Genetic Recombination

Why are we so diverse?

1. Mutation

2. Random Fertilization

3. Recombination

Mechanisms of Genetic Recombination

1. Requires 2 DNA molecules that are similar but non-identical

2. Homology allows DNA on different molecules to line up and recombine precisely

3. Enzymatic cutting and pasting of both DNA backbones from each of 2 DNA molecules required for recombination

Genetic Recombination in Bacteria

1. Genetic recombination occurs in E. coli

2. Bacterial conjugation brings DNA of two cells into close proximity

3. Transformation and transduction provide additional sources of DNA for recombination

Some bacteria genetically reshuffle

• Genes transferred from one individual to another recombine with existing DNA

How did they figure this out?

• Genetically identical clones allow molecular genetics studies

Genetic Recombination in E. coli what are the definitions of Prototrophs and Auxotrophs, along with complete medium and minimal media

Prototrophs - Bacteria grow on minimal media because they make their own amino acids

Auxotrophs - Bacteria with mutations does not grow on minimal medium

• Three letter gene name:

• +normal - mutated allele

  • Ex. arg+ and arg_

Complete medium - full complement of nutrients

Minimal media - missing some nutrients

What is Replica Plating and what does it do

Replica plating - is a technique that is used to:

1. Identify prototrophs versus auxotrophs

2. Identify and count genetic recombinations in bacterial colonies

How is Bacterial recombination by conjugation done

• Bacteria are haploid

• Sex pilus/Pilus connects two bacteria 

• Donor sends DNA via cytoplasmic bridge to recipient

Recipient undergoes recombination

• Plasmids Circular, nonchromosomal transferable DNA

• R plasmids confer resistance to antibiotics

Horizontal Gene Transfer

F-Factor and conjugation what is F factors what does it do

Donor cell must have F Factor (fertility) plasmid

• F+ cells are donors with F Factor

• F- cells are recipients without F Factor

F Factor has genes that encode for sex pilus cytoplasmically connects F+ cell to F- cell

• F- cell converts to F+ cell

Fill in the Blanks

No chromosomal DNA is transferred

No DNA recombination

Hfr Cells and Recombination what do Hfr Cells do and what are they

Hfr (high frequency) cells integrate F factor into bacterial chromosome through recombination

• Hfr cells can conjugate with F- cells

• Recipient becomes  F-

Genetic recombination occurs by double crossing over in recipient

• New generations have new gene combinations

FIll in the Blanks

Yes partial chromosomal DNA transferred

Yes DNA recombination

What happens when Mapping Genes by Conjugation

Mated Hfr and F- cells that differed in number of alleles

At regular intervals after conjugation commenced, remove cells and break apart mating pairs

Cultured separated cells and analyzed for recombiants

Greater time to conjugate before separation the greater number of donor genes into recipient

• Therefore greater number of recombinants

Note order and time at which genes were transferred 

→ able to map and assign relative positions of several genes of E. coli chromosome

Transformation occurs when bacteria take up DNA from disintegrated bacteria what are the steps

• Linear fragments recombine by double crossovers

• Transformation bacteria usually have DNA binding protein in wall

Artificial transformation

• Alters cell membrane for DNA penetration

• Electroporation

Transduction occurs when bacterial phages transfer DNA from one bacteria to another what does the bacteria DNA do to the host cell

Virus incorporates DNA fragments from host cell

• If DNA fragments are homologous

→ bacteria become partial diploid

• Recombination by double crossovers

Horizontal Gene Transfer

FIll in the Blank

Phage with Bacterial DNA

Virulent vs. Temperate Bacteriophage

Virulent Bacteriophage - uses only lytic cycle of infection

→kills/lyses host bacteria

Temperate Bacteriophage - uses both lysogenic and lytic cycle of infection

→ may or may not kill host bacteria

Prophage - bacteriophage integrated into host DNA

Genetic Recombination in Eukaryotes: Meiosis

1. Meiosis occurs in different places in different organismal life cycles

2. Meiosis changes both chromosome number and DNA sequence

3. Meiosis produces four genetically different daughter cells

4. Several mechanisms contribute genetic diversity

What happens in Sexual Reproduction

Sexual reproduction produces offspring by union of male and female gametes (sperm and egg)

