Cellular reproduction part 2

Chromosomes are matched in homologous pairs MEIOSIS AND CROSSING OVER Chromosomes are matched in homologous pairs Somatic cells of each species contain a specific number of chromosomes Human cells have 46, making up 23 pairs of homologous chromosomes Chromosomes Centromere Sister chromatids Figure 8.12

Homologous Chromosomes Humans have 23 pairs of homologous chromosomes 22 pairs – autosomes – found in both males and females 1 pair – sex chromosomes, XX = female, XY= male

Homologous Chromosomes Matched pairs of chromosomes Similar in size, shape, and banding pattern Both carry genes controlling the same inherited characteristics (the version of the gene may be different) The genes are located at the same locus One chromosome of each pair is inherited from the mother, the other from the father

Multicellular diploid adults (2n = 46) Mitosis and development The human life cycle Haploid gametes (n = 23) Egg cell Sperm cell MEIOSIS FERTILIZATION Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46) Mitosis and development Figure 8.13

Human Life Cycle Diploid cells (2n) – cells that contain both homologous chromosomes. In humans the diploid number is 46. Haploid cells (n) – cells with one copy of each homologous chromosome. The gametes (egg and sperm) are haploid. In humans the haploid number is 23.

Meiosis Meiosis The division that reduces the number of chromosomes by half. In animals, meiosis results in the formation of haploid egg and sperm cells.

Figure 8.15

Figure 8.16.2

Two nuclear divisions occur: Meiosis I During prophase I homologous chromosomes pair – synapsis During prophase I the paired chromosomes exchange chromosome parts – crossing over Homologous chromosomes are separated 2 cells produced each containing one copy of each homologous chromosome

Figure 8.16.3

Meiosis Meiosis II Not preceded by the replication of DNA Sister chromatids of each chromosome are separated Produces 4 haploid cells

Meiosis Meiosis produces 4 cells that Are haploid Chromosome makeup of each is different from each other and the parent cell

Figure 8.17

Meiosis Spermatogenesis Formation of sperm by meiosis Occurs in special cells (spermatogonia) in the testes All 4 haploid cells become sperm

Meiosis Oogenesis Formation of an egg by meiosis Occurs in special cells (oogonia) in the ovaries Unequal divisions of the cytoplasm during meiosis I and meiosis II result in the formation of 1 haploid egg and 3 haploid polar bodies Only the egg can be fertilized

Genetic Recombination Genetic Recombination – the production of gene combinations different from those carried by the parent There are 4 processes that contribute to genetic recombination.

Figure 8.18

Independent Assortment of Chromosomes The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes This results in 2n possible combinations of gametes For humans 2n = 223 = 8 million possible combinations http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__random_orientation_of_chromosomes_during_meiosis.html Random fertilization also increases variation in offspring

C E C E C E c e c e c e Coat-color genes Eye-color genes Brown Black White Pink Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes Figure 8.17A, B

Homologous chromosomes carry different versions of genes The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci

Tetrad Chaisma Centromere Figure 8.18A

How crossing over leads to genetic recombination Coat-color genes Eye-color genes How crossing over leads to genetic recombination Tetrad (homologous pair of chromosomes in synapsis) 1 Breakage of homologous chromatids 2 Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase I 3 Separation of chromatids at anaphase II and completion of meiosis 4 Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Figure 8.18B Gametes of four genetic types

Crossing over a closer look Meiosis II Animation (must insert Miller and Levine lecture cd ch11) Meiosis 1 Crossing over a closer look Meiosis II http://bcs.whfreeman.com/thelifewire/content/chp09/0902002.html Figure 8.19

Genetic Recombination Crossing over The exchange of genetic information between 2 homologous chromosomes. Occurs during prophase I. Random fertilization Depends on which sperm cell and its chromosome combinations fertilizes which egg

Preparation of a karyotype Fixative Packed red And white blood cells Hypotonic solution Blood culture Stain White Blood cells Centrifuge 3 2 1 Fluid Centromere Sister chromatids Pair of homologous chromosomes 4 5 Figure 8.19

A karyotype is a photographic inventory of an individual’s chromosomes ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE A karyotype is a photographic inventory of an individual’s chromosomes To study human chromosomes microscopically, researchers stain and display them as a karyotype A karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes

8.21 Accidents during meiosis can alter chromosome number Abnormal chromosome count is a result of nondisjunction Either homologous pairs fail to separate during meiosis I http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.swf Nondisjunction in meiosis I Normal meiosis II Gametes n + 1 n + 1 n – 1 n – 1 Number of chromosomes Figure 8.21A

Or sister chromatids fail to separate during meiosis II Normal meiosis I Nondisjunction in meiosis II Gametes n + 1 n – 1 n n Number of chromosomes Figure 8.21B

Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Egg cell n + 1 Zygote 2n + 1 Sperm cell n (normal) Figure 8.21C

Connection: An extra copy of chromosome 21 causes Down syndrome This karyotype shows three number 21 chromosomes An extra copy of chromosome 21 causes Down syndrome Figure 8.20A, B

The chance of having a Down syndrome child goes up with maternal age Figure 8.23

Alterations of Chromosomes In most cases abnormal chromosome number results in spontaneous abortion long before birth. Nondisjunction in the sex chromosomes has less of an affect on survival

Connection: Abnormal numbers of sex chromosomes do not usually affect survival Nondisjunction can also produce gametes with extra or missing sex chromosomes Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes

Table 8.1

Connection: 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

Duplication – the fragment joins to a homologous chromosome. Deletion Deletion – a chromosome breaks and a fragment is lost. Seems to have the greatest affect. Duplication Duplication – the fragment joins to a homologous chromosome. Homologous chromosomes Inversion – the fragment reattaches to the original chromosome but in reverse orientation. Least likely to produce harmful affects. Inversion Reciprocal translocation Translocation– attachment of a chromosome fragment to a nonhomologous chromosome. May/may not be harmful. Nonhomologous chromosomes Figure 8.23A, B

Alterations of Chromosomes Abnormalities in the structure of the chromosome may cause disorders (Figure 8.23A) Deletion – a chromosome breaks and a fragment is lost. Seems to have the greatest affect. Duplication – the fragment joins to a homologous chromosome.

Alterations of Chromosomes Inversion – the fragment reattaches to the original chromosome but in reverse orientation. Least likely to produce harmful affects. Translocation (Figure 8.23B) – attachment of a chromosome fragment to a nonhomologous chromosome. May/may not be harmful.

Chromosomal changes in a somatic cell can cause cancer A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia Chromosome 9 Reciprocal translocation Chromosome 22 “Philadelphia chromosome” Activated cancer-causing gene Figure 8.23C