Biology Chapter on Chromosomes

Chromosomes

• DNA is located within the nucleus.
• A chromosome is DNA tightly coiled around proteins called histones.
• Histones help with support and control of gene activity within a cell.
• Genes are specific segments of DNA that direct protein synthesis, which plays a structural or functional role in the cell.

Chromatin, Chromosomes, and DNA

• Chromatin is DNA and histones coiled into thin, thread-like structures that exist in the nucleus between cell divisions.
• Chromosomes are the same DNA and histones super-condensed into thicker, visible structures that occur during cell division.

Chromosomes & Somatic Cells

• All somatic cells (body cells except germ cells - eggs and sperm) have 46 chromosomes.
• Two sets (pairs) of 23 chromosomes, with one set from each parent.
• Each pair is called a homologous pair.
• One chromosome of the pair is from the mother, and one chromosome of the pair is from the father.

Diploid & Haploid Cells

• Diploid means a cell with two sets of chromosomes (one from each parent).
• All cells in the human body are diploid (2n) except sperm and egg cells (haploid – n).
• Genes also occur in pairs in diploid cells.
• Each gene pair is located at the same position on homologous chromosomes.

Autosomes & Sex Chromosomes

• 23 pairs of chromosomes total.
• 22 pairs are called autosomes and determine expression of most inherited characteristics.
• The last pair is sex chromosomes, determining the biological sex.
  • XX = genetic female
  • XY = genetic male

Karyotype

• A karyotype is a complete set of chromosomes from a cell.
• Shows the exact number and visual appearance of chromosomes, and can show anomalies.

The Cell Cycle

• The cell cycle is a predictable set of events that a cell goes through from its origin to its own division into two daughter cells.
• Two major phases:
  • Interphase - Growth and preparation for cell division.
  • Mitosis - Cell division for somatic cells.
• Cytokinesis is the division of the cytoplasm into two distinct cells.

Interphase

• Interphase is a period of growth and preparation for cell division.
• Interphase has three phases: G1, S, and G2.
  • G1 phase - Major growth prior to DNA synthesis.
  • S phase - DNA is replicated, and the copies are called chromatids, which remain attached at the centromere.
  • G2 phase - Period after DNA is synthesized and before mitosis begins.

G0

• Cells that have temporarily or permanently stopped dividing are in the G0 phase.

Cell Division

• Consists of two processes:
  • Mitosis - Division of the nucleus.
  • Cytokinesis - Division of the cytoplasm.

Stages of Mitosis

• Mitosis occurs in four phases:
  • Prophase - Centrioles migrate to the poles. DNA forms short, thick, double-stranded chromosomes. Nuclear envelope breaks down.
  • Metaphase - Mitotic spindle attaches to the centromere. Chromosomes align in the center of the cell along the equatorial plane.
  • Anaphase - Mitotic spindle pulls the chromosomes apart and sister chromatids move to opposite ends of the cell. The cell begins to elongate.
  • Telophase - Chromosomes uncoil. Nuclear envelope reforms around chromatin. Spindles break down.

Cytokinesis

• Cytokinesis begins during telophase.
• Physical dividing of cytoplasm, organelles, and cell membrane into two separate cells.
• A ring of microfilaments contracts at the midline of the cell, eventually pinching the cell in two.

Cell Cycle Control

• There are checkpoints where the genetic material is checked for errors and mutations.
• Internal and external controls provide "stop" and "advance" signals to the cells.
• If errors or mutations are discovered:
  • DNA repair occurs, if possible.
  • The cell cycle is arrested permanently if not repairable.
  • Apoptosis occurs if severe errors/mutations are discovered.

Checkpoints - Locations

• G1 checkpoint - Determines whether the cell should divide and enter the S phase.
• G2 checkpoint - Leads to mitosis.
• M checkpoint - Occurs during metaphase and triggers the exit process from the M phase and entry into the G1 phase.

Regulation of the Cell Cycle

• Two types of genes regulate normal cell division:
  • Proto-oncogenes - Stimulate cell division.
  • Tumor-suppressor genes - Inhibit cell division.
• Programmed cell death (apoptosis) is initiated when genes regulating cell division are damaged.

Cancer

• Cancer is a growth disorder of cells.
• Abnormal cells grow uncontrollably and can damage surrounding tissues.
• A growing cluster of cells is called a tumor or neoplasm.
• There are two types of tumors:
  • Benign - Abnormal mass of tissue surrounded by healthy cells. Remains at the site where it forms.
  • Malignant - Not encapsulated. Invasive. Spreads to multiple locations via metastasis.

Mutations that Lead to Cancer

• Cancer results from damaged genes that fail to control cell division.
• Mutations in proto-oncogenes can speed up cell division.
• Mutations in tumor-suppressor genes prevent the cell cycle from stopping at checkpoints when DNA is damaged.

Cancer Cells - Malignant Tumors

• Cancer cells usually have 2 or more of the following characteristics:
  • Up-regulated oncogenes (mutated proto-oncogenes) or down-regulated tumor-suppressor genes, causing increased cell division and tumor development.
  • Do not undergo cell death and become "immortalized."
  • Spread to other regions of the body.
  • Cause growth of new blood vessels for nutrient access (angiogenesis).

Known Causes of Cancer

• DNA mutations can be random, inherited, or caused by carcinogens.
• Carcinogens are anything that fosters development of cancer:
  • Chemical carcinogens - May cause mutations in genes that regulate cell division.
  • Viruses - Cause about 5% of cancers in the U.S.
  • Radiation - Causes mutations in DNA, including cosmic rays, UV light, and radon.

Cancer Treatments

• Surgery - When a tumor is accessible and can be removed without damaging vital tissue.
• Radiation & Chemotherapy - Goal is to damage DNA in rapidly dividing cells.
• Immunotherapy - Goal is to boost natural immune responses against cancer cells.
  • Immune checkpoint inhibitors allow the immune system to respond more vigorously than usual.
  • T-cell transfer is when T-cells attacking the tumor are removed, altered in a lab, and placed back into the tumor.
• Gene therapy - Insert normal tumor-suppressor genes into cancerous cells, or a gene to prevent accelerated cell division.

Cancer Treatment Side Effects

• Radiation and Chemotherapy have side effects because they kill healthy cells also.
  • Side effects include temporary sterility, nausea, anemia, and hair loss.

The importance of early detection in treating cancer

• Early detection of cancer is crucial because the cancer is less likely to have spread to other parts of the body, which greatly increases the chances of successful treatment and increases the likelihood of complete recovery.

Explore the essential concepts of chromosomes, including their structure, function, and the differences between chromatin, diploid, and haploid cells. This quiz covers crucial topics regarding DNA organization and the role of histones in gene activity. Test your knowledge of somatic cells and genetic inheritance patterns.