ap bio: heredity

Heredity/Inheritance

Heredity/Inheritance

transmission of traits from one generation to the next

Inherited Similarity

similarity, but not identicality, between characteristics of children and parents

Variation

slight differences in children and parents

Genetics

scientific study of heredity and hereditary variation

Genome

organism’s entire complement of DNA

Genes

coded information in the form of hereditary units that genetically link us to our parents and account for family resemblances (program specific traits)

DNA

polymer of four difference nucleotides that allows inherited information to be passed down in the form of a specific DNA sequence

What Genes Do

programs cells to synthesize specific enzymes and proteins which produce an organism’s inherited traits

Transmission of Hereditary Traits

precise replication of DNA produces copies of genes passed down from parents to offspring

Gametes

reproductive cells that are the vehicles transmitting genes from one generation to the next (sperm and egg)

Somatic Cells

cells of body except gametes are precursors (46 chromosomes)

Chromosome

DNA of a eukaryotic cell is packaged into chromosomes within the nucleus (except for small amounts of DNA in mitochondria and chloroplasts)

Chromosomes Contain

single long DNA molecule elaborately coiled in association with various proteins, coding several hundred to a few thousand genes via specific sequences of nucleotides

Locus

gene’s specific location along the length of a chromosome

Genetic Endowment

consists of all the genes that are a part of the chromosomes we inherited from our parents

Asexual Reproduction

single individual is sole parent and passes down copies of all genes to offspring w/o fusion of gametes (so offspring are genetically identical to parents)

Example of Asexual Reproduction

single celled eukaryotic organisms reproduce asexually via mitosis, where DNA is copied and allocated equally to daughter cells and genomes are identical copies of parent genome

Clone

group of genetically identical individuals

Sexual Reproduction

two parents give rise to offspring w/ unique combination of genes inherited from both parents, which allows their offspring to vary genetically from both siblings and parents (not replicas, variations w/ common resemblance, so causes genetic variation)

Life Cycle

generation ot generation sequence of stages in reproductive history of organism (concept >> production of offspring)

Somatic Cell Chromosome Count

46 chromosomes found in a diffused state throughout the nucleus, but 23 specific chromosomes, with two of each (maternal and paternal set)

Somatic Cells Differences

in mitosis, chromosomes become condensed enough to be distinguished microscopically from each other, and they differ in size, centromere position, and pattern of bands produced by chromatin binding stains

Karyotype

an arrangement of chromosomes in pairs, starting with longest to shortest prepared from isolated somatic cells that stimulate mitosis and are arrested in metaphase (most condensed chromosomes) and stained to via

Homologous Chromosomes (homologs)

two chromosomes of a pair having the same length, centromere position, and staining pattern, and carrying genes controlling the same inherited characteristics on equivalent loci (not 100% the same)

Sister Chromatids

sister chromatids are two copies of one chromosome, associated along all their lengths, and making up one duplicated chromosomes

Sister Chromatid Cohesion

sister chromatids are associated along all their lengths

Nonsister Chromatids

two sister chromatids of two different chromosomes which are not identical

Pair of Homologous Chromosomes

individual chromosomes inherited from different parents (has different versions of genes (alleles) at corresponding loci)

Centromere

middle of the sister chromatids

X and Y Chromosomes

only small parts are the same on both, in actuality most of the genes on X do not have counterparts on the Y, and vice versa

Sex Chromosomes

X and Y chromosomes

Female Sex Chromosomes

homologous pair of X chromosomes (XX)

Male Sex Chromosomes

differing pair of XY chromosomes (XY) where only small parts of X and Y are homologous

Autosomes

other chromosomes other than X and Y

Set of Chromosomes

23 chromosomes in maternal and paternal sets (46 total, 2n = 46)

n

number of chromosomes in a single set (different in each organism)

Haploid Cell

cell w/ single set of chromosomes

2n

number of chromosomes in two sets (different in each organism)

Diploid Cell

cell w/ two sets of chromosomes

Beginning of Human Life Cycle

haploid sperm from father fuses with haploid egg from mother

Fertilization

fusion of sperm and egg in union of gametes, culminating in the fusion of nuclei

Zygote

resulting from fertilization, a fertilized egg that is diploid (two haploid sets of chromosomes bearing genes representing maternal/paternal family lines)

