Genetics and Cell Division: Chromosomes, Meiosis, and Genetic Variation

chromosome

Any of the usually linear bodies in the cell nucleus that contain the genetic material. Made up of DNA and proteins

centromere

Area where the chromatids of a chromosome are attached

haploid

An organism or cell having only one complete set of chromosomes.

diploid

2 sets of chromosomes

diploid number

46 chromosomes (2n)

haploid number

23 chromosomes (n)

ploidy

number of sets of chromosomes in a cell

sex chromosomes

Chromosomes that determine the sex of an individual (X & Y)

autosomes

Any chromosome that is not a sex chromosome

homologous chromosomes

Pair of chromosomes that are the same size, same appearance and same genes.

sister chromatids

Identical copies of a chromosome; full sets of these are created during the S subphase of interphase.

non-sister chromatids

chromatids belonging to homologous chromosomes

fertilization

Process in sexual reproduction in which male and female reproductive cells join to form a new cell (egg meets sperm cell)

zygote

fertilized egg

synapsis

the pairing of homologous chromosomes during meiosis

chiasma

site of crossing over

crossing over

the exchange of genes between homologous chromosomes, resulting in a mixture of parental characteristics in offspring.

spindle apparatus

the whole structure, including the spindle fibers, centrioles, and aster fibers

kinetochore

A specialized region on the centromere that links each sister chromatid to the mitotic spindle.

cytokinesis

division of the cytoplasm

describe each phase of interphase

G1- Growth

S- DNA synthesis

G2- Growth and preparation for mitosis

M- Mitosis (cell divison): formation of 2 daughter cells

describe the roles of mitosis and meiosis

Meiosis is to make genetic variety and create sex chromosomes. Mitosis produces body and identical cells by cell division, it's great for the repair of damage.

explain how genetic variation arises from crossing over

exchanging segments of DNA by paired homologous chromosomes during prophase 1 of meiosis

independent assortment

One of Mendel's principles that states that genes for different traits can segregate independently during the formation of gametes

random fertilization

source of genetic variation caused by the unlimited number of possible sperm & egg combinations

prophase/prometaphase (mitosis)

- chromosomes condense and become visible

-spindle fibers emerge from the centrosomes

-nuclear envelope breaks down and then nucelolous disappears

- chromosomes continue to condense

-kinetochores appear at the centromeres

- mitotic spindle microtubules attach to kinetochores

- centrosomes move toward opposite poles

interphase (mitosis)

G1, S, and G2 phase of cell cycle

DNA is replicated in the S phase. its Used for growth, DNA replication, and cell functions.

metaphase (mitosis)

- mitotic spindle is fully developed, centrosomes are opposite poles of the cell

- chromosomes are lined up in the middle

- each sister chromatid is attached to a spindle fiber originating from opposite poles

anaphase (mitosis)

- cohesin proteins binding the sister chromatids together, break down

- sister chromatids (now are chromosomes) are pulled toward opposite poles

- spindle fibers are lengethened and elongate the cell

telophase (mitosis)

- chromosomes arrive at opposite poles and begin to decondense

- nuclear envelope material surronds each set of chromosomes

- mitotic spindle breaks down

nondisjunction

Error in meiosis in which homologous chromosomes fail to separate.

anueploidy

Abnormal number of chromosomes.

monosomy

missing a chromosome (2n-1)

trisomy

gained a chromosome; 3 copies

haploinsufficiency

The appearance of a mutant phenotype in an individual cell or organism that is heterozygous for a normally recessive trait

trisomy 18

edward's syndrome; second-most common autosomal trisomy; seen with clenched fists

trisomy 13

Patau syndrome - 50% die within 1 month; 95% die by age 3

trisomy 21

Down syndrome caused by an extra chromosome 21.

gene dosage

The number of times a given gene is present in the cell nucleus.

