Chapter 9: Cell Cycle

_{Cell cycle: the entire life of a cell from formation to death.}

_{Why do we need the cell cycle?}

• _{Repair damage}

• _{Make a copy/replicate(unicellular)}

• _{Growth Development}

_{Interphase: 90% of the cell’s life.}

• _{Three parts—or}

• _Checkpoint

_{Mitosis: “Spitting of the nucleus}

_{Cytokinesis: Splitting of the cytoplasm}

_{Interphase stages:}

• _{Gap1(G1): the first gap comes after the Split. Or Cell to rows, performing regular functions}

• _{Synthesis(S): make new DNA. Or DNA replicates itself for future division.}

• _{Checkpoint: The cell looks for damaged chromosomes}

• _{Gap 2(G2): replicate to organelles and prepare to split the nucleus—or more growth, which reaches twice its size.}

• _{Checkpoint: Cell prepares organelles.}

• _{Mitosis (M): The cell divides into two identical cells. Ends with cytokinesis (full split)}

• _{Gap 0 (G0): leave the cell cycle, and continue t, leave the cell cycle, and continue to routine A stage where the cells stop dividing, leave the cell cycle, and continue to perform normal functions. This does NOT mean that the cell will kill itself.}

_{^{Checkpoints are to make sure everything ensures proper progression! If a cell develops abnormally, it enables apoptosis (cell death homeostasis, a mechanism that prevents dangerous mutations. *}}

_Mitosis:

• _{Prophase: nuclear membrane disappears, and chromosomes condense, and Sindle: fiber appears.}

• _{Metaphase: Chromosomes lines in the middle because of spindle fiber}

  _{are attached}

• _{Anaphase: chromosomes are pulled apart}

• _{Telophase: two new nuclei form.}

• _{Cytokinesis: Cytoplasm are dived}

  • _{Animals: membrane pinches}

  • _{Plants: build a new cell wall}

_{Chromosome Structure:}

_{leave the cell cycle, and continue t, leave the cell cycle, and continue to routine A stage where the cells stop dividing, leave the cell cycle, and continue}

_{Chromosome (Sister) Chromatid: 1/2 of a Chromosome}

• _{Centromere: Region where the sister chromatids attach/ connect}

• _{Chromatin: the long, loose state of a chromosome}

• _{Diploid (2n): Having 2 sets of chromosomes -Ex: Human somatic cells, which is where mitosis takes place.}

_{^{*Very important!! The daughter cells will have the exact SAME genetic makeup as the parent cells after mitosis*}}

_{Has the cell cycle gone wrong?!}

• _{Normal: enzymes control the checkpoint, and the growth signals}

• _{Abnormal: mutation in any enzyme may lead to uncontrolled growth (Cancer)}

• _{If a cell develops abnormally, it enables apoptosis (cell death homeostasis, a mechanism that prevents dangerous mutations.}

_{DNA structure and replication:}

• _{1953, James Waston and Francis Crick published a paper describing the shape of DNA}

• _{double helix}

• _{Outside (backbone)}

  _{Altering sugar (deoxyribose) and phosphate}

  

• _{Inside: nitrogenous-based pairs}

  _{A: Adenine}

  _{T: Thymine}

  _{C: Cytosine}

  _{G: Guanine}

  • _{10 base pairs per twist}

_{DNA replication:}

• _{Occur in the “s” phase of interphase.}

• _{uses many enzymes}

• _{occurs in a semi-conservative manner}

_{Steps for DNA replication:}

1. _{Topoisomerase unwinds and straightens—the DNA strand. (Start at a sequence called the origin of replication)}

2. _{Helicase “unzips” the double helix by breaking the hydrogen bonds between the base pair.}

3. _{DNA polymerase adds new nucleotides in one direction (leading stand/lagging)}

4. _{Ligase fills in any cracks (lagging strands)}

_{Chapter 10: Meiosis}

_{Chromosome: one piece of DNA that carries genetic information}

_{Chromatid: One copy of a duplicated Chromosome}

_{Haploid: one copy of each chromosome}

• _{Gametes are haploid}

_{Diploid: Two copies of each chromosome}

• _{Body cells are diploid (copy from each parent)}

• _{Meiosis is similar to mitosis, except the result is four non-identical cells.}

• _{Copied chromosomes are separated and moved into—each new cell.}

• _{Meiosis increases genetic diversity by randomly distributing chromosomes into gametes.}

• _{More genetic diversity occurs when “homologous” (the same number) Chromosomes line up and swap portions, a process called crossing over.}

• _{If chromosomes don’t separate correctly:}

  • _{extra copy of chromosomes}

    

_{Sexual reproduction}

• _{Meiosis is a type of a cell division where a diploid cell splits into 4 haploid gametes (sperm/egg cells).}

• _{n = number of chromosomes in ONE set (ex: humans have 23, so a diploid cell would have 46)}

• _{Gametes only have I set known as haploid}

• _{Fertilization: haploid sperm cell fuses with a haploid egg cell, creating a diploid zygote.}

• _{That's how we get one set of chromosomes from our father and the other from our mother!}

_{Genetic variation}

_{Why are there variations in offspring?}

1. _{crossing over Occurs in prophase 1. Creates recombinant chromosomes, with mixed genes from both parents.}

2. _{Independent Assortment Occurs in metaphase 1 and 2. Random orientations creates many possible combinations of chromosomes in the gametes.}

3. _{Random Fertilization During sexual reproduction, it is complete random which male gamete will fuse with a female gamete.}

_{Nondisjunction}

• _{When homologous pairs or sister chromatids fail to separate properly during anaphase, a gamete may receive any extra copy of a chromosome while the other gamete receives 0.}

• _{This can result in genetic conditions such as Trisomy 21 (Down Syndrome), where there is an extra copy of the chromosome 21}