Genetics Units 1-4

Nuclear DNA is present as

linear chromosomes inside the nucleus at interphase

nuclear envelope

a distinguishing feature of eukaryotes

Organelle DNA

present as circular chromosomes in the mitochondria and chloroplasts.

G1 (gap 1)

cell growth

S-Phase

synthesis, DNA replication

G2 (Gap 2)

cell growth pt2

M-Phase

mitosis + cytokinesis. Eukaryotic cell division

Interphase

Contains G1, S, and G2

quiescence

Known as G0. The cell may exits the division cycle during G1 and become a non-dividing cell.

checkpoints

steps at which the progression through the cell division cycle is monitored and regulated

During G1 or G0

each chromosome consists of a single DNA molecule

After S-Phase and during G2

each chromosome consists of two DNA molecules (two identical sister chromatids)

Telomeres

stable ends of linear chromosomes

centromere

the defined region of a chromosome at which sister chromatids are joined following DNA replication

decondensed interphase chromosomes

occupy their own distinct territories within the nucleus

Prior to cell division

the chromatin condenses into visible chromosomes due to the tightening of interactions between DNA and chromatin proteins

histones

chromatin proteins

gene

sequence that encodes a trait. A unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic

chromatin

fibers of decondensed DNA chromosomes. occurs during the non-divisional phases of the cell cycle

p arm

“petite”, the short arm of the chromosome

q arm

the longer arm of the chromosome, typically bellow the centromere

Chromosome

Before DNA replication: A single molecule of DNA

After DNA replication in preparation for cell division: Chromosomes consist of two sister chromatids

metacentric

the centromere is at the center of the chromosome

submetacentric

the centromere is near the center of the chromosome

acrocentric

the centromere is near the telomere of the chromosome

telocentric

the centromere is at the telomere of the chromosome

haploid (N)

one copy of genetic material subdivided into chromosomes

Diploid (2N)

Two copies of genetic material subdivided into chromosomes

A diploid organism has

two sets of chromosomes organized as homologous pairs

Alleles

alternative forms of a gene found on the same position (locus) of homologous chromosomes

homologous chromosomes

chromosomes with the same length and centromere location. Contain the same linear sequence of loci but not necessarily identical DNA sequences. Some of the genes may be allelic, but they are not sister chromatids.

transmission genetics

a unit of heredity that is transferred from parent to offspring

molecular genetics

a region of a chromosome that codes for a functional product

locus

a specific location on a chromosome

all genes are in loci, but

not all loci contain genes

Karyotype

a visual representation of the complete set of chromosomes in an individual organism

normal human karyotype

shows 22 homologous pairs of autosomal chromosomes and 1 pair of sex chromosomes

Ploidy

the number of sets of chromosomes in an organism

polyploid

3n, 4n, 5n, etc

Polyploid

none of the above (EX humans not having 46 chromosomes)

kinetochore

protein structure that is assembled on the centromere

spindle microtubules

attach to the kinetochore and mediate the disjunction of the sister chromatids

centrosome

a pair of centrioles. It is the microtubule-organizing center of animal cells. Replicated before the beginning of mitosis.

prophase

the chromatin condenses into the visible chromosomes. The centromeres move away from each other to become poles of the mitotic spindles made of microtubles.

prophase/metaphase

the nuclear envelope breaks down, the mitotic spindles attach to the centromeres of the chromosomes, and the chromosomes line up at the metaphase plate.

anaphase

the chromatids are pulled apart by the spindles and are disjoined. They are now individual chromosomes

Telophase

the chromosomes are segregated and their DNA decondenses. They become the chromosomes of the daughter cells. The nuclear envelopes reassemble.

Mitosis in diploid cells

Number of chromosomes and DNA per cell

mitosis of a diploid cell

two diploid (2n) daughter cells with the same chromosomal content of the original cell. Each chromosome consists of a single DNA molecule

Meiosis

a special type of eukaryotic cell division cycle that occurs during the generation of gametes and spores. Creates four haploid (1n) cells.

Meiosis I

1st division of meiosis, also known as the reduction division. The tetrads are disjoined, the separated homologous chromosomes with two chromatids each are segregated into daughter cells.

synaptonemal complex

an ordered protein structure that forms between homologous chromosomes during meiotic prophase 1. Mediates the pairing of the chromosomes and facilitates crossing over between non-sister chromatids.