• Meiosis produces gametes with half chromosome number

• Evolutionary advantage: Genetic shuffling

What happens in Fetilization

Fertilization fuses nuclei of egg and sperm → Zygote

• Restores parental chromosome number

What happens in Animal Life Cycles

Diploid phase dominates animal life cycles 

• Meiosis followed directly by gamete formation

• Haploid phase is reduced and short, no mitosis

In males, all four nuclei from meiosis form separate sperm cells

In females, only one nucleus becomes an egg

Homologous Chromosome Pairs what are they and where do they come from

Paternal chromosomes from male parent

Maternal chromosomes from female parent

• Homologous chromosome pairs

• Alleles may be different within homologous pairs

What does meiosis do to homologous pairs

Meiosis separates homologous pairs

• Before meiosis - diploid (2n)

• After meiosis - haploid (n)

What happens in meiosis 1

Meiosis l: First meiotic division

• Recombination exchanges segments between homologues

• Produces two haploid cells with chromatids attached

What happens in meiosis 2

Meiosis ll: Second meiotic division

• Sister chromatids separate into separate cells

• Produces 4 recombined haploid cells

Steps in the Meiotic Cell Cycle

Prophase l - Sister chromatids condense chromosomes

Synapsis → pairing of homologs

Tetrads → fully paired homologs

Recombination → mixes alleles across tetrads

What happens in Prometaphase 1

• Nuclear envelope breaks down

• Kinetochores attach to polar spindles

What happens in Metaphase l and Anaphase 1

• Tetrads align on metaphase plate

• Homologs segregate - move to poles (sister chromatids attached)

• Nondisjunction creates abnormal chromosome numbers

What happens in Meiotic Divisions Telophase 1 and Interkinesis

• NO change in chromosomes 

• Spindle disassembles

Meiotic Cell Cycle what happens in Prophase 2 and Prometaphase 2

• Chromosomes condense, spindles form

• Nuclear envelope breaks, kinetochores attach to microtubules

What happens in Metaphase 2

• Chromosomes align on metaphase plate

What happens in Anaphase ll and Telophase ll

• Spindles separate chromatids

• Spindles disassemble

• New nuclear envelopes form

4 genetically different haploid cells

What is nondisjunction and what are the results

Both members of pair of homologous chromosomes connect to spindles from same pole

Following anaphase, one pole then receives both copies of pair and other pole receives none

Result is gametes that have 2 copies of a chromosome

After fertilisation, zygote has 3 copies of chromosome instead of 2

Ex. Trisomy 21

How do Sex Chromosomes relate to Meiosis

Meiosis and sex chromosome inheritance:

• Gametes produced by females may receive either X chromosome

• Gametes produced by males may receive either X or Y chromosome

Compare Meiosis and Mitosis

• Both: Similar cell divisions, meiosis divides twice

• Mitosis: Two identical daughter cells (diploid)

• Meiosis: Four genetically different cells (haploid)

Premeiotic interphases similar to mitotic interphase

• Chromosomes copied into sister chromatids

What are the Four main things for Genetic Variability

1. Genetic recombination

2. Random segregated at anaphase l 

3. Alternative combo at anaphase ll

4. Random Fertilization

What happens in Genetic Recombination

Recombination (crossing-over)

• Key genetic shuffle of prophase l

Tetrads held together at synaptonemal complex

• 2 of 4 chromatids exchange alleles

• Chiasmate or crossovers are points of exchange

What happens in Random Segregation

• Key genetic shuffle of metaphase l
  
  

Each chromosome of a homologous pair may randomly end up at either spindle pole

• Any combo of maternal and paternal chromosomes → segregated to gametes

• 2^x number of possible combinations

Alternative Combo at Anaphase ll

What is Random Fertilization

Random chance of male and female gamete forming zygote

Meiosis allows randomness necessary for Mendelian laws of inheritance