Chapter 4 The Transmission of DNA at Cell Division. DNA Replication Cell Division Mitosis Meiosis

Size: px

Start display at page:

Download "Chapter 4 The Transmission of DNA at Cell Division. DNA Replication Cell Division Mitosis Meiosis"

Sophie Chase
6 years ago
Views:

1 Chapter 4 The Transmission of DNA at Cell Division DNA Replication Cell Division Mitosis Meiosis

2 The faithful transmission of genotype at cell division, illustrated by the behaviour of tumor cells

3 Cell division, nuclear division, chromosome division, and DNA replication occur in a coordinated manner, producing daugther cells with identical genetic material.

DNA Replication DNA replication is prerequisite to cell division. DNA is replicated by using each strand as template for synthesis of the complementary strand.

4 DNA Replication DNA replication is prerequisite to cell division. DNA is replicated by using each strand as template for synthesis of the complementary strand. one strand is synthesized continuously, the other discontinuously all nucleotides are added at the 3 end of the growing strand Replicated chromosomes are partitioned to nuclei of daughter cells in mitosis. Meiosis distributes replicated chromosomes to haploid daughter cells in two nuclear and cell divisions.

5 A DNA replication fork, nucleotides are added to the 3' ends

6 DNA Replication is semiconservative and semidiscontionous. DNA synthesis proceeds by continous synthesis on the leading strand and discontinous synthesis on the lagging strand. All DNA polymerases synthezise only in the 5' to 3' direction.

7 Meselson and Stahl (1958!) showed that DNA replicates semiconservatively. They used the heavy isotope of nitrogen ( 15 N) to label DNA of E. coli.

10 Replication fork

11 Lagging strand synthesis

12 Replication for at the beginning (a) and at the end (b) of the synthesis of an Okazaki fragment. A complex containing helicase and 2 DNA polymerases carries out the coordinated synthesis of both strands. The two polymerases are attached to each other and move with the replication fork. The fidelity of DNA polymerization is very high, one error per 10 9 bases (proofreading!).

13 DNA replication initiates at specific sites called origins of replication. Each origin produces two replication forks that move bidirectionally in opposite directions.

14 Replication of circular DNA: generation of θ (theta) structures

15 Replication of circular DNA: the rolling circle model

16 Replication of linear DNA: the problem at the ends RNA primers are required for DNA replication. If a priming site was located at the end of a chromosome, this DNA segment would remain unreplicated after the primer is degraded and would not be available during the next round of DNA replication.

17 Telomeres protect the ends of chromosomes

18 Telomerases contain specific RNA molecules that are part of the enzyme complex and serve as complementary templates for the telomeric repeat unit. Telomerase is a type of reverse transcriptase.

19 Telomere lenght can vary during development

20 Vizualization of telomeres The chromosomes have formed sister chromatides. An unbroken nucleus is shown at the bottom.

21 Cell division Asexual cell division Sexual cell division

22 Cell cycle - G1: gap (growth) - S: DNA replication, forms sister chromatids - G2: gap (growth) - M: mitosis (nuclear division) and cytokinesis

25 The life cycle of humans (animals)

26 The life cycle of plants

27 Mus musculus has genes and is the model closest to humans.

28 The plant model Arabidopsis thaliana has genes.

29 The life cycle of fungi

30 During S phase the DNA of each chromosome is replicated. Both daughter DNA molecules become bound to histone proteins. At this stage two chromosomes consists of two sister chromatides, each of which contains one of the replicated DNA molecules. The two sister chromatides are always identical.

31 DNA and gene transmission during asexual and sexual cell divisions in eukaryotes. S phase and the main stages of mitosis and meiosis are shown. The diagrams emphasize the DNA content of each cell and chromosome. The first two panels in each column show DNA replication, which occurs during the S phase; the remaining panels show mitosis or meiosis. The alleles A and a of one gene are used to show how genotypes are transmitted during cell division.

32 The stages of mitosis

33 The stages of meiosis The process of meiosis starts with diploid meiocytes in the reproductive tissue and produces an array of haploid cells with diverse genotypes.

34 Sexual cell division and crossover Just before the first nuclear division, homologous chromosomes pair along their lengths, so that now for each chromosomal type there are two pairs of sister chromatids juxtaposed, making a bundle of four also called a tetrad. At the tetrad stage a remarkable process occurs: paired nonsister chromatids exchange homologous sections of DNA through breakage and reunion of their arms at points called crossovers.

DNA replication: Enzymes link the aligned nucleotides by phosphodiester bonds to form a continuous strand.

DNA replication: Enzymes link the aligned nucleotides by phosphodiester bonds to form a continuous strand. DNA replication: Copying genetic information for transmission to the next generation Occurs in S phase of cell cycle Process of DNA duplicating itself Begins with the unwinding of the double helix to expose