Leptotene

DNA begins to partially condense

zygotene

homologous chromosomes pair to form tetrads (Also known as bivalents)

pachytene

condensation continues and the individual chromatids become more visible. Crossing over occurs

Diplotene

the homologous chromosomes begin to partially separate. The chiasmata become visible

chiasmata

points at which crossing over has already occurred

diakinesis

the homologous chromosomes pull further apart (but remain together). Further condensation occurs in preparation for metaphase.

meiosis 2

2nd division, the equational division. The chromatids are disjoined and become individual chromosomes. They are segregated into daughter cells

mitosis events

• homologous chromosomes do not pair at any time

• during anaphase, the chromosomes are disjoined and the chromatids become individual chromosomes, which are segregated into daughter cells

meiosis events

• homologous chromosomes pair to form tetrads during prophase 1

• in anaphase 1, the tetrads are disjoined and the homologous chromosomes are segregated into daughter cells

• in anaphase 2, the chromosomes are disjoined and the chromatids become individual chromosomes, which are segregated into daughter cells.

spermatogenesis

equal cytokinesis, results in four gametes. EX Sperm

Oogenesis

unequal cytokinesis, one cell receives all the cytoplasm. The rest of the cells become polar bodies. Only results in one gamete (EX Egg)

Ferritization

produces a diploid (2n) zygote

miotic cell division outcome (redefined)

a cell divides to generate two daughter cells with the same chromosomal content of the original cell. The dividing cell may have any chromosomal content

meiotic cell division outcome (redefined)

a cell undergoes two rounds of division, and generates 4 daughter cells with half the chromosomal content of the parent cell. The dividing cell must have an even number of chromosomes.

meiosis fails when

organisms do not contain chromosome compliments in an even number, unbalanced gametes.

nondisjunction

occurs if chromosomes or chromatids do not separate during anaphase 1 or 2. Both chromosomes migrate to the same gamete, and after fusion trisomic, monosomic aneuplodies may occur

trisomy

three copies of a chromosome are present

monosomy

only one copy of a chromosome is present

partial monosomy

the loss of a piece of one of the two copies of a chromosome

complete autosomal monosomies

are not viable in humans

Pisum sativum

garden pea plant

Pea plants are

self-fertilizing in nature, therefore their traits remain pure in the wild, but they can also be cross-fertilized easily

The character _____ exhibits only two contrasting traits: _____ and _____

seed color; green, yellow

characters studied by Mendel

1. seed color

2. seed shape

3. seed coat

4. pod color

5. pod shape

6. flower position

7. stem length

monohybrid cross

the experimental mating of two individuals with contrasting forms of a character, but only one character is studied in the experiment.

Each trait studied by Mendel is determined by

a unit factor. EX) R and r are both unit factors

Mendel’s first three postulates

1. Unit factors in pairs

2. Dominance/Recessiveness

3. Segregation

Unit factors in pairs

Genetic characters are controlled by unit factors that exist in pairs in individual organisms

Dominance/Recessiveness

When two unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant to the other, which is said to be recessive.

Segregation

During the formation of gametes, the paired unit factors separate randomly so that each gamete receives one or the other with equal likelihood.

Genes considered allelic

they must be different variants of a gene and must map to the same locus on both homologous chromosomes

genes

units of inheritance (Mendel’s unit factors)

Alleles

alternative variants of a gene, resulting in contrasting traits for a character

polymorphic

genes and loci with multiple versions

genotype

the genetic makeup of an individual (the alleles present in an individual)

homozygote (homozygous)

an individual with two copies of the same variant (allele) of a gene

heterozygote (heterozygous, hybrid)

an individual with two different variants (alleles) of a gene

phenotype

the physical expression of a trait (the consequence of the individuals genotype)

locus (or loci plural)

the exact location a gene is on the chromosome

segregation occurs

in anaphase 1 of meiosis

A heterozygote cross results in

two different types of gametes

testcross (Tt x tt)

a cross of an individual showing a dominant trait(s) and an individual that is homozygous recessive for the same character(s). It is a mating experiment to determine the genotype of an individual that exhibits the dominant phenotypes. The ratio for this cross is 1:1

dihybrid cross

two independent characters are studied at the same time. The characters behave as in two separate monohybrid crosses. Ratio is 9:3:3:1

product rule

when two independent events occur simultaneously, the combined probability of the two outcomes is equal to the product of their individual probabilities of occurrence

Mendel’s fourth postulate: independent assortment

during the formation of gametes, segregating pairs of unit factors assort independently from each other.

How to make a branch diagram

don’t forget your fractions

Mendel’s law of independent assortment applies to

characters controlled by loci on separate chromosomes, and the events that occur in metaphase and anaphase of meiosis 1.

an individual with the genotype RrYr

produces four different types of gametes (Ry, rY, ry & RY) with equal probability due to independent assortment. he gametic combinations depend on the orientation of the homologous chromosomes in the tetrads of metaphase I of meiosis, and their disjunction in anaphase 1