Date post: | 15-Jan-2016 |
Category: |
Documents |
Upload: | cecelia-tennant |
View: | 212 times |
Download: | 0 times |
Cellular reproduction part 2
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Somatic cells of each species contain a specific number of chromosomes
– Human cells have 46, making up 23 pairs of homologous chromosomes
MEIOSIS AND CROSSING OVER
Chromosomes are matched in homologous pairs
Chromosomes
Centromere
Sister chromatids Figure 8.12
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The human life cycle
Figure 8.13
MEIOSIS FERTILIZATION
Haploid gametes (n = 23)
Egg cell
Sperm cell
Diploidzygote
(2n = 46)Multicellular
diploid adults (2n = 46)
Mitosis anddevelopment
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.15
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.16.2
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Meiosis
Two nuclear divisions occur:
1. Meiosis I
a. During prophase I homologous chromosomes pair – synapsis
b. During prophase I the paired chromosomes exchange chromosome parts – crossing over
c. Homologous chromosomes are separated
d. 2 cells produced each containing one copy of each homologous chromosome
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.16.3
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Meiosis
2. Meiosis II
a. Not preceded by the replication of DNA
b. Sister chromatids of each chromosome are separated
c. Produces 4 haploid cells
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Meiosis
Meiosis produces 4 cells that
• Are haploid
• Chromosome makeup of each is different from each other and the parent cell
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.17
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Meiosis
Spermatogenesis
• Formation of sperm by meiosis
• Occurs in special cells (spermatogonia) in the testes
• All 4 haploid cells become sperm
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.18
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.17A, B
Coat-color genes Eye-color genes
Brown Black
C E
c e
White Pink
C E
c e
C E
c e
Tetrad in parent cell(homologous pair of
duplicated chromosomes)
Chromosomes ofthe four gametes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci
Homologous chromosomes carry different versions of genes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.18A
TetradChaisma
Centromere
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• How crossing over leads to genetic recombination
Figure 8.18B
Tetrad(homologous pair ofchromosomes in synapsis)
Breakage of homologous chromatids
Joining of homologous chromatids
Chiasma
Separation of homologouschromosomes at anaphase I
Separation of chromatids atanaphase II and completion of meiosis
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
1
2
3
4
Coat-colorgenes
Eye-colorgenes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.19
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Preparation of a karyotype
Figure 8.19
Blood culture
1
Centrifuge
Packed redAnd white blood cells
Fluid
2
Hypotonic solution
3
Fixative
WhiteBloodcells
Stain
4 5
Centromere
Sisterchromatids
Pair of homologouschromosomes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE
A karyotype is a photographic inventory of an individual’s chromosomes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
8.21 Accidents during meiosis can alter chromosome number
Figure 8.21A
Nondisjunctionin meiosis I
Normalmeiosis II
Gametes
n + 1 n + 1 n – 1 n – 1
Number of chromosomes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
– Or sister chromatids fail to separate during meiosis II
Figure 8.21B
Normalmeiosis I
Nondisjunctionin meiosis II
Gametes
n + 1 n – 1 n n
Number of chromosomes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome
Figure 8.21C
Eggcell
Spermcell
n + 1
n (normal)
Zygote2n + 1
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• This karyotype shows three number 21 chromosomes
• An extra copy of chromosome 21 causes Down syndrome
Connection: An extra copy of chromosome 21 causes Down syndrome
Figure 8.20A, B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.23
The chance of having a Down syndrome child goes up with maternal age
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Connection: Abnormal numbers of sex chromosomes do not usually affect survival
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Table 8.1
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer
– Four types of rearrangement are deletion, duplication, inversion, and translocation
Connection: Alterations of chromosome structure can cause birth defects and cancer
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.23A, B
Deletion
Duplication
Inversion
Homologouschromosomes
Reciprocaltranslocatio
n
Nonhomologouschromosomes
Deletion – a chromosome breaks and a fragment is lost. Seems to have the greatest affect.
Duplication – the fragment joins to a homologous chromosome.
Inversion – the fragment reattaches to the original chromosome but in reverse orientation. Least likely to produce harmful affects.
Translocation– attachment of a chromosome fragment to a nonhomologous chromosome. May/may not be harmful.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Alterations of Chromosomes
Abnormalities in the structure of the chromosome may cause disorders (Figure 8.23A)
1. Deletion – a chromosome breaks and a fragment is lost. Seems to have the greatest affect.
2. Duplication – the fragment joins to a homologous chromosome.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Alterations of Chromosomes
3. Inversion – the fragment reattaches to the original chromosome but in reverse orientation. Least likely to produce harmful affects.
4. Translocation (Figure 8.23B) – attachment of a chromosome fragment to a nonhomologous chromosome. May/may not be harmful.