the significance of meiosis

Why is meiosis biologically significant?

• Produces genetically different gametes

• Increases genetic variation within a population

• Genetic variation provides the raw material for natural selection

• Improves a population’s ability to adapt to environmental change

What mechanisms in meiosis increase genetic variation?

• Crossing over (during prophase I)

• Independent assortment of homologous chromosomes (during metaphase I)

• Random fusion of gametes at fertilisation

What is crossing over?

• The exchange of DNA (alleles) between non-sister chromatids

• Occurs between homologous chromosomes

• Results in new combinations of alleles

At what stage of meiosis does crossing over occur?

• Prophase I

• When homologous chromosomes pair up to form bivalents

What are chiasmata?

• The physical points where non-sister chromatids cross

• Visible under a microscope

• Sites where genetic material is exchanged

How does crossing over happen?

• Homologous chromosomes pair closely

• Non-sister chromatids cross and become entangled

• DNA strands break and rejoin with the other chromatid

• Leads to recombinant chromatids

Why is crossing over important?

• Produces new allele combinations

• Increases variation between gametes

• Makes offspring genetically different from parents

Where on a chromosome is crossing over more likely?

• Further from the centromere

• Less likely near the centromere due to reduced flexibility

What is independent assortment?

• The random alignment of homologous chromosome pairs

• Occurs at the equator during metaphase I

• Leads to different combinations of maternal and paternal chromosomes in gametes

How does independent assortment increase variation?

• Each homologous pair lines up randomly

• Orientation of one pair is independent of all others

• Results in different chromosome combinations in daughter cells

What happens after random alignment in metaphase I?

• Homologous chromosomes are separated in anaphase I

• Each daughter cell receives a random mix of chromosomes

How is the number of possible chromosome combinations calculated?

• Using the formula 2ⁿ

• n = number of chromosomes in a haploid cell

How many chromosome combinations are possible in human gametes?

• Humans have 23 chromosomes

• 2²³ = 8,324,608 possible combinations

• This excludes variation from crossing over

What is meant by random fusion of gametes?

• Any male gamete can fuse with any female gamete

• Occurs at fertilisation

• Further increases genetic variation in offspring

Why are offspring from sexual reproduction rarely genetically identical?

• Crossing over produces recombinant chromosomes

• Independent assortment creates varied gametes

• Random fertilisation combines alleles unpredictably

• Probability of identical offspring is almost zero