Biology 305 - Exam 1 - UM

Biology 305 - Exam 1 - University of Michigan

Homologous Chromosomes

Two chromosomes carrying the same genes (although not necessarily the same alleles)

Only possible in diploid cells

Dyad

Duplicated chromosomes

A chromosome after DNA replication; consists of two identical copies of the original chromosome (sister chromatids) which remain attached (at the centromere)

Still counts as one chromosome

Doesn’t change ploidy

Ploidy

Measure of the number of chromosome in a cell

Chromatid

Monad

½ of a duplicated chromosome; each chromatid is essentially equivalent to the chromosome prior to duplication

Bivalent

Tetrad

Homologous dyad synapsed to form a complex

Required for proper meiosis I, allows crossing-over (recombination) between chromatids

3 Steps of Interphase

1. G₁ - Generalized growth & metabolism of the cell

2. S - Synthesis - continued growth & metabolism. replication of DNA; production of histones

3. G₂ - Continued growth of metabolism, centrosomes duplicate; tubular synthesized

Genome

The full (haploid) set of an organism’s genes (all of the heritable factors that define an organism; does not include non-essential genetic elements like plasmids)

Locus

The site on a particular chromosome where a give gene is typically located

Haploid vs. Diploid

Haploid - One set of chromosome (one copy of every gene; one possible allele)

Diploid - Two sets of chromosomes (two copies of every gene; two possible alleles)

Allele

Variant of a gene

Gene

A hereditary determinant of a trait; fundamental unit of biological information

Genetics

The study of heredity

Chromosome

A double-stranded DNA molecule encoding the information for many genes

Citokinesis

Cell constricts (or cell plate forms) to separate daughter cells

Meiosis II

Analogous to haploid mitosis (duplicated chromosome) dyad>haploid

Meiosis I

The reduction division
Starts at 4n (bivalents) but ends at 2n (diploid)

Chiasma

The site of crossing over

Mixing and matching pieces of chromosomes

Recombination

Breakage and rejoining of DNA

Homogametic sex

Can only contribute one type of sex chromosome

Human/fruit fly females; bird/butterfly males

Heterogametic sex

Can contribute two types of sex chromosome (X or Y)

Human/fruit fly males; bird/butterfly females

Haploinsufficiency

Without full level of protein activity, some critical process cannot occur/occur fast enough

Dominant Negative

When a mutant protein interferes with the function of good copy

Polygenic Inheritance

When the phenotype is determined by more than one gene.

Examples - height, weight, skin color

Recessive lethals/steriles

Occurs when a mutation is recessive but lethal/sterile in homozygous condition.

Mutant allele may be silent or may cause distinct phenotype in the heterozygote.

Organelles that have their own DNA

Mitochondria and chloroplast

They have small circular chromosomes and carry several dozen genes.

All mitochondria are inherited from your mother.

Uniparental Inheritance

Progeny inherit organelle genes exclusively from one parent but not the other.

Phenylkentonuria

An autosomal recessive disease that is the inability to convert phenylalanine to tyrosine.

Leads to mental disabilities, seizures; has no cure and is managed through a strict diet low in phenylalanine.

Achondroplasia

Dwarfism

Autosomal dominant

Lethal if homogenous

Color-blindness

Sex-linked recessive

More males suffer than females

1% recombination frequency

=1 map unit (MU) = 1 centimorgan

Interference

Cross-over at one point lessens cross-over in adjacent regions

Interference = 1 - (double recomb)/(f(single))*(f(double))

Called the coefficient of coincidenc

Pleiotropic

Any allele that affect s several properties of an organism

Mendel’s First Law

Law of Equal Segregation - In meiosis the members of a gene pair separate equally into the eggs and sperm

Meiosis Overview

Start - 2 homologs

Replication - 2 dyads

Pairing - tetrad

First division - one dyad to each daughter cell

Second division - One chromatid to each daughter cell

End - 4 daughter cells with genetic differences

Chromosome Terminology

Chromatid + chromatid = dyad

Homologous dyads = tetrad = bivalent

Mitotic Division

Dyads line up in the middle and chromatids are pulled apart.

Meiotic Division 1

Pairs of homologous chromosomes (bivalents) line up in the middle and dyads are pulled apart.

Meiotic Division 2

Dyads line up in the middle and chromatids are pulled apart.

Crossover Timeframe

4-chromatid stage to produce 2 recombinant and 2 parental.

Repeated selfing

Leads to an increased proportion of homozygotes (to create pure lines)

General rule for n-hybrid

Genotype = 3ⁿ

Neurospora

Model system because A/a meiocyte undergoes meiosis followed by mitosis resulting in equal numbers of A and a products.

Null alleles

Proteins encoded by them completely lack function

Haploinsufficient

One wild type dose is not enough to achieve normal levels of function.

Dominant Negative Mutation

Binds to wildtype polypeptides and destroys/interferes.

Haploid (x)

Amount of DNA per genome after meiosis 2

Diploid (2x)

Amount of DNA per genome after meiosis 1/ in G₁ phase

Diploid (4x)

Amount of DNA per genome after replication in S phase

Meiosis ONLY

Chromosome pairing (synapsis)

Calculating Polygenic Classes

If four heterozygous polygenes, then 2(4)+1=9, phenotype classes possible

Segregation if NO recombination

Segregation of different alleles happens in meiosis one.

Segregation if recombination

Segregation of different alleles happens in meiosis two.

Mendel’s Law of Segregation

A diploid individual possesses a pair of alleles for any particular trait and each parent passes one of these randomly to its offspring.

• Each gamete acquires one of the two alleles as chromosomes separate into different gametes during meiosis.

Linkage

If genes are on the same chromosome and close together, crossover is uncommon.