Chromosome Chr Duplica Duplic t a ion Pixley

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1 Chromosome Duplication Pixley

2 Figure 4-6 Molecular Biology of the Cell ( Garland Science 2008)

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7 Figure 4-72 Molecular Biology of the Cell ( Garland Science 2008)

8 Interphase During mitosis (cell division), all of the DNA (chromatin) is duplicated and then condensed into chromosomes. Then, the nucleus dissolves and the chromosomes are split between the 2 daughter cells. After the cytoplasm is divided, then the nucleus reforms. Then, the condensed, d mitotic i chromosomes Become less condensed. Interphase

9 The mitotic chromosome is the most condensed form of DNA and it is ONLY found At mitosis or meiosis. That is the only time that we can see whole chromosomes with a light microscope, as is seen below in a chromosome spread. Humans have 46 chromosomes (DNA molecules), which are actually 22 pairs of very Similar (homologous) chromosomes, the 22 autosomal chromosomes PLUS the 23 rd chromosome set, which consists of the sex chromosomes: If you have XX you are female, XY (male). Figure Molecular Biology of the Cell ( Garland Science 2008)

10 Chromosomes (Information in [] is extra, not info on the final exam) [To make a chromosome spread : Get some cells from either cell culture or the body, making sure first that most of the cells are dividing. Drop the cells from about waist hih height onto a glass slide. The cells will burst open from the drop. Fix the slide by putting it in alcohol. Then stain with a special stain. Take pictures of all the loose chromosomes and arrange them on a piece of pp paper.]

11 Chromosomes Each pair of homologous chromosomes is very similar (and different from the another pairs). The pairs actually encode for the same genes, which means there are TWO copies of every gene! These copies are called alleles. One allele is usually dominantand and expressed, while the other isrecessive and not expressed (meaning it is not transcribed and translated into a protein). During mitosis, the DNA is first duplicated, resulting in 92 chromosomes. Each duplicate is called a sister chromatid. Sister chromatids are not separated quickly, but instead they remain bound for a time (a few hours) at one place by a protein complex called the centromere. A rough average time for mitosis is 24 hours (varies from a few hours to times like 40 days or never!)

12 Figure 4-50c Molecular Biology of the Cell ( Garland Science 2008)

13 Diagram of a duplicated and condensed metaphase eukaryotic chromosome. (1) Chromatid one of the two identical parts of the chromosome after S phase. (2) Centromere the point where the two chromatids touch, and where the microtubules attach. (3) Short arm. (4) Long arm.

14 Figure 4-15 Molecular Biology of the Cell ( Garland Science 2008)

15 Interphase During mitosis (cell division), all of the DNA (chromatin) is duplicated and then condensed into chromosomes. Then, the nucleus dissolves and the chromosomes are split between the 2 daughter cells. After the cytoplasm is divided, then the nucleus reforms. Then, the condensed, d mitotic i chromosomes Become less condensed. Interphase

16 Meiosis In specialized eukaryotic cells, the germ cells that give rise to eggs and sperm, a different form of cell division takes place, called meiosis. In meiosis, each resulting daughter cell (gamete) gets only half the chromosomes. To be specific: each daughter cell gets ONE OF EACH PAIR of homologous chromosomes. These specialized cells are germ cells. They reside in the ovaries and testes. Their daughter cells are gametes, which we know as the egg and sperm. All other cells are called somatic cells. Gametes have ½ of the DNA of somatic cells. Thus, gametes are HAPLOID, while all somatic cells are DIPLOID. Meiosis actually requires two rounds of formation of two new cells from one (two divisions). Meiosis results in 4 gametes.

17 Figure 21-5 Molecular Biology of the Cell ( Garland Science 2008)

18 Figure Molecular Biology of the Cell ( Garland Science 2008)

19 Gametes are haploid Gametes have one copy each from each pair of homologous chromosomes. This means that they have only one allele or copy of each gene. When gametes (egg and sperm recombine, their nuclei fuse and reform one nucleus Containing now the normal, diploid, 46 chromosomes.

20 Differences between Mitosis and Meiosis Mitosis Meiosis Where does it occur? Almost all tissues Reproductive organs What are the parent cells called? Somatic cells Germ cells How many cellular divisions? 1 2 What separates during cell division(s)? i i Division i i 1: sister Division i i 1: Homologous chromatids separate chromosomes separate Division 2: sister chromatids In terms of the daughter cells: What are they called? Somatic cells Gametes How many daughter cells? 2 4 What is their DNA state? Diploid Haploid

21 Mutations For each gene, you have two copies = alleles. One came from your mother (maternal, One came from your father (paternal). In genetic work, we mutate (change) )genes. The terminology is that the normal is Wild Type, the animal with the altered gene is a mutant. Because you have two copies of each gene, a normal animal will have two wild type genes, one on each homologous chromosome and we call it: WT/WT for that particular gene (but it is also said: wild type at that locus on the chromosome). The gametes will be WT alone, because they have only one copy. The mutant will be: mut. We designate WT as + and the mutant as if the resulting mutant gene does not work (no active protein = loss of function). So a WT/WT is gene X +/+ (i.e. for the gene encoding EGF, the wild type animal is EGF +/+ ). The mutant that has both genes altered and is a complete gene knockout is mut/mut = EGF knockout = EGF /. Each of these is homozygous because they have two similar genes. Remember: the gametes will have only one copy: WT or mut, because they have only one copy of each gene. (remember they are haploid) An animal lthat t inherits it one WT gene from one parent and one mut from the other is WT/mut (i.e. EGF +/ ). We call these heterozygous (n. heterozygotes).

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23 Mutations: Dominant/Recessive

24 DOMINANT mutation: If you have WT/mut, then The mutant gene is Expressed NOT the WT. The mut PHENOTYPE shows up in 3/4 of The offspring, those with Mut/mut = A*A* genotype And WT/mut = AA* genotype

25 RECESSIVE mutation: RECESSIVE mutation: Shows up only in 1/4 of The offspring, those with Mut/mut = A*A* genotype (homozygous for A*)

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