Mitosis

mitosis of zygote and descendant cells generates all the somatic cells within the body, which have chromosomes sets in the zygote and genes that these chromosomes carried (haploid and diploid cells can undergo this)

Meiosis

cell division that produces gametes, which undergoes no further cell division prior to fertilization (only diploid cells can undergo this, bc they are the only ones with a second set of chromosomes that are reduced)

Why Meiosis

so that doubling is counterbalanced and gametes are haploid rather than diploid (and so that normal chromosome number does not double from generation to generation)

Germ Cells

gametes develops from meiosis in these specialized cells in the gonads (ovaries and testes)

Male vs Female

males produce four viable gametes in every one of their divisions, in females, only one of the gametes produced goes onto become an egg

Fertilization and Meiosis

hallmarks of sexual reproduction and alternates in sexual life cycles

Types of Sexual Life Cycles

(1) humans and most animals have only gametes as haploid cells, and meiosis occurs in germ cells to create gametes, and then they divide after fertilization by meiosis (2) plants and some algae have alternation of generations (3) fungi and protists have meiosis produces just haploid cells that divide by mitosis and give rise to unicellular descendents, or haploid multicellular adult organism that produces further mitoses, and produces cells developing into gametes that fuse and form a zygote which restarts the cycle

Commonality Between All Three

genetic variation among offspring

Alternation of Generations

both diploid and haploid stages that are multicellular, with sporophyte and gametophyte generations alternating

Sporophyte

multicellular diploid organism which produces haploid cell called spores

Spores

does not fuse with another cell but divides mitotically, generating gametophyte

Gametophyte

multicellular haploid organism that give rise to gametes via mitosis, and fusion of these gametes at fertilization developes a zygote

Overview of Meiosis

for a single pair of homologous chromosomes in a diploid cell, both are duplicated and the copies are sorted into four haploid daughter cells

Prophase I

(1) chromosomes begin to condense, and homologs pair around their lengths, aligned gene by gene (2) paired homologs become physically connected along their lengths by synaptonemal complex (3) crossing over occurs (4) centrosome movement, spindle formation, and nuclear envelope breakdown (5) microtubules from one pole or the other attach to two kinetochores of homologs, and the homologous pairs move towards metaphase plate

Kinetochores

protein structures at centrosomes of homologs

Synapsis

when paired homologs become physically connected to each other along their lengths by synaptonemal complex (a zipper-like protein structure) (ends w/ disassembly of synaptonemal complex)

Crossing Over

genetic rearrangement between non-sister chromatids involving exchange of corresponding segments of DNA molecules (begins during pairing and is completed during synapsis)

Chiasmata

x shaped regions that exit at the points where crossover has occurred, due to sister chromatid cohesion holding sister chromatids together, even where they are part of the other homolog, which holds homologs together as spindle forms for meiotic division

Metaphase I

(1) pairs of homologous chromosomes attach at metaphase plate, with one chromosome facing each pole (2) chromatids of homolog are attached to kinetochore microtubules from one pole, and the others are attached to the others

Anaphase I

(1) breakdown of proteins responsible for sister chromatid cohesion along chromatid arms allows homologs to separate (2) homologs move towards opposite poles, guided by spindles (3) sister chromatid cohesion persists at centromere, causing chromatids to move as a unit towards the same pole

Telophase I and Cytokinesis

(1) each half of the cell has a complete haploid set of duplicated chromosomes, with each having two sister chromatids and regions of nonsister chromatid DNA (2) cytokinesis occurs simultaneously w/ telophase I, forming two haploid daughter cells (3) in animal cells, cleavage furrow forms, in plant cells, cell plate forms (4) chromosomes decondense and nuclear envelopes form (5) chromosome duplication does not occur again

Prophase II

(1) spindle apparatus forms (2) chromosomes, still composed of chromatids associated at centromere, move towards metaphase II plate

Metaphase II

(1) chromosomes are positioned at metaphase plate as in mitosis (2) two sister chromatids are not genetically identical due to crossing over (3) kinetochores of sister chromatids are attached to microtubules extending from opposite poles

Anaphase II

(1) breakdown of proteins holding sister chromatids at centromere allows them to separate, and they move towards opposite poles as individual chromosomes

Telophase II and Cytokinesis

(1) nuclei form, chromosomes decondense, and cytokinesis occurs (2) four daughter cells, distinct from each other and from parent cell and with a haploid set of unduplicated chromosomes, are produced

