Exam 3

Allele

Alleles are two versions of a gene that segregate during gamete formation and are rejoined at fertilization.

Homozygous

Homozygous dominant = two dominant alleles (DD)

Homozygous recessive = two recessive alleles (dd)

Heterozygous

One dominant and one recessive allele (Dd)

Dominant

A dominant allele expresses its trait even when only one copy is present.

Shows up in every generation.

Recessive

Recessive traits can skip one or more generations.

A recessive allele is only expressed when two copies are present.

Genotype

The genetic makeup of an organism.

The total set of alleles the organism contains.

Phenotype

The physical appearance of an organism

Examples: hearing or deafness

Reason for performing a test cross

A testcross is used to determine whether an organism is: homozygous dominant (“purebred”) or heterozygous (“mutt”)

What is needed to perform a testcross

Cross the individual with the unknown genotype with a homozygous recessive organism.

Polygenic Inheritance

More than one gene affects a single trait

Examples: Human height, Eye color, and Skin color

Pleiotropy

A single gene affects more than one trait

Examples: Sickle cell anemia or Cystic fibrosis

Co-dominance

Two different alleles are both expressed

Neither allele overpowers the other

Example: AB blood type in humans

Incomplete Dominance

A heterozygous genotype produces an intermediate phenotype.

Example: Pink flowers in four o’clock plants

Epistasis

One gene affects the expression of another gene.

Example: Coat color in Labrador retriever’s gene Y determines whether pigment is produced gene X determines black or brown color if pigment is produced

What is the difference between heterochromatin and euchromatin?

Heterochromatin= tightly packed, usually not actively transcribed

Euchromatin= Loosely packed, actively transcribed into RNA

What are histones?

Positively charged proteins that DNA wraps around

-Helps package DNA and regulate gene expression

What are nucleosomes?

Consists of DNA histone proteins

What is the function of condensin and when is it needed?

Condensins compact chromosomes in mitosis

What is the function of cohesions and when are they needed?

Cohesions hold sister chromatids together. They are needed from DNA replication to anaphase.

-Ensures chromatids separate correctly

What is the difference between Diploid and Haploid?

Diploid= (2n) two sets of chromosomes

Haploid= (n) One set of chromosomes

What are homologous chromosomes?

Pais of chromosomes with the same genes (one from each parent)

What is the difference between autosomes and sex chromosomes?

Autosomes: not sex chromosomes

Sex chromosomes: Determine biological sex X and Y

What is a karyotype?

An organized pictorial display of chromosomes

Cell cycle phase: G0

Resting phase, cell is not dividing.

Cell cycle phase: G1

Cell grows and performs normal functions

Cell cycle phase: S-phase

DNA replication occurs

Cell cycle phase: G2

Cell prepares for mitosis

Cell cycle phase: M-phase

Mitosis and cytokinesis occur

Phases of mitosis: Prophase

Chromosomes condense spindle forms

Phases of mitosis: Prometaphase

Nuclear envelope breaks down, Spindle attaches.

Phases of mitosis: Metaphase

Chromosomes align at the cells center

Phases of mitosis: Anaphase

Sister chromatids separate

Phases of mitosis: Telophase

Nuclear envelopes reform, chromosomes decondense.

What is a cleavage furrow and where is it formed?

A groove that pinches the cell into two

-formed in animal cells

What is the difference between cytokinesis in plants and animals?

Animals: Cleavage furrow forms

Plants: Cell plate forms

When do the three checkpoints occur?

G1 checkpoint: Before DNA replication

G2 checkpoint: Before mitosis

M checkpoint: Before chromatid separation

How are cyclins controlled?

Cyclins are synthesized and degraded cyclically.
Explanation: Their levels rise and fall to regulate cell cycle timing.

How are cyclin dependent kinases (CDK’s) controlled?

CDK’s are activated by binding cyclins and phosphorylation

-CDK’s regulate progression through the cell cycle

What is the difference between proto-oncogenes and tumor suppressor genes?

Proto-oncogenes: Promote cell division

Tumor suppressor genes: Inhibit cell division

What is the function of p53?

p53 halts the cell cycle or triggers apoptosis if DNA is damaged.
Explanation: It prevents damaged cells from dividing and is known as the “guardian of the genome.”

Gametes (germ-line cells)

egg and sperm

are produced by meiosis

haploid (n)

Somatic cells (body cells)

produced by mitosis

diploid (2n)

What is a zygote?

A zygote is a diploid (2n) cell produced by the fusion of sperm and egg during fertilization (syngamy).

Synapsis

The pairing of homologous chromosomes during Prophase I.

Synaptonemal complex

A layer of proteins that holds two homologous chromosomes together during Prophase I.

Crossing over

The exchange of DNA between non‑identical sister chromatids of homologous chromosomes during Prophase I.

Chiasmata

The physical sites where crossing over occurs.

Tetrad

A structure formed when homologous chromosomes pair, consisting of four chromatids.

Independent assortment

The random orientation of tetrads on the metaphase plate during Metaphase I, leading to genetic variation.

Where are the two places’ chromatids within a tetrad are held together?

Chromatids are held together:

1. By the synaptonemal complex (between homologous chromosomes)

2. By cohesion proteins (holding sister chromatids together)

Differences of mitosis and meiosis

• Mitosis

  • Produces identical daughter cells

  • Cells remain diploid

• Meiosis

  • Produces non‑identical daughter cells

  • Cells are haploid

  • Crossing over and independent assortment occur

Similarities of mitosis and meiosis

Both involve:

• Chromosome condensation

• Spindle formation

• Separation of chromosomes

Telomeres

repetitive DNA sequences found at the ends

Keep chromosomes protected during replication

*when telomeres become too short they can no longer replicate*

Chromosome packing sequence

nucleosomes

solenoids

chromatin loops

rosettes

Solenoids

Coiled nucleosome fibers

Chromatin loops

loops anchored to a protein scaffold

Rosettes

tightly compressed chromosomal structures seen in mitosis

What does H1 do??

Links one end of the nucleosomal DNA to another.