where can nondisjunction occur

- during anaphase of meiosis 1 and 2

- during anaphase of mitosis

meiosis vs mitosis

- meiosis has 2 cell divisions, mitosis only one

- in meiosis homologous chromosomes pair up on cell's equator, in mitosis homologous chromosomes never pair up

- in anaphase 1 of meiosis sister chromatids are still paired, in anaphase in mitosis, sister chromatids are separated

- meiosis results in a haploid cell, mitosis results in a diploid

- meiosis has two cell divisions, mitosis only one

why is anueploidy in animals usually lethal

causes massive gene dosage imbalances; it causes genetic disorders and even death

why does nondisjunction in meiosis occur more frequent in women than men

since women are born with all their eggs and remain "stuck" in prophase 1 for decades, it can lead to misalignment during the final division.

example of human autosomal aneuploidy

Down syndrome, Edwards syndrome, and Patau syndrome

example of sex-chromosome aneuploidy

Turner syndrome and Klinefelter syndrome

gene

A segment of DNA on a chromosome that codes for a specific trait

locus

Location of a gene on a chromosome

allele

Different forms of a gene

trait

specific characteristic of an individual

dominant allele

An allele whose trait always shows up in the organism when the allele is present.

recessive allele

An allele that is masked when a dominant allele is present

homozygous

Having two identical alleles for a particular gene

heterzygous

having two different alleles for a trait

genotype

genetic makeup of an organism

phenotype

physical characteristics of an organism

testcross

cross between an organism with an unknown genotype and an organism with a recessive phenotype

incomplete dominance

A pattern of inheritance in which two alleles, inherited from the parents, are neither dominant nor recessive. The resulting offspring have a phenotype that is a blending of the parental traits.

complete dominance

a relationship in which one allele is completely dominant over another

multiple alleles

three or more forms of a gene that code for a single trait

recessive lethal allele

an allele that negatively affects the survival of a homozygote

dominant lethal allele

the presence of just one copy of the allele results in the death of the individual

autosomal dominant

inheritance pattern of a dominant allele on an autosome; Hungtion's disease

autosomal recessive

two copies of an abnormal gene must be present in order for the disease or trait to develop

prophase 1

Chromosomes become visible; the nuclear envelope breaks down; and homologous chromosomes undergo crossing-over. they begin to transfer their DNA to each other.

metaphase 1

Paired homologous chromosomes line up across the center of the cell

anaphase 1

Homologous chromosomes are being pulled away by the spindle fibers

telophase 1

2 daughter cells are formed, each daughter cell contains only one chromosome of the homologous pair.

prophase 2

A new spindle forms around the chromosomes, but no homologous chromosome pairs or crossing over

metaphase 2

Centromeres of chromosomes line up randomly at the equator of each cell.

anaphase 2

sister chromatids separate and move to opposite poles

telophase 2

A nuclear membrane forms around the chromosomes in each of the 4 new cells then cytokinesis splits the cytoplasm

wild type allele

Generally, but not always a dominant allele. It's the most prevalent allele in the population.

loss of function mutation

causes a complete or partial loss of function

gain of function mutation

produces a new trait or causes a trait to appear in inappropriate tissues or at inappropriate times in development.

null mutation

results in complete loss of function

neutral mutation

a mutation that has no effect on survival or reproduction

masking genes can

alter the appearance of the expected phenotype from a genotype

Aa x Aa phenotypic ratio

3:1

Aa x aa phenotypic ratio

1:1

Aa x Aa (recessive lethal) phenotypic ratio

2:1 (lethal so it doesn't survive)

AaBb x AaBb (no gene interaction) phenotypic ratio

9:3:3:1

AaBb x AaBb (recessive epistasis) phenotypic ratio

9:3:4

AaBb x AaBb (dominant epistasis) phenotypic ratio

12:3:1

AaBb x AaBb (complementary gene interaction) phenotypic ratio

9:7

AA x aa phenotypic ratio

1:0