Mistakes During Cell Division

1. THIS DIAGRAM SHOWS THE PROCESS OF MEIOSIS IN A CELL WITH A SINGLE PAIR OF HOMOLOGOUS CHROMOSOIES. IN THIS DIAGRAM, LABEL A PAIR OF HOMOLOGOUS CHROMOSOIES BEFORE MEIOSIS BEGINS THE SISTER CHROMATIDS IN A CHROMOSOME THE HAPLOID GAMETES. Mekrns ( I Meio$K I Diploid cell Haploid gametes DNA \ replication Homologous Homologous chromosome chromosomes pairing Sister chromatids 2. ONE TYPE OF MISTAKE II CELL DIVISION IS CALLED IEIOTIC NONDISIUNCTION. IF MEIOTIC NONDISJUNCTION OCCURS DURING MEIOSIS I, THE HOMOLOGOUS CHROMOSOMES FAIL TO SEPARATE. DRAW A DIAGRAM THAT SHOWS THE PROCESS OF MEIOSIS WHEN MEIOTIC NONDISJUNCTION OCCURS DURING MEIOSIS I. BE SURE TO SHOW HOW MANY COPIES OF THE CHROMOSOME EACH GAMETE WOULD HAVE. NONDISJUNCTION IN MEIOSIS I NONDISJUNCTION IN MEIOSIS II Meiosis I * » Nondisjunction: Pair of homologous chromosomes fails to separate. Meiosis II Nondisjunction: Pair of sister chromatids x fails to separate. to Number of chromosomes i Abnormal gametes Abnormal gametes I i n n ! T Normal gametes I Figure 8.20-3 3. EXPLAIN HOW MEIOTIC NONDISJUNCTION CAN RESULT IN 1 BABY WITH TRISOMY 31 (THREE COPIES OF CHROMOSOME 21 IN EACH CELL). (HINT: THINK ABOUT WHAT WOULD HAPPEN WHEN A GAMETE PRODUCED BY MEIOTIC NONDISJUNCTION IS FERTILIZED TO FORM A ZYGOTE AND HOW THE ZYGOTE DEVELOPS INTO A BABY.) ▪ An egg cell with 2 copies of chromosome TRISOMY 21 (NONDISJUNCTION CELL DIVISION) 21 (produced by meiotic nondisjunction) is fertilized by a normal sperm cell with 1 copy of chromosome 21. chromosome 21 is fertilized by a sperm cell with 2 copies of chromosome 21 (produced by meiotic nondisjunction). SPERM 46 ch 46 ch v r OR ▪ A normal egg cell with 1 copy of ECC HH nmn nmn L - 47 chr nondisjunction < fertilization mitosis ▪ 5. When a cell has three copies of a chromosome, the extra copies of the genes on this chromosome result in extra copies of the proteins coded for by these genes, and this causes abnormal cell function. As a result, a zygote that has three copies of a chromosome will develop abnormally. For example: ▪ If a zygote has an extra copy of any of the chromosomes in the top row of the karyotype, the embryo develops such severe abnormalities that it dies and the pregnancy ends in a miscarriage. (A miscarriage occurs when a pregnancy ends spontaneously before the 20th week, usually during the first trimester). Therefore, no babies are born with trisomy 1, 2, 3, 4, or 5. ▪ If a zygote has an extra copy of chromosome 21 (trisomy 21), the embryo may die or a baby may be born with Down syndrome which includes mental retardation, a broad flat face, a big tongue, short height, and often heart defects. ▪ What do you think is the reason that trisomy 1, 2, 3, 4 or 5 always results in miscarriage, but trisomy 21 has less serious effects on development so some babies are born with trisomy 21? A cell cannot function properly when there are too many copies of some types of proteins due to an extra copy of one of the (large) chromosomes. 6. DOWN SYNDROME DUE TO TRISOMY 21 IS II GENETIC CONDITION WHICH IS NOT INHERITED. IN OTHER WORDS, MOST BABIES WITH DOWN SYNDROME HAVE PARENTS WITH NORMAL CHROMOSOMES. EXPLAIN HOW DOWN SYNDROME CAN BE GENETIC, BUT NOT INHERITED. ▪ Trisomy 21 arises due to meiotic nondisjunction, which is a mistake during the formation of a gamete, and is not inherited within the genes of a chromosome. It is not caused by the genetic makeup of a parent. ▪ 7. Normally, each cell in a person has 2 sex chromosomes + 44 autosomes = 46 chromosomes. Normal females are 46XX (i.e. 44 autosomes + 2 X chromosomes in each cell). Normal males are 46XY. However, in some cases the chromosomal makeup is abnormal. ▪ Some females are 47XXX (i.e. 44 autosomes + 3 X chromosomes), 48XXXX, or ▪ 45X0 (i.e. 44 autosomes +1 X chromosome). ▪ Some males are 47XYY, 47XXY, or 48XXYY. ▪ No babies, children or adults are 46YY, 47YYY or 45Y0. ▪ Based on this information, which chromosome is crucial for determining the sex of a person? ▪ Y chromosome ▪ Which sex chromosome must be present in order for a zygote to develop into a baby? ▪ X chromosome ▪ Explain why this sex chromosome is required in order for a zygote to develop into a baby. ▪ The X chromosome is a relatively large chromosome with many crucial genes which are needed for the development of a baby. 8. BOTH 47XXX EMBRYOS AND 45 X0 EMBRYOS RESULT FROM MEIOTIC NONDISJUNCTION. EXPLAIN HOW THE SAME PROCESS CAN RESULT IN EMBRYOS WITH TOO MANY OR TOO FEW SEX CHROMOSOMES IN EACH CELL Nondisjunction of Sex Chromosomes XX x gamete XX Y XXY (Klinefelter syndrome) or XX x X O x Y x X nondisjunction XXX ( “m e t a f e m a l e ” ) XX V gamete o (not viable) or O meiosis and gamete formation YO possibilities at fertilization XO ( Turner syndrome) genotype (p h e n o t y p e ) ▪ 9. If more than one X chromosome is present in a cell, all but one is inactivated, so both male and female cells have the same balance between a single active X chromosome and 44 autosomes. A small part of each X chromosome has the same genes as part of the Y chromosome, and these genes remain active even if the rest of the X chromosome is inactivated. ▪ Based on this information, explain why 45X0 embryos can survive and develop into relatively normal girls and women, but an embryo with an autosomal monosomy (a single copy of one of the autosomes in each cell) has such severe abnormalities that it does not survive to birth. ▪ Even in females with two X chromosome, one X chromosome is inactivated, which provides the basis for normal cellular function both for the cells in males and the cells in females. So most females are operating with one active X chromosome. ▪ 45X0 girls or women have Turner syndrome; their ovaries are usually small and not functional and they have a substantial risk of other abnormalities such as heart defects or kidney problems. Provide a genetic explanation for these abnormalities in 45X0 females. ▪ Both the X and Y chromosome have similar genes in their pseudoautosomal regions, and these genes are active even on an inactive X chromosome. 45X0 females will lack the second copy of these genes.