Meiosis

Genes

Genes

The units of heredity and are made up of segments of DNA

Gametes

• The reproductive cells that allow genes to be passed down  

• Sperm or egg; contains a single set of chromosomes and is haploid (n) 

• Only type of human cells produced by meiosis rather than mitosis  

• Fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism 

Somatic Cell

the cells of the body except for gametes and their precursors  

• In humans have 23 pairs of chromosomes 

Locus

specific locations for each gene on the chromosome

Asexual Reproduction

reproduction in which a single individual passes gene to its offspring without the fusion of gametes 

Clone

a group of genetically identical individuals from the same parent, produced asexually  

Sexual reproduction

reproduction in which two parents give rise to offspring that have unique combinations of genes inherited from two parents 

Karyotype

an ordered display of the pairs of chromosomes from a cell  

homologous chromosome

the two chromosomes in each pair 

• The chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters  

Sex chromosome

a package of DNA with part of/all of the genetic material  

• Determine the sex of the individual; called X and Y 

• Females have a homologous pair of X chromosomes (XX) 

• Males have one X and one Y chromosome 

Autosome

the remaining pairs of chromosomes aside from the sex chromosomes 

Diploid

(2n) an organism has two complete sets of chromosomes 

• 23 from the mother; 23 for the father; total of 46 

Haploid

an organism that has one set of unpaired chromosomes; 23n  

• Each set of 23 consists of 22 autosomes and a single sex chromosome 

Fertilization

the union of gametes (the sperm and the egg cell)  

• Zygote (egg after union) has one set of chromosomes from each parent and so is diploid  

• Zygote produces somatic cells by mitosis   

Meiosis

• Takes place in two sets of cell divisions, which result in 4 daughter cells each of which only have half as many chromosomes as the parent cell  

• Produce gametes 

• Results in one set of chromosomes in each gamete 

• This, when combined with fertilization, maintain chromosome number 

• Reduces the number of chromosomes sets from two (diploid) to one (hapoloid), producing cells that differ genetically from each other and from the parent cell  

• Produces 4 new haploid cells 

Meiosis I

• Prophase I: synapsis and crossing over 

• Homologous chromosomes physically connect and exchange genetic information  

• Two members of a homologous pair associate along their length, allele by allele 

• Metaphase I: alignment of homologous pairs; homologous pairs of chromosomes are positioned there in the first phase of this 

• Homologous pairs line up at the platem with one chromosome facing each pole 

• Microtobules from one pole are attatched to the kinetochore of one chromosome of each tetrad 

• Anaphase I: Separation of homologs 

• One chromosome moves toward each pole, guided by the  spindle apparatuus 

• Telophase I: at the beginning of this stage, each half of the cell has a haploid set of chromosomes 

• Each chromosome still consists of two sister chromatids 

• Cytokinesis usually occurs simultaneously, forming two haploid daughter cells  

Meiosis II

• Occurs in four phases: 

• Prophase II: 

• Spindle apparatus forms 

• Later in this stage chromosomes (each still comprised of two chromatids) move toward the metaphase plate 

• Metaphase II: the sister chromatids are arranged at theplate 

• Because of crossing over in in the first version of this stage, the two sister chromatids of each chromosome are no longer genetically identical 

• The kinetochores of sister chromatids attatch to microtubles extending from extending opposite poles 

• Anaphase II:  

• In this stage, the sister chromatids separate 

• The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles  

• Telophase II + cytokinesis  

• Nuclei form, and the chromosomes begin decondensing  

• At the end of meiosis, there are four daughter cells, each with a haploid set of unduplicated chromosomes 

• Each daughter cell is genetically distinct from the others and from the parent cell  

Chromosomes

Each one replicated consists of two identical sister chromatids 

Sister chromatid

identical copies of a chromosome that are joined together by a centromere 

Crossing Over

nonsister chromatids exchange DNA segments  

• Produces recombinant chromosomes- combine DNA inherited from each parent  

• Contributes to genetic variation by combining DNA, producing chromosomes with new combinations of maternal and paternal alleles 

Independent Assortment

one of the mechanisms that contributes to genetic variation 

• Homologous pairs of chromosomes orient randomly at metaphase I of meiosis 

• Each pair of chromosomes sorts maternal and paternal homologs into daughter cells independetly of the other pairs  

• Number of combinations possible when chromosomes assort independently into gametes is 2n 

Recombinant Chromosome

combine DNA inherited from each parent 

Random Fertilization

adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) 

• Each zygote has a unique genetic identity  

Prophase I

Metaphase I

Anaphase I

Telophase

Meiosis II