cell division-meiosis Flashcards
meiosis starts with what kind if cell and ends with what kind of cell?
meiosis starts with a diploid cell and ends with four haploid gametes
what are haploid cells?
cells with a single set of chromosomes
are the cells produced in mitosis and meiosis genetically identical or different?
in mitosis the two diploid cells are identical, in meiosis the gametes are genetically different.
what is a homologous chromosome?
a set of one maternal and paternal chromosome which pair up inside a cell during fertilisation. The genes are the same.
how is inheritance determined?
in a homologous pair, the genes are the same but the alleles are different. The more dominate allele will determine the characteristic inherited.
how are the gametes produced by meiosis genetically different (crossing-over)?
During prophase 1, the two chromosomes in a homologous pair come together and the chromatids of the two chromosomes cross over. The attached pair of chromosomes is called a bivalent and the points where the chromatids are joined are called the chiasmata. Parts of the chromatids break off and exchange so the maternal and paternal DNA have been exchanged- recombinate chromosomes. Exchanging of DNA means exchanging alleles which is major for genetic variation as there are new COMBINATIONS of alleles.
meiosis involves two round of nuclear division: meiosis 1 and meiosis 2. What occurs in meiosis 1 and 2?
in meiosis 1, homologous chromosomes are separated from each other and in meiosis 2 sister chromatids are operated from each other.
Before meiosis starts, interphase begins. What occurs during interphase?
-the cell copies the chromosomes and organelles
Describe prophase 1 in meiosis (4 points)
- chromosomes condense and become visible
- homologous chromosomes link to form chiasmata and bivalents causing the exchanging of alleles
- nuclear membrane breaks down
- centrioles move to opposite poles leaving spindle fibres which start to assemble into the spindle apparatus.
Describe metaphase 1 in meiosis (1 point)
-pairs of homologous chromosomes line up on the equator of the spindle apparatus
Describe anaphase 1 in meiosis (2 points)
- spindle fibres shorten
- homologous chromosomes move to opposite poles causing the chiasmata to break
where does meiosis occur?
occurs in diploid germ cells (reproductive cells) in the gonads (reproductive organs)
describe telophase 1 in meiosis(4 points)
- chromosomes have reached the poles
- nuclear membrane reforms
- chromosomes uncoil back to their chromatin state
- cytokinesis occurs- the cells are now haploid
Describe prophase 2 in meiosis (3 points)
- chromosomes condense and become visible again-sister chromatids joined at centromere
- nuclear membrane breaks down
- spindle fibres begin to develop as centrioles move to opposite poles
Describe metaphase 2 in meiosis (1 point)
-chromosomes line up on the equator of the spindle apparatus
Describe anaphase 2 in meiosis (2 points)
- centromere of each chromosome divides
- spindle fibres shorten and chromatids are pulled towards opposite poles
Describe telophase 2 in meiosis
- chromotids reach opposite poles. They are now called chromosomes.
- nuclear membrane reforms and chromosomes uncoil back to chromatin state
- cytokinesis- two haploid cells formed
meiosis is also known as?
reduction division
what other ways in meiosis increases genetic variation (independent assortment)?
IN PROPHASE 1, when homologous chromosomes line up on the spindle, they are Independently assorted into their gametes which is random
how many chromosomes do humans have?
46
how do you calculate the number of genetically different gametes produced by independent assortment?
2 to the power of n, where n is the number of homologous pairs
how else is genetic variation increased?
random fertilisation as alleles mix causing different expressions
explain three ways in which meiosis gives rise to genetic variation (3 points)
- independent assortment during metaphase of chromosomes
- crossing over of alleles in a homologous chromosome pair during prophase and metaphase (chiasmata formation)
- random fertilisation
what is a synapsis?
pairing of homologous chromosomes in early prophase
why is DNA replication semi-conservative?
DNA replication is semi-conservative because each helix that is created contains one strand from the helix from which it was copied.
what is discontinuous variation?
clear-cut alternatives of a given trait with no intermediate forms e.g blood group
what is continuous variation?
a given trait that has many variations with only minor differences between them e.g height in humans
explain the origin of continuous and discontinuous variation
continuous variation occurs when there is a variety of alleles but discontinuous variation occurs if the gene only has two alleles for the expression.
