Cell Division. Chapter 10 04/25/2016

Transcription

1 Cell Division Chapter 10 04/25/2016

2 Why do Cells Need to Divide? Make more cells Replace cells Grow a larger organism Repair damage to organism Reproduction of the organism

3 Cell Division Enables Asexual Reproduction

4 DNA is the instruction manual of the cell Codes for all proteins Codes directions for how proteins will be used Relies entirely on the sequence of nucleotide bases Permanent changes in this sequence are called mutations

5 Dealing with the DNA During Cell Division Human DNA measures about 6 ft in length In non-dividing cells DNA needs to accessible for making RNA Termed uncondensed DNA need to be copied prior to cell division New cells will need DNA DNA need to be divided evenly between the new cells No tangling, messing up

6 Humans Have 23 Pairs of Chromosomes A 1 set came from female parent 1 set came from male parent We are diploids (2 copies of each chromosome) Often given number/letter names chromosome 1A chromosome 1B Chromosomes 1A and 1B are homologous chromosomes sex chromosomes Chromosomes with different numbers (like 1A and 4A) are non-homologous chromosomes

7 Genome A cell s entire DNA, packaged as a doublestranded DNA molecule

8 Gene A segment of DNA the codes for a protein

9 Chromatin All of the DNA molecules and their associated proteins present in the nucleus Typically present as diffused strands throughout the nucleus Heterochromatin tightly packed DNA that is inaccessible Euchromatin lightly packed DNA that is enriched with genes

10 DNA is Tightly Wound Into Chromosomes for Cell Division 1 DNA double helix histone proteins 2 DNA wound around histone proteins 3 Coiled DNA/histone beads 4 Loops attached to a protein scaffold; this stage of partial condensation typically occurs in a nondividing cell protein scaffold 5 Folded chromosome, fully condensed in a dividing cell

11 Nucleosome Basic unit of DNA packaging in eukaryotes Segment of DNA wound in sequence around 8 histone protein This structure is often compared to thread wrapped around a spool

12 Chromosome A double-stranded linear DNA molecule Human body (somatic) cells have 46 chromosomes (23 pairs) Human gametal cells have 23 chromosomes

13 Chromosomes Are Copied Before Cell Division Original cell 1 chromosome replication p arm centromere 1 duplicated chromosome q arm mitosis 2 daughter cells Each has 1 chromosome

14 Chromatid All of the chromatin that is associated with a single chromosome

15 Ploidy Ploidy: number of sets of chromosomes in a cell Human somatic cells are diploid Human germ cells are special diploid cells that will divide and make gametal cells Human gametal cells are haploid

16 Definitions 1A 1A 1A 1A 1B Single Duplicated chromosome Chromosome, sister chromatids Homologous pair of Chromosomes

17 Homologous Homologous 1A 1A 1B 1B 2A 2A 2B 2B Sister chromatids Non homologous

18 The Cell Cycle An orderly set of stages that take place between the birth of a cell and the time it reproduces another identical cell.

19 Cell Division is Only Part of the Cell G 2 : cell growth and preparation for cell division; organelles are duplicated Cycle S: synthesis of DNA; chromosomes are duplicated G 1 : cell growth and preparation for synthesis Fig.

20 Interphase The phase in a cells life when it performs growth and DNA replication The longest stage in a cell s life Some cells never complete the cell cycle and are permanently arrested e.g. nerve and muscle cells Stages: G1 stage (G stands for gap or growth ) S stage (S stands for DNA synthesis ) G2 stage

21 Gap 1 stage Cell continues to produce proteins and accomplish its normal functions Cell s organelles are doubled (mitochondria, ribosomes, etc.) Cell accumulates needed materials for DNA replication Cell grows

22 Cell replicates DNA S stage Results in two identical sister chromatids for each chromosome

23 Gap 2 stage Follows DNA replication (S Stage) and last to onset of Mitosis Cell synthesizes materials needed for cell division (mitotic stage)

24 M stage Stage when mitosis and cytokinesis occur Cytokinesis: Division of the cytoplasm Daughter chromosomes are distributed to two daughter nucleii When complete, two daughter cells are present

25 Control of the Cell Cycle Cells initiate M stage after receiving either external and internal protein signals A signal is a molecule that stimulates or inhibits a metabolic response Growth factors are external signals received at the plasma membrane that ask the cell to divide Size of the cell is an internal factor that can lead to cell division

26 Cell cycle checkpoints Points in the cell cycle when the cell ensures that all components needed to make a new cell are perfect

27 G 1 Checkpoint Cell evaluates reserves and size DNA is evaluated, and if not perfect, the cell may die (apoptosis unless repairable)

28 G 2 Checkpoint DNA is checked to ensure that it has successfully replicated and is not damaged Occurs at G2/M transition

29 M Checkpoint Mitosis stops if chromosomes are not properly aligned Occurs during metaphase AKA the spindle checkpoint Are all the sister chromatids correctly attached to the spindle microtubules?