Mitosis v Meiosis

(meiosis) reduces chromosome set number, produces cells that differ genetically from parent cell (mitosis) conserves number of chromosomes and produces genetically identical daughter cells

Events Unique to Meiosis

(1) synapsis and crossing over (2) homologous pairs at metaphase plate (3) separation of homologs

Synapsis and Crossing Over

prophase I, duplicated homologs pair up, and formation of synaptonemal complex between them holds them in synapsis, and crossing over occurs

Homologous Pairs at Metaphase Plate

metaphase I, chromosomes positioned at metaphase plate as homologs pairs, rather than individual chromosomes

Separation of Homologs

anaphase I, duplicated chromosomes of each homologous pair move towards opposite poles, but sister chromatids are attached

Cohesins

sister chromatids attached along their lengths by this, and is released in two steps in anaphase I and anaphase II

Reductional Division

meiosis I, because it halves chromosomes

Equational Division

meiosis II, because only sister chromatids separate (identical almost in meiosis II and mitosis)

Mechanisms Contributing to Genetic Variation

(1) independent assortment of chromosomes (2) crossing over (3) random fertilization

Independent Assortment of Chromosomes

random orientation of pairs of homologous chromosomes at metaphase of meiosis I, resulting in gametes differing greatly in combination of chromosomes

Assortment of Chromosomes

each pair of homologous chromosomes may orient w/ maternal or paternal homolog closer to a given pole (50% chance of daughter cell getting maternal or paternal chromosome)

Independent Assortment

each pair of homologous chromosomes is positioned independently of other pairs, so each pair is sorted independently of every other pair, and each daughter cell is one outcome of all possible combinations of maternal and paternal chromosomes

Possible Combinations

2n (two possible arrangements x two possible arrangements) (n = 23)

Crossing Over

genetic rearrangement between non-sister chromatids involving exchange of corresponding segments of DNA molecules (begins during pairing and is completed during synapsis)

Consequence of Independent Assortment

each of us has gametes differing greatly in combination of chromosomes

Recombinant Chromosomes

individual chromosomes carrying genes (DNA) derived from both parents

Parental Chromosomes

individual chromosomes with just the parent DNA

Crossover Count

1-3 crossover events occur per chromosome pair, depending on their size and position of centromeres

Crossover Event

(1) begins when homologous chromosomes pair loosely along their lengths in prophase I, where each gene aligns perfectly with corresponding gene (2) DNA of non sister chromatids (maternal/paternal chromatid of homologous pair) is broken by specific proteins at specific points (3) two segments beyond the crossover point are joined to other chromatid, creating new combos of material (4) at metaphase II, chromosomes w/ recombinant chromatids oriented in two alternative, non equivalent ways, because sister chromatids are not identical

Random Fertilization

fusion of male gamete w/ female gamete during fertilization produces a zygote out of so many different possible combinations of gametes

Evolutionary Significance of Genetic Variation

genetic variations produce people who are both suited or not suited to a local environment, and people who are suited produce more offspring, and their genes are transmitted to the population, which means genetic variation increases the chances of a population to survive

Mutations

original source of different alleles

The Blending Model

parents traits mix together to form new traits for child

The Gene Idea

parents traits are chosen between to form collection of picked set of traits for child

Gregor Mendel

monk who documented mechanism for inheritance

Mendel’s Experiments

mendel breed garden peas in carefully planned experiments, with many different varieties of peas, and controlled mating between plants

Character

heritable feature varying among individuals (ex: flower color)

Trait

each variant for a character (ex: purple flowers, white flowers)

Mendel’s Controlling of Mating Between Plants

each pea flower has stamens (pollen producing) and carpel (egg bearing) so pea plants self fertilize (pollen grains from stamen land on carpel of same flower, and sperm from pollen grains fertilize eggs in carpel) but mendel achieved cross pollination by removing immature stamens before they produced pollen, and then dusting pollen from another plant to the altered flowers, causing the zygotes to develop into a plant embryo in a seed (a pea)

True Breeding

plants that produce the same varieties as their parent plants

Breeding Experiment of Mendel

cross pollination of two contrasting, true breeding pea varieties (ex: purple v white)

Hybridization

crossing of two true-breeding varieties

P generation

parental generation