Genetics: Chromosomes, DNA, and Cell Division

• Learning Guide 6 focuses on cell division
• Cell division is related to heredity
• Cell division relates to mitosis and meiosis

Heredity Basics

• Life was believed to arise from nonliving things in the past
• Life arises only from the division of living cells
• Traits pass from one generation to the next by DNA combinations from their parents

Cell Replacement

• Cells are constantly being replaced from fertilization onward
• Daughter cells should be identical, carrying the same genetic information as parent cell
• DNA replication/cell division achieves this

DNA Structure

• DNA contains the genetic information in the nucleus of every cell within the body, around 3m
• Chromosomes are tightly coiled DNA, wrapped around histones, and inherited in pairs
• Unpackaged DNA is chromatin that has not been coiled for division
• Genes are DNA segments on a chromosome that code for certain traits
• Each gene has a location on its specific chromosome

Somatic Cells

• Somatic cells are body cells
• They are diploid and contain two sets of chromosomes (2n)
• Pairs of chromosomes are inherited from parents

Gametes

• Gametes are sex cells
• They are haploid cells, denoted as n
• They have half the number of chromosomes
• Egg and sperm unite during fertilization in creating a diploid zygote
• The zygote divides into a baby with 2n somatic cells

Chromosome Types for Autosomes

• Autosomes are chromosome pairs #1-22
• These are found in homologous pairs, where the gene sequences are the same
• One is maternal, the other paternal and not connected like sister chromatids
• They may have the same gene set, but different forms of each gene, called alleles
• Chromosomes in pairs make us diploid

Chromosome Types for Sex Chromosomes

• Sex chromosomes are chromosome # 23
• These are non-homologous but paired together
• This pairing determines the genetic sex of an individual

Karyotype

• Karyotypes arrange chromosomes in homologous pairs
• This helps observe an individual's sex and abnormalities
• Chromosome numbers vary by species
• Organism complexity does not correspond to chromosome number

Polyploidy

• Polyploidy exists when there is more than one complete set of chromosomes
• It is common in plants
• Strawberries can be diploid (2n) to octoploid (8n)
• For a species is 3n = 33, then the haploid cell has 11 chromosomes
• A haploid of 8n = 48 has 6 chromosomes

Mitosis Overview

• Through mitosis, identical daughter cells are created
• It carries on the life of cells through maintenance, growth, and repair using autosomes

Cell Cycle Overview

• Goal is to produce daughter cells with the same DNA as parent cells
• It is separated into interphase and mitosis

Interphase Basics

• It is the non-division of the cell cycle
• It has the largest part of cell cycle to prepare the cell before it can be divided
• It consists of Gap 1, Synthesis (S phase), and Gap 2
• At this time, DNA is uncondensed

Interphase Details for Gap 1

• The cell goes through a period of growth
• The Restriction point determines where the cell commit to division

Interphase Details for Synthesis

• DNA doubles creating double-stranded/replicated chromosomes
• Two sister chromatids exist as one replicated chromosome at one centromere

Interphase Details for Gap 2

• This is a period of cell prep for division

Mitosis in Depth

• Following DNA replication in interphase, cells undergo four mitosis stages before splitting of cytoplasm
• Prophase is the first stage
• Metaphase is the second stage
• Anaphase is the third stage
• Telophase is the fourth and last stage

Prophase Specifics

• Each pair of homologous chromosomes is duplicated from the Synthesis phase
• The DNA is twice the normal amount
• Replicated DNA condenses into chromosomes, held by cohesin
• The nucleus and nucleolus disappear
• Centrioles go towards poles
• Spindle fibers begin to form

Metaphase Specifics

• Spindle fibers attach to chromosomes via centromere
• Chromosomes line up in a single row
• The chromosomes are lined up atop of one another along the metaphase plate

Anaphase Specifics

• Centromeres divide and split
• Sister chromatids move apart towards different poles
• Chromosomes reaching the poles are now single-stranded and unreplicated
• They are still diploid
• One replicated chromosome = two chromatids
• Two chromatids split = one unreplicated chromosome

Telophase Specifics

• Chromosomes reach opposite poles
• Spindle fibers disappear
• Chromosomes unravel into chromatin
• The nucleus and nucleolus reappear at each pole
• Cytoplasm begins to divide

Cytokinesis

• Cytokinesis results in the division of cytoplasm to separate the cell
• Parent cells divide into two daughter cells
• Daughter cells are identical to parent cell due to replicated chromosomes
• Animal cells divide through a cleavage furrow
• Plant cells divide through a forming cell plate

