genetic diversity (section 4) Flashcards
what is a diploid
a cell with 46 chromosomes or 23 pairs of chromosomes
what is a haploid
a cell with 23 chromosomes(no pairs yet)
what is chiasmata
the points where the chromosomes break and rejoin
what does it mean to condense
chromosomes become visible
what is a homologous chromosome
two chromosomes in a pair - normally one inherited from the mother and one from the father
what is cytokinesis
a process of cell division , which divides the cytoplasm of a parental cell into two daughter cells forming a new cell membrane
what is a centromere
links a pair of sister chromosomes toghether during cell division . this is usually in the middle of the homologous pairs
what is a chromatid
one of the two identical halves of a chromosome that has been replicated in preparation for cell division
what is chromatin
a mixture of loosely packed DNA and proteins that form the chromosomes
what is meisosis
meiosis is a type of cell division that produces 4 gametes (haploid cells) that are used in sexual reproduction
where does meiosis occur
meiosis occurs in the ovaries and testes of animals and the ovaries and anthers of flowering plants (so animals and plants)
how many chromosomes do the gametes produce have
23 chromosomes
how is meiosis different from mitosis
meiosis has 2 divisions , meiosis 1 and meiosis 11
what stages are in both meiosis 1 and 11
prophase , metaphase , anaphase and telophase
what are the two important roles of meiosis
-results in haploid cells
-creates genetic variation
how does it produce variation
meiosis produces daughter cells that are genetically different from eachother and the parent cell , this is due to two processes that happen
-independent segregation
-crossing over
what happens at independent segregation
this is when homologous chromosomes line up in random orientations at the middle of the cell during metaphase. when pulled to either side of the cell this will result in different combinations of alleles in each gamete
what happens at crossing over
the process whereby a chromatid breaks during meiosis and joins to the chromatid on its homologous chromosome so that its alleles are exchanged
what is crossing over
chromosomes in each homologous pair twist around each other , chromatids break and rejoin to the chromatid on sister chromosome
what are sister chromatids
exactly the same ‘replicated’
what is meant by bivalent
a pair of homologous chromosomes ( 4 chromatids )
describe the process of meiosis
interphase - DNA replication occurs which doubles the amount of chromosomes
prophase 1 - DNA condenses and becomes visible as chromosomes , nuclear envelope breaks down, the chromosomes are arranged side by side in homologous pairs ( bivalent ) , crossing over can occur exchanging alleles , centroles migrate to oppositte poles and the spindle is formed
metaphase 1 - the pairs of homologous chromosomes (bivalents) are aligned at the centre of the cell . independent segregation occurs where they can align in different orientations
anaphase 1 - spindle fibres pull homologous chromosomes towards oppositte poles
telophase 1 and cytokinesis - nuclear envelope forms around the two groups of chromosomes and nuclei reform . cytokinesis is when the division of the cytoplasm occurs
prophase 2 - chromosomes condense and become visible again . nuclear membrane breaks down and spindle fibres develop
metaphase 2 - the chromosomes are aligned at the centre of the cell in single file this time
anaphase 2 - centromere of each chromosome divide and the chromatids are pulled to opposite poles of the cell by spindle fibres
telophase 2 and cytokinesis - the chromatids are now at either side of the cell and each cell undegoes cytokinesis to produce 4 haploid cells