Exam #3 Review

Overview of Cell Cycle and Mitosis

Cell Cycle Introduction

  • The cell cycle follows mitosis.

  • A newly created cell enters the cell cycle.

Interphase

  • Interphase consists of three main phases: G1, S, and G2.

G1 Phase

  • Also known as the "grow and go" stage.

  • The cell undergoes growth:

    • Increases in size.

    • Gains plasma membrane and cytoplasm.

    • Replicates organelles and proteins necessary for division.

  • The cell carries out normal functions such as:

    • Gene expression.

    • Synthesis of hormones and secretions.

  • Contains a checkpoint called the restriction point.

  • At the restriction point, the cell assesses the readiness for division.

  • Cyclins and CDKs (cyclin-dependent kinases) play a role but not heavily emphasized for exams.

S Phase

  • The key event is DNA synthesis and replication.

  • Each chromosome transitions from a single chromatid to a duplicated form (two sister chromatids).

G2 Phase

  • Prepares for cell division by ensuring all necessary machinery and components are present.

  • The process is similar whether preparing for mitosis or meiosis.

  • The cell synthesizes DNA and doubles cellular components.

  • Another checkpoint is present before mitosis.

M Phase

  • Comprises mitosis and cytokinesis.

  • Mitosis involves the division of the nucleus:

    • Followed by the steps:

    • Prophase

    • Prometaphase

    • Metaphase

    • Anaphase

    • Telophase

  • Cytokinesis involves the division of the cytoplasm.

  • Collectively, these phases ensure that the genetic material is accurately distributed.

Chromatin and DNA Structure

  • In interphase, nuclear contents are referred to as chromatin.

    • Chromatin is composed of DNA and proteins.

  • During mitosis, DNA condenses and organizes into chromosomes.

  • Chromosomes exist as a duplicated form held together at the centromere.

DNA Structure Details

  • DNA is a double helix.

  • Nucleotides consist of four bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).

  • Base pairing occurs via hydrogen bonds:

    • A pairs with T.

    • G pairs with C.

  • Genes are segments of DNA consisting of thousands of nucleotides.

Alleles and Genotypes

  • Alleles represent different variants of a gene located at specific loci.

    • Example: Capital 'A' (dominant allele) and lowercase 'a' (recessive allele) pertaining to traits like freckles.

  • Genotypes are combinations of alleles that determine the phenotype.

    • E.g., Freckles are influenced by the expression of the dominant allele.

Mitosis versus Meiosis

  • Mitosis and meiosis involve interphase which occurs only once.

    • DNA replication occurs in S phase for both processes.

Comparing Mitosis and Meiosis

  • Mitosis:

    • Number of divisions: 1

    • Number of daughter cells: 2 (identical to the parent)

    • Purpose: Growth, repair, and development of somatic (body) cells.

  • Meiosis:

    • Number of divisions: 2

    • Number of daughter cells: 4 (unique and different from the parent)

    • Purpose: Produces gametes for sexual reproduction.

Additional Details on Meiosis

  • Synapsis occurs in prophase I of meiosis, where homologous chromosomes pair up and may exchange genetic material via crossing over (chiasmata).

  • Independent assortment also occurs, providing genetic variation.

  • Unique combination of alleles leads to distinct daughter cells.

Gene Expression and Traits

  • The phenotypic expression arises from gene activity.

    • Example: Freckles due to melanin production from gene expression.

Genetic Probability and Punnett Squares

  • Use the rule of multiplication for genetic probability.

  • Set up a two-by-two Punnett square to calculate inheritance probabilities.

  • Example with alleles A (yellow) and a (green):

    • If both parents are heterozygous (Aa), compute ratios based on their allele combinations (1:2:1 genotypic ratio).

  • If alleles are linked, traits travel together. If unlinked, independent assortment occurs, showing all possible combinations.

Conclusion

  • Understanding the distinction between mitosis and meiosis is fundamental in genetics and cellular biology.

  • Insight into gene expression is crucial for exploring traits and inheritance patterns.