Unstable when not bound (bc RNA) but can't bind unless there' RNA/protein near it 

     • Can't really move 

   • Recruits chromatin remodeling protein complexes to facilitate chromatin condensation (condensing DNA so RNA polymerase can't access it) 

3. Tsix – negative regulator, represses expression of Xist 

What does XIST stand for?

X-inactive specific transcript