Chromosomes and Cell Division

• Chapter 8 explores chromosomes, cell division, and related concepts.
• This chapter also looks at sexual reproduction, genetic variation, and chromosomal disorders.

Cell Division and Telomeres

• Normal cell division is essential for maintaining organisms.
• Telomeres in eukaryotic cells shorten with each cell division, tracking the number of divisions.
• Excessively short telomeres lead to the loss of essential DNA and cell death.

DNA's Role in Reproduction

• DNA utilizes the cell to gather energy and carbon for replication.
• DNA, not the cell, is the main character in the story of life
• DNA reproduces as often as possible, requiring the cell to do so.

Mitosis Review

• Chromosomes replicate and sister chromatids condense before mitosis.
• The spindle forms as mitosis begins.
• Spindle fibers attach to centromeres, pulling sister chromatids to the cell's center.

Interphase and Mitosis Stages

• Interphase: Chromosomes replicate.
• Prophase: The nuclear membrane breaks down and sister chromatids condense, with the spindle forming.
• Metaphase: Sister chromatids align at the cell's center.
• Anaphase: Sister chromatid pairs separate and move to opposite sides via spindle fibers.
• Telophase: Chromosomes uncoil, the nuclear membrane reforms, and cytokinesis begins.
• Cytokinesis: The cytoplasm divides, resulting in two daughter cells.

Inheritance and Common Ancestry

• Traits are inherited from a common ancestor
• Traits are in addition to those acquired since then.

Evolutionary Concepts

• Living things change slowly and gradually
• Maintaining the past while adding to it.
• Homology describes common ancestry.

Lancelet Significance

• The lancelet represents the earliest chordate ancestors.
• It provides insight into the appearance of our common ancestor.

Homologous Structures

• Homologous structures are inherited from a common ancestor.
• These structures change over time.

Ploidy in Chromosomes

• Ploidy refers to the number of homologous chromosomes.
• Having more than a haploid set is considered redundant.

Homologous Chromosomes

• Homologous chromosomes share the same evolutionary origin.
• These chromosomes contain the same genes but may have different forms.

Genetic Recombination

• Recombination moves genes between evolutionary lineages.
• Horizontal gene transfer, facilitates genetic recombination between bacteria with the assistance of viruses, plasmids, or uptake of floating DNA.
• Evolution is not always linear; endosymbiosis is horizontal gene transfer.

Cancer and Cell Division

• Cancer is characterized by unrestrained cell growth and division.
• It can lead to serious health issues, and is the second leading cause of death in the United States.
• Cancer occurs when DNA disruption interferes with cell division regulation.
• Cancer cells ignore contact inhibition.
• Cancer cells divide indefinitely and exhibit reduced "stickiness".
• Benign tumors are masses that don't spread and can be safely removed.
• Malignant tumors shed and spread cancer cells, known as metastasis.
• Cancer can kill organisms when tumors metastasize.
• Cancer treatments, like chemotherapy and radiation, can be painful.
• Cancer mutations stimulate or restrain cell growth.

Asexual vs Sexual Reproduction

• Asexual reproduction occurs when a single parent produces identical offspring.
• Sexual reproduction produces offspring from the fusion of two reproductive cells (gametes) in fertilization.
• Some species, particularly plants, use both methods of reproduction.
• Asexual reproduction is efficient, but can lead to genetically identical offspring
• Sexual reproduction creates genetically diverse offspring.

Meiosis

• Meiosis enables organisms to produce haploid gametes.
• Diploid cells have two copies of each chromosome.
• Two haploid cells merge to create a diploid individual.
• Meiosis turns a diploid cell into a haploid cell and produces genetically different gametes.
• Meiosis occurs only in the gonads, commencing with 46 chromosomes.
• It is preceded by DNA replication, involving, two rounds of cellular division.
• Meiosis results in four genetically dissimilar haploid gametes in diploid organisms
• Meiosis changes the ploidy from haploid (1N) to diploid (2N).

Meiosis Details

• It reduces ploidy from 2 to 1.
• Stage 1-3 include prophase 1 where replicated chromosomes condense, metaphase 1, where homologues line up and anaphase I where homologues separate.
• This leads to Telophase I and cytokinesis, where chromatids arrive at the cell poles and the cell divides.
• Then prophase II, metaphase II, anaphase II, and lastly telophase II and cytokinesis.

Gametes

• Gamete production occurs only in gamete-producing cells.
• Females produce the larger gamete egg, while males produce smaller motile sperm.
• Each gamete ends up with just one copy of each chromosome.

Genetic Recombination and Variation

• Crossing over creates new allele combinations, it doesn't create new versions of any alleles
• Independent assortment shuffles parental lineages.
• Both contribute to genetic variation

Sex Determination

• Human sex chromosomes are X and Y.
• Males are XY, while females are XX.
• No information on the Y chromosome is necessary for a functioning human.
• Females lack a Y chromosome
• Sex can be determined by number of chromosome e.g. ants, bees and wasps, and environment e.g. turtles
• Other species determine sex through hermaphroditism (both male and female gametes), or through the female parent

Chromosomal Disorders

• A karyotype helps assess chromosomal abnormalities in a fetus.
• Life is possible with too many or too few sex chromosomes.
• Trisomy 21 (Down syndrome) is marked by an extra copy of chromosome 21.
• Unequal chromosome distribution during cell division is known as nondisjunction.
• Nondisjunction occur during metaphase 1, leading to homologues not separating.
• Then metaphase 2 where both gametes have an extra chromosome and others are missing a chromosome
• Problems with reproduction increases with a woman's age.
• Although it's usually fatal to have too many or too few non sex chromosomes, individuals are born with only a single sex chromosome.

Telomeres and Immortality

• Telomeres are noncoding, repetitive DNA sections.
• Telomeres act as protective caps on chromosome tips.
• Telomere rebuilding and uncontrolled cell division is known as cancer
• Telomeres can lead to Hutchinson-Gilford progeria
• Eukaryotes and cells rebuild their telomeres