MEIOSIS AND CROSSING OVER

Chromosomes are matched in homologous pairs Homologous chromosomes: the 2 members of a pair of chromosomes—contain genes for the same traits Somatic cells of each species contain a specific number of chromosomes Human cells have 46, making up 23 pairs of homologous chromosomes Chromosomes Sister chromatids

Paired chromosomes Homologous chromosomes both chromosomes of a pair carry “matching” genes control same inherited characters homologous = same information diploid 2n 2n = 4 homologous chromosomes double stranded homologous chromosomes eye color (brown?) eye color (blue?)

Gametes have a single set of chromosomes Gametes~ egg or sperm Cells with two sets of chromosomes are said to be diploid (2n) somatic cells(46 in humans) Gametes are haploid, with only one set of chromosomes, (1n)(23 in humans)

Human female karyotype 46 chromosomes 23 pairs XX diploid = 2 copies 2n

46 chromosomes 23 pairs XY Human male karyotype diploid = 2 copies 2n

Life Cycle At fertilization, a sperm fuses with an egg, forming a diploid zygote Repeated mitotic divisions lead to the development of a mature adult The adult makes haploid gametes by meiosis All of these processes make up the sexual life cycle of organisms

Why meiosis? When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells. Imagine if mitosis were the only means of cell division. IF the parent organism has 14 chromosomes, it would produce gametes that contained a complete set of 14 chromosomes The offspring would have cell nuclei with 28 chromosomes, and the next generation would have cell nuclei with 56 chromosomes

Meiosis reduces the chromosome number from diploid to haploid Meiosis, like mitosis, is preceded by chromosome duplication However, in meiosis the cell divides twice to form four daughter cells In the first division, meiosis I, homologous chromosomes are paired While they are paired, they cross over and exchange genetic information The homologous pairs are then separated, and two daughter cells are produced

Meiosis I In the first division, meiosis I, homologous chromosomes are paired As the chromosomes coil, homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad.Here synaspsis occurs: the meeting of two homologous pairs While they are paired, they cross over and exchange genetic information The homologous pairs are then separated, and two daughter cells are produced Division in meiosis I occurs in four phases: prophase, metaphase, anaphase, and telophase

Meiosis 1 overview 1st division of meiosis  4 chromosomes  diploid  2n  2 chromosomes  haploid  1n double stranded Copy DNA before meiosis Line Up 1 Divide 1 gamete prophase 1metaphase 1 telophase 1

Meiosis II Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is four haploid daughter cells

2nd division of meiosis looks like mitosis Meiosis 2 overview  2 chromosomes  haploid  1n 4 gametes Line Up 2 Bye Bye 2 telophase 1 metaphase 2 telophase 2

Review: A comparison of mitosis and meiosis For both processes, chromosomes replicate only once, during interphase

Review: A comparison of mitosis and meiosis

Genetic variation Each chromosome of a homologous pair comes from a different parent The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes Random fertilization also increases variation in offspring

Crossing over further increases genetic variability Crossing over is the exchange of corresponding segments between two homologous chromosomes Genetic recombination results from crossing over during prophase I of meiosis This increases variation further

Errors of Meiosis Chromosomal Abnormalities

Chromosomal abnormalities Incorrect number of chromosomes nondisjunction chromosomes don’t separate properly during meiosis breakage of chromosomes deletion duplication inversion translocation

ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE A karyotype is a photographic inventory of an individual’s chromosomes Human female karyotype

An extra copy of chromosome 21 causes Down syndrome This karyotype shows three number 21 chromosomes: trisomy 21 An extra copy of chromosome 21 causes Down syndrome The chance of having a Down syndrome child goes up with maternal age

Down syndrome & age of mother Mother’s age Incidence of Down Syndrome Under 30<1 in in in in in in in in in in in in in 12 Rate of miscarriage due to amniocentesis:  1970s data 0.5%, or 1 in 200 pregnancies  2006 data <0.1%, or 1 in 1600 pregnancies

Accidents during meiosis can alter chromosome number Nondisjunction~ The failure of homologous chromosomes to separate properly during meiosis Abnormal chromosome count will result.

Nondisjunction Problems in meiosis cause errors in daughter cells – chromosome pairs do not separate properly during Meiosis 1 – sister chromatids fail to separate during Meiosis 2 – too many or too few chromosomes 2n n n n-1 n+1

Abnormal numbers of sex chromosomes do not usually affect survival Nondisjunction can also produce gametes with extra or missing sex chromosomes A man with Klinefelter syndrome has an extra X chromosome A woman with Turner syndrome lacks an X chromosome

XXY male –one in every 2000 live births –have male sex organs, but are sterile –feminine characteristics some breast development lack of facial hair –tall –normal intelligence Klinefelter’s syndrome

Turner syndrome Monosomy X or X0 –1 in every 5000 births –varied degree of effects –webbed neck –short stature –sterile

Nondisjunction When a gamete with an extra set of chromosomes is fertilized by a normal haploid gamete, the offspring has three sets of chromosomes and is triploid.(3n) The fusion of two gametes, each with an extra set of chromosomes, produces offspring with four sets of chromosomes—a tetraploid. (4n) This is polyploidy.

Alterations of chromosome structure can cause birth defects and cancer Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer –Four types of rearrangement are deletion, duplication, inversion, and translocation Deletion Duplication Inversion Reciprocal translocation Nonhomologous chromosomes

Changes in chromosome structure deletion – loss of a chromosomal segment duplication – repeat a segment inversion – reverses a segment translocation – move segment from one chromosome to another