30 Regulator molecules Molecules that control the progress of the cell cycle

31 Cyclins and Cdks Positive regulators Cyclins and cyclin-dependent kinases (Cdks) are responsible for the progress of the cell through the various checkpoints

32 Cyclins and Cdks Cyclins are only active when bound to Cdks and are phosphorylated

33 Negative regulators The best understood are retinoblastoma protein (Rb), p53, and p21 All three of these regulatory proteins were discovered in cancerous cells Act mainly at the G 1 checkpoint

34 Negative regulators p53 checks DNA integrity Triggers p21 and apoptosis Rb monitors cells size

35 Telomeres do not shorten Too much telomerase enzyme

36 Interphase: Main Features: DNA not visible as chromosomes Cell carries out normal cell functions Cell spends most of its time here DNA present as chromatin DNA is copied

37

38 Prophase: Main features: Start of mitosis DNA visible as chromosomes Duplicated chromosomes stuck together Nuclear envelope fragments Nucleolus disappears Chromosomes attach to spindle Spindle microtubules form

39 Prophase

40 Metaphase: Main Features: Chromosome line up in the middle of the cell Duplicated chromosome still stuck together

41 Metaphase

42 Anaphase: Main Features: Duplicated chromosomes pulled apart 1 copy to each end of the cell Cell elongates as spindle fibers disassociate

43 Anaphase

44 Telophase: Main features: Chromosomes arrive at cell ends New nuclear membranes form around chromatin Chromosone uncondense Nucleoli appear Cleavage furrow appears Ends with cytokinesis (division of cytoplasm) Final division into 2 new cells

45 Telophase

46 Cytokinesis in animal cells Actin filaments (contractile ring) constrict around cleavage furrow Filaments draw tight until two daughter cells are formed

47 Cytokinesis in plant cells No cleavage furrow Builds new PM and cell wall between daughter cells

48 Return of Interphase Main Features: DNA not visible as chromosomes Cell carried out normal cell functions DNA is copied for the next round of cell division

49 Overview of Mitosis Fig.

50 Summary: the Stages of Mitosis Prophase: chromosomes become visible Metaphase: chromosomes line up Anaphase: chromosomes move apart Telophase: two distinct cells form

51 G 0 Phase Exiting the cell cycle After G1, some cells enter a quiescent state Some re-emerge after a time Some remain quiescent for the life of the organism (heart muscle cells, cortical neurons)

52 Prokaryotic cells divide too! Peokaryotic fission Escherichia coli doubles in 20 minutes under ideal conditions

53 Big Topic in Cell Division Control: Cancer Tumor ( -oma ) Uncontrolled cell growth Benign stationary Malignant (cancer) invasive Uncontrolled growth Loss of adhesion Loss of cell cycle control Uncontrolled cell division Several genes must mutate to form cancer

54 How does cancer begin? Proto-oncogenes when mutated in certain ways, become oncogenes A proto-oncogenes is a gene that codes for a positive cell cycle regulator An oncogene is any gene that, when altered, leads to an increase in the rate of cell cycle progression In most instances, the alteration of the DNA sequence will result in a less functional protein Occasionally, however, a gene mutation causes a change that increases the activity of a positive regulator

55 How does cancer begin? Tumor suppressor genes or negative cell cycle regulators can stop the progress A functional p53 will deem the cell unsalvageable and trigger apoptosis

56 What happens when the tumor suppressor gene is mutated? Mutated p53 genes have been identified in more than one-half of all human tumor cells A cell with a faulty p53 may fail to detect errors present in the genomic DNA Even if a partially functional p53 does identify the mutations, it may no longer be able to signal the necessary DNA repair enzymes

57 Big Topic in Cell Division Control: Stem Cells Stem cells are non-differentiated cells (no specific function) Able to divide and multiply indefinitely renew themselves give rise to differentiated cells Stem cell Two types: Embryonic Stem Cells Adult Stem Cells Non Stem cell Stem cell

58 Embryonic Stem Cells (ESC) Cell Potential Pluripotent cell any cell type Multipotent cell multiple cell types Unipotent cell one cell type ESC are pluripotent cells ESC are cells isolated from 5-6 day old embryos that can become any cell in the human body

59 Multipotent and unipotent cells found in small numbers within specific tissues bone marrow Adult Stem Cells

60 Plants Have Stem Cells Too Tip of shoot and root have stem cells (meristem) Divide then give: new stem cells (maintain meristem size) make differentiated cells (the root and shoot) Overproduction of stem cells leads to abnormal plant growth