Lecture 2

Rounds of mitosis are required for the diploid zygote to form diploid gamete progenitor cell, during which mutations can occur: Animals 

• In higher animals, germline is set aside early during embryogenesis to minimize DNA-damaging mitosis: all eggs are eggs are formed early but they must be stored for decades 

• Mitosis to form sperm is life long 

Rounds of mitosis are required for the diploid zygote to form diploid gamete progenitor cell, during which mutations can occur: Plants 

• Embryonic cells are located at the apex of each shoot 

• Shoot apical meristem, which forms the leaves/stems/branches 

• Within the SAM are low rate mitotic stem cells known as quiescent center (QC) 

• QC cells convert SAM into florals, giving rise to a carpel (egg nuclei form) and stamen (sperm nuclei form) 

• Germline is NOT set aside early and there are many mitotic divisions during which mutations can occur 

• More successive rounds of DNA-damaging mitosis are required to make gametes in higher plants than in higher animals 

Gamete formation 

Requires meiosis, during which there is rare recombination (crossing over) within homologous chromosomes inherited from parents to diversify sperm or eggs  

Meiosis 

• There is independent assortment of entire homologous chromosomes inherited from parents to diversify sperm and eggs 

 

Meiotic chromosomes complete 3 major tasks 

• Reduce homologous chromosome from two to one (diploid to haploid) 

• Chromosome recombination (crossing over): Major drive of genetic diversity 

• Random assortment of chromosomes: Arguably most important driver of genetic diversity in eukaryotes 

Meiosis 

• Chromosome replication  

• Cell division (meiosis I) 

  • Only one chromosome homolog of each pair ends up in daughter cells which will give rise to sperm and eggs 

• Mother and father chromosome pair and recombine (cross over) 

• Cell division II (meiosis II) 

  • Half of every replicated chromosome (chromatid) ends up in a sperm or egg 

• Final result 

  • Due to independent assortment and recombination, each sperm and egg are potentially different 

Crossing over 

• Relatively rare 

• It can becomes dangerous to host 

• It involves two double strand breaks (DSBs) of DNA followed by accurate fusion of the broken chromatids 

• If this goes wrong, entire chromosome arm could be lost 

• Can occur nearly anywhere on chromosome 

• Each human chromosome encodes ~1000 genes, thus there are >1000 possible loci combinations that can be mixed and matched 

• In higher animals, large regions of opposite sex chromosomes (x to y) and possibly the entire chromosome don’t recombine, however x-x chromosomes do recombine, x-y chromosomes don’t  

Why we don’t possess only one set of chromosomes 

• Can't have independent assortment 

• Can recombine but there's only one possibility 

• Sexual organisms divide their genes into multiple chromosomes to permit independent assortment 

• Random assortment involves entire chromosomes, thousands of alleles are mixed/matched 

How many possible combinations are there for 23 sets of human chromosomes 

2^23 = ~8 million 

At every sexual act, two unique gametes and their alleles from different ancestral lineages are combined 

• Chromosomes of a human are the fusion of a large number of ancestors 

• Calculate how ancestors DNA originate from 15 generations ago, assuming none were ever related to one another? 

  • 2^15 = ~30,000 

Genetic diversification is both additive and exponential across generations: it reoccurs every generation independently, also increases exponentially as progeny multiply over generations 

• Short lifespan organisms have the potential to diversify faster 

• Thus high fecundity organisms (those that have move children) have the potential to diversify the lineage faster