Confusion about Meiosis

[lboogy]

Hi everyone,

 

I was hoping someone could explain something I was reading? Below is the text I read and I have put my question in bold.

 

Before meiosis begins, the chromosomes are copied exactly. The DNA of each chromosome is replicated to form two chromatids. They then arrange themselves into homologous pairs (both coding for the same characteristics), and prepare for cell division. At this point maternal and paternal chromatids can exchange bits of DNA to recombine their genetic material and increase the potential for variation (If the chromosomes are copied exactly, why would they exhange bits of DNA as the are just swaping like for like? Also if this is before meiosis how can there be both maternal and parental chromatids? Surely this can only happen when the sperm and egg meet?).

 

 

 

thanks so much in advance

Gav

[Revenged]

independent assortment and crossing over of DNA both increase variation...

[CharonY]

(If the chromosomes are copied exactly, why would they exhange bits of DNA as the are just swaping like for like?

 

The description is for (at least diploid) organims. Meaning that you got a pair of chromsomes, one from each parent. Recombination occur between them (and mechanistically between chromatids of the same chromosomesm but these are really neutral exchanges).

 

 

Also if this is before meiosis how can there be both maternal and parental chromatids? Surely this can only happen when the sperm and egg meet?).

 

As above already implied, this refers to the parent of the organism carrying the chromomes undergoing meioisis. Everyone carries one chromosome (consisting of two identical chromatids) from the father (paternal, not parental, btw) and from the mother.

[lboogy]

thanks CharonY - understood now.