Chromosomes and Meiosis

DNA and Chromosomes Some Interesting Facts: The DNA molecule in a single human cell is 99 cm long, so is 10 000 times longer than the cell in which it resides (<100mm) Since an adult human has about 1014 cells, all the DNA in one human would stretch about 1014 m, which is a thousand times the distance between the Earth and the Sun. 22/02/2019 J. Gilbert

So how does the DNA fit into the cell? The DNA is cut into shorter lengths Each length is tightly wrapped up with histone proteins to form a complex called chromatin. Just before cell division the DNA is replicated. Following replication the chromatin then coils up even tighter to form short fat bundles called chromosomes. 22/02/2019 J. Gilbert

How does the DNA fit into the cell? (cont..) Each chromosome is roughly X-shaped because it contains two replicated copies of the DNA. The two arms of the X are therefore identical. They are called chromatids, and are joined at the centromere. Chromatin DNA + histones at any stage of the cell cycle Chromosome compact X-shaped form of chromatin formed (and visible) during mitosis Chromatid single arm of an X-shaped chromosome 22/02/2019 J. Gilbert

Homologous Chromosomes Chromosomes come in pairs called homologous pairs (‘same shaped’) The homologous pairs are the maternal (inherited from the mother) and paternal (inherited from the father) versions of the same chromosome, so they have the same sequence of genes. 22/02/2019 J. Gilbert

Homologous Chromosomes (cont…) One pair of chromosomes is different in males and females. These are called the sex chromosomes, and are non-homologous in one of the sexes. In humans the sex chromosomes are homologous in females (XX) and non-homologous in males (XY). (In birds it is the other way round!) The non-sex chromosomes are sometimes called autosomes, so humans have 22 pairs of autosomes, and 1 pair of sex chromosomes. 22/02/2019 J. Gilbert

Meiosis A form cell division that produces gametes Two important functions: To form haploid cells with half the number of chromosomes To re-arrange the chromosomes with a unique combination of genes There are two successive divisions, without DNA replication in between 22/02/2019 J. Gilbert

Meiosis Meiosis in detail http://www.thelifewire.com/content/ch09/ch09xe02.htm OR go to www.thelifewire.com Click on ‘animated tutorials’ Scroll down to ‘9.2 Meiosis’ 22/02/2019 J. Gilbert