CHAPTER 12 – THE CELL CYCLE

SCIENCE STARTER Write a short response in your notebook to the following questions: What is cell division? What are the 3 key functions of cell division? Cell division is a process by which a cell divides into 2 new daughter cells. The 3 key functions of cell division are reproduction, growth, and repair. Mitosis results in asexual reproduction in unicellular organisms. Mitosis results in growth and repair of tissues in multicellular organisms.

THINK-PAIR-SHARE Write a short response to the following question: Why do cells divide rather than just growing larger and larger? There are 2 main reasons why cells divide rather than continuing to grow larger and larger: The larger a cell becomes, the more demands the cell places on its DNA. As the cell grows too large, the DNA cannot keep up with the demands of running a larger cell. If the cell grows too large, it will have trouble moving enough nutrients and wastes across the cell membrane. The larger the surface area to volume ratio, the better.

Learning Objective I can…describe the structural organization of the genome. There are 2 main reasons why cells divide rather than continuing to grow larger and larger: The larger a cell becomes, the more demands the cell places on its DNA. As the cell grows too large, the DNA cannot keep up with the demands of running a larger cell. If the cell grows too large, it will have trouble moving enough nutrients and wastes across the cell membrane. The larger the surface area to volume ratio, the better.

THE KEY ROLES OF CELL DIVISION Cell division distributes identical sets of chromosomes to daughter cells The total hereditary endowment of a cell of a particular species is called its genome. The genomes of some species are quite small (e.g., prokaryotes), while the genomes of other species are quite large (e.g., eukaryotes).

THE KEY ROLES OF CELL DIVISION Cell division distributes identical sets of chromosomes to daughter cells The replication, division, and distribution of the large genomes of eukaryotes is possible because the genomes are organized into multiple functional units called chromosomes.

THE KEY ROLES OF CELL DIVISION Eukaryotic chromosome have the following characteristics: They are supercoils of a DNA-protein complex called chromatin. Each chromosome consists of: A single, long, double-stranded molecule of DNA, segments of which are called genes. The DNA is associated with various proteins that maintain chromosome structure and help control the activity of the genes.

THE KEY ROLES OF CELL DIVISION Eukaryotic chromosome have the following characteristics: They exist in a characteristic number in different species (e.g., human somatic cells have 46). Gamete cells (sperm and ova) possess half the number of chromosomes of somatic cells (e.g., human gametes have 23).

THE KEY ROLES OF CELL DIVISION Eukaryotic chromosome have the following characteristics: They exist in different states at different stages of the cell cycle. E.g. During interphase, the chromosomes are loosely folded. During the mitotic phase, chromosomes are highly folded and condensed

THE KEY ROLES OF CELL DIVISION In preparation for eukaryotic cell division, the complete genome is duplicated. As a result, each chromosome consists of two sister chromatids. The two sister chromatids possess identical copies of the chromosome’s DNA. The two sister chromatids are initially attached to each other at a specialized region called the centromere.

THE KEY ROLES OF CELL DIVISION Cell division usually proceeds in two sequential steps: Mitosis = nuclear division Cytokinesis = division of cytoplasm In mitosis, the sister chromatids are pulled apart, and this results in the segregation of two sets of chromosomes, one set at each end of the cell. In cytokinesis, the cytoplasm is divided and two separate daughter cells are formed, each containing a single nucleus with one set of chromosomes.

THE KEY ROLES OF CELL DIVISION MEIOSIS A variation of cell division that yields daughter cells that have half as many chromosomes as the parent cell Occurs ONLY in the gonads (ovaries and testes) Fertilization joins the gametes and doubles the chromosome number

Draw a picture of chromosome including: sister chromatids centromere proteins DNA

Draw a picture of chromosome including: sister chromatids centromere proteins DNA

QUICKWRITE On a post-it note, write a short response to the following question: Explain why an organized chromosome structure is an important adaptation for eukaryotic organisms.

QUICKWRITE On a post-it note, write a short response to the following question: Explain why an organized chromosome structure is an important adaptation for eukaryotic organisms. Having an organized structure helps cells use and pass on large amounts of DNA in multiple strands exactly and efficiently. (e.g. It is easier to sort two spools of thread than two long, tangled threads.)