* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download HL#1 Meiosis - hutchhighIBbiology
Survey
Document related concepts
Site-specific recombinase technology wikipedia , lookup
Epigenetics of human development wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
Genomic imprinting wikipedia , lookup
Polycomb Group Proteins and Cancer wikipedia , lookup
Gene expression programming wikipedia , lookup
Genome (book) wikipedia , lookup
Designer baby wikipedia , lookup
Skewed X-inactivation wikipedia , lookup
Hybrid (biology) wikipedia , lookup
Microevolution wikipedia , lookup
Y chromosome wikipedia , lookup
X-inactivation wikipedia , lookup
Transcript
IB HL #10 •Meiosis reduces chromosome number by copying the chromosomes once, but dividing twice. •The first division, meiosis I, separates homologous chromosomes. •The second, meiosis II, separates sister chromatids. •In prophase I, the chromosomes condense and homologous chromosomes pair up to form tetrads • Centrioles move to the poles in animal cells •Spindle microtubules forms from each pole to the equater • Nucleoli break down • Nuclear membrane starts to break down Crossing-over occurs •At metaphase I, the bivalents line up at the equater. • Spindle microtubules from the two poles attach to the centromeres on different chromosomes in each pair. • At the end of metaphase I the chromosomes start to move • In anaphase I, the homologous chromosomes separate and are pulled toward opposite poles. • This halves the chromosome number. • Each chromosome still consists of two chromatids. • Because of crossing-over the two chromatids are not identical • At the end of anaphase I the chromosomes reach the poles. •In telophase I, movement of homologous chromosomes continues until there is a haploid set at each pole. • Each chromosome consists of linked sister chromatids. • Nuclear membranes form around the groups of chromosomes at each pole. •Cytokinesis by the same mechanisms as mitosis usually occurs simultaneously. • The chromosomes partially uncoil. • The cell divides to form two haploid cells • The two cells may enter a brief period of Interphase or immediately proceed to the next division. DNA is not duplicated. • Meiosis II is very similar to mitosis. • During prophase II chromosomes become shorter and thicker again by uncoiling. • Centrioles move to the poles in animal cells and spindle microtubles form. • At the end of Prophase II the nuclear membranes break down. •At metaphase II, spindle fibers attach to the centomeres and the sister chromatids line up at the equater • At the end of metaphase II the centromeres divide •At anaphase II, the centomeres of sister chromatids separate and sisters chromatids travel toward opposite poles. •In telophase II, separated sister chromatids arrive at opposite poles and uncoil. •Nuclear membrane forms around the chromatids. •Cytokinesis separates the cytoplasm. •Nucleoli appear •At the end of meiosis, there are four haploid daughter cells. SUMMARY OF MEIOSIS 1. Meiosis involves two divisions. One cell or nucleus divides to form four cells or nuclei. 2. The chromosome number is halved, from diploid to haploid. 3. An almost infinite amount of genetic variety is produced, as a result of crossing-over in Prophase I and the random orientation of bivalents in Metaphase I. Mendel’s law of segregation Mendel’s law of segregation states that alleles of a gene on homologous chromosomes separate when gametes are formed. Mendel’s law applies to two (or more) alleles (alternate forms of a gene). The allele pairs separate in Meiosis I, when a diploid cell divides into two haploid cells, before the formation of gametes. So, for example each sperm or egg will get just one big R, or one little r, or one big T, or one little t. Mendel’s law of independent assortment • The law of independent assortment says that alleles of genes (that are not linked) located on different chromosomes separate independently. • In other words alleles for different traits are transmitted independently. • During Meiosis I, the homologous chromosomes line up randomly on the equator during metaphase ensuring random distribution of alleles.