8. Mitosis, Meiosis and Sex Selection

meiosis is a fundamental problem

a paradox for nature

resolution of the paradox thru

sexual reproduction and meiosis

sexual reproduction

1. production of special reproductive cells or gamete (1n)

2. fusion of gametes (fertilization) - zygote 2n

gametes produced in specialized tissues through

reductional (2n → n) cell division = meiosis

three types of sexual life cycles

1. animals

2. plants and some algae

3. most fungi and some protists

after dna duplication

chromatid formation

during mitosis - chromatid segregation

(pseudo 4n to 2n, or half the DNA from G2/after S) through separation of centromeres in anaphase

Meiosis I - chromosome segregation

(reduction of pseudo 4n to pseudo 2n), chromatids are still attached through centromere regions (chromosomes are segregating, no separation of centromeres in anaphase I)

Meiosis II - chromatid segregation

(this part is similar to mitosis: pseudo 2n to n or half the DNA after anaphase) - through separation of centromeres in anaphase II

reduction of chromosome number

• only one round of DNA replication

• but two meioitic divisions

homologous chromosomes

containing duplicated DNA , separate 2n → n

mitosis equational division

maintains the ploidy/ of chromosomes int he cell

mitosis one division per cell cycle

one cytoplasmic division per equational chromosomal division

meiosis the first stage meiosis I is a reductional division that separates homologous chromosomes

sister chromatids separated in an equational division during the second stage meiosis II

meiosis two divisions per cell cycle one cytoplasmic division follows reductional division (from 2n to n)

second cytoplasmic division follows equational chromosomal division

mitosis normally occurs in

almost all somatic cells

mitosis begins at the

zygote stage and continues thorugh the life of organism

daughter cells from meiosis cannot undergo

additional meiotic division although they may undergo subsequent mitotic divisions

meiosis occurs only after higher organisms

have begun to mature (in majority of higher organisms)

number of possible new combinations of chromosomes for a species

2^n where n = haploid number of chromosomes for the species

gametophyte

haploid form of an organism - can grow through mitotic division of 1n spore

gametophytes

cells differntiate and form different tissues

sporophyte

zygote (mitosis and differentiate ) into diploid form

caution in asexual reproduction

exchange of genetic material can occur (partial)

sexual reproductions

1. diploid organisms

2. gametes meiosis

3. fertilization = fusion of 1n gametes

hermaphrodite

both sets of reproductive organs

parthenogenesis

some lizards produce diploid eggs asexually

chromosomal sex determination

1. XY humans

2. organisms with alteration of generations

3. ZW butterflies, birds

4. XO some insects

5. haplodiploidy bees ants

6. environment Reptiles

more than 90% of plants are either

hermaphrodites or monoecious, less than 10% of plants are dioecious

approx 15% of animal species have a

a haplo-diploid sex determining system

female germ cell will go through abt 20-30 cell divisions before it stops dividing

only up to 12 eggs per year complete meiosis

3 mistakes per cell division

95% of human dna is noncoding and non conserved, therefore low probability that any of the 30,000+ proteins will have a mutation

dna protein interactions

1. enzymatic - nucleic acids as substrates

2. structural - change in dna/rna structure

3. regulatory - binding to nucleic acids

when protein binds to nucleic acid

positively charged ions are displaced (energy requirement etc)

amino acids (protein) interacting with

nitrogenous bases (DNA)

nucleic acid interactions

1. ionic

2. hydrogen bonds

3. van der waals

4. hydrophobic interactions

bacterial genomes and plasmids often have only one

replication origin

replicon

entire region of DNA replicated from one origin = a piece of dna which replicates as a single unit

pulse-chase autoradiography experiment

The symmetrical pattern of dark spots extending from ORIs in both directions proves that DNA replication proceeds bidirectionally in eukaryotic cells.

discovery of enzymes involved in DNA replication

This cell-free assay proved that DNA replication could be reconstructed outside the cell using purified components, and helped identify which enzymes were required.

DnaA

initiation

Single-strand binding proteins

protection

Helicase and Primase

DnaB and DnaG, Rna polymerase

Clamp loader

DnaC

sliding clamp

B clamp

Replisome

combination of all the proteins that funcation at the replication fork and undertake the synthesis of DNA

why is replication so complicated

1. dna polymerase cannot break inter-chain hydrogen bonds at the point of origin

2. dna polymerases cannot start chains, only elongate them - needs primer → oligo-ribonucleotide made by specific rna polymerase

3. dna polymerase cna add nucleotides ony at 3’ OH end

alternate ways to generate 3’ ends

1. specific rna polymerase - synthesize small segment of RNA - eu and pro

2. nicked dna - duplex dna is nicked to provide free end for dna polymerase - some phages

3. priming nucleotide - some viruses do this also happens at end of eu replication telomerase

control of initiation

dna-protein interactions DNA

DNA replication is controlled at

INITIATION

most important decision every cell must make

1. whether to replicate dna

2. when to replicate dna

cis elements

cis acting sites; specific nucleotide sequences - precies distribution of acceptors and donors, sites or sequences on dna

trans factors

trans-acting functions; diffuse thorugh the cells and nucleus - proteins, recognize cis elements and bind to them

initiation of replication in e.coli

1. Oric recognized by specific proteins

2. AT rich 13 bp, 3 sequences adject to ORiC helps denaturation

3. repetitive 9bp (Dna A boxes, 4 of them)

the dam methylase

maintains methylation, activating oriC

methylation of OriC

N6 of adenine is methylated in the sequence GATC

DnaA in Dna replication

1. initiates replication in e.coli at OriC

2. REcognizes 9-mers in OriC

3. 10-20 DnaA and 4 DnaA boxes form an initial complex

4. initiates only if dna is negatively supercoiled (easy to melt stored energy)

5. opening occurs at the 13bp sequences 13-mers and requires ATP

DnaB: Helicases in dna replication

1. move along dsDNA and separate the strands (require ATP) DnaB = E coli helicase

2. hexamer could clamp around either single strand of DNA

3. Requires DnaC to be escorted to DNaA (to form pre-priming complex)

DnaB is key Helicase in DNA replication and is processive

doesn’t fall off until it reaches the end of the strand or it is unloaded by another protein

cooperative binding

binding of one molecule promotes the binding of the next one

SSB small protein that binds to ssDNA

prevents internal pairing and double helix from reforming, ssDNA is coated by SSB; forces DNA to have extended conformation

primosome

lagging strand Primase (DnaG) functions with the helicase (dnaB) to formt he enxt priming site

one primer for the leading and

hundreds to thousands for the lagging strand

DnaG: DNA primase

Dna polymerase can only elongate existing nucleotide strnads; solution is rna primers ( small, 5-10 mer, but can be longer) synthesized by rna polymerase

what steps require atp in replication

DnaA strand separating at 13bp repeats and DNaB/DnaC joining the complex to form replication forks