Mitotic Index Lab

• Being able to identify mitosis stages from microscope slides is important

Cancer

• Cancers result from uncontrolled cell division caused by a mutation from a mutagen
• Normal cells will be destroyed in apoptosis if there is a mutation
• Cancer cells still grow and divide
• They do not die on their own

HeLa Stem Cells

• Stem cells are retrievable from living beings

Mitosis and Aging

• Telomeres protect the end of each chromosome and shorten over time
• Once telomeres are short enough, the cell dies
• Telomere length is a molecular clock for cellular aging
• Constantly long telomeres can indicate cancer

Telophase

• Chromosomes have typically begin to reach opposite poles
• Spindle fibers start to disappear
• Unraveling into chromatin
• Nucleus and nucleolus can reappear at poles
• Division of the cytoplasm is typically starts

Intro to Meiosis

• Meiosis is the process of gamete formation

Gametes

• Sex cells are formed in male and females
• These are haploid created from spermatogenesis or oogenesis
• Meiosis creates gametes

Meiosis Compared to Mitosis

• Humans must create gametes with half the genetic content
• This ensures a complete chromosome set that combines genetic information
• Forming these gametes = meiosis
• Gamestes fuse together thru fertilization, with any sperm able to fertilize
• This increases genetic variability

Meiosis Breakdown

• Meiosis uses TWO division stages
• Meiosis I includes reduction division, diving the chromosome number in half
• Meiosis II is similar to mitosis; ends stage as same number of chromosome
• Results in four unique daughter cells different from their parent

Meiosis Interphase

• Meiosis starts with interphase like mitosis
• DNA is replicated creating homologous chromosomes

Meiosis Prophase I

• Prophase I involves replicated chromosomes
• They condense into homologous chromosomes
• Spindle fires start to form and the nuclear membrane dissolves
• Pairs of homologous chromosomes line up for crossing over as a tetrad in synapsis
• This results in genetic recombination

Crossing Over

• Lined-up chromosomes are connected by chiasma
• In a tetrad of 4 chromatids, non-sister chromatids can swap genetic material
• Alleles may switch from paternal to maternal and increase genetic diversity

Meiosis Metaphase I

• Spindle fibers join at the centromere
• Chromosomes join as homologous pairs, not as individual files like in mitosis
• Chromosomes from maternal/paternal origin end up at opposite poles
• How homologous chromosomes pair up is independent
• How each lines up to the metaphase plate is random

Meiosis Anaphase I

• Spindle fibers shorten to pull chromosomes
• Sister chromatids stay together w/ replicated DNA
• New poles each get a full copy of DNA
• Cells are well on the way to being officially considered haploid

Meiosis Telophase I

• Homologous chromosome begins to unwind
• Spindle fibers disappear
• Cytoplasm is divided
• Nuclear membrane reforms around each new set of chromosomes
• Each new cell is then considered haploid, and contains EITHER the maternal/paternal set of homologous chromosomes
• The genetic content is half of the original count

Meiosis II

• Undergoes stages like mitosis (PMAT)
• Chromosomes do line up in a single file akin to mitosis
• Cells however are not genetically identical

Meiosis Anaphase II

• Chromosomes at poles are not genetically identical

Spermatogenesis Basics

• It product of cytokinesis creates four functional sperm
• It begins with undifferentiated germ cell that undergoes mitosis to become a spermatocyte
• Spermatids are immature sperm

Oogenesis Basics

• The cytoplasm is not equally divided
• Starting with an oogonium, it goes through mitosis to produce a single viable ovum and three polar bodies

Nondisjunction

• This relates when chromosomes improperly separate during either meiosis I or II resulting in gametes with too many or too few chromosomes
• The end products are
• Trisomy: Having an extra copy of a chromosome (2n+1)
• Monosomy resulting when monosomy occurs (2n-1)

Aneuploidy Disorders

• Aneuploidy disorders come from more or less chromosomes
• Monosmy means that individual is missing a chromosome
• Trisomy means that individual has an extra chomosome
• Common eamples are;
• Down syndrome with type 21
• Klinefelter syndrome with gender chromosome
• Trisomy autosomal has Cri Du Chat

Karyotype Application

• A karyotype is a visual organized arrangement of chromosomes

Alternate of Generations Overview

• Plant life cycles have two generations
• The diploid sporophyte generation produces spores through meiosis that spread and develop without fertilization
• The haploid gemetophyte generations have spores grow into a plant body that makes gemetes
• Male and female gemetes then fertilize
• They develop into a sporophyte and cycle repeats

Explore the relationships between genes, chromosomes, and DNA. Understand cell division processes like mitosis and meiosis, and their impact on chromosome number and genetic variation. This covers diploid and haploid chromosome numbers in sexual reproduction.