Honors Biology Final Study Guide - Unit 4

Cell Cycle

life of a cell from its formation until it divides into two cells

Functions of Cell Division

Reproduction, Growth, and Tissue Repair

Duplicated chromosome

2 sister chromatids attached by a centromere

Somatic Cells

Body Cells

Diploid (2n)

2 of each type of chromosome

Mitosis

Divide by mitosis

Humans: 2n

46

Gametes

Sex cells (sperm/egg)

Haploid (n)

1 of each type of chromosome

Meiosis

Divide by meiosis

Humans: n

23

Phases of the Cell Cycle

The mitotic phase alternates with interphase: G1 → S → G2 → mitosis → cytokinesis

Interphase

90% of cell cycle

G1 Phase

cell grows and carries out normal functions - production of proteins needed for DNA replication

S Phase

duplicates chromosomes (DNA replication); after DNA replication, each chromosome consists of two identical sister chromatids attached at the centromere.

G2 Phase

prepares for cell division; makes proteins needed for cell division

M Phase (mitotic)

Mitosis: nucleus divides; Cytokinesis: cytoplasm divides

Steps of Mitosis

Prophase → Metaphase → Anaphase → Telophase

G0 Phase

the "resting" phase of the cell cycle. A cell enters G0 when it is not needed to divide and may enter G0 due to various factors like cellular signals or environmental cues.

G1 Checkpoint

This checkpoint occurs near the end of the G1 phase and checks overall cell conditions. If conditions are good, the cell is signaled to move on to the S phase (DNA replication).

G2 Checkpoint

This checkpoint occurs between the G2 cycle and mitosis and checks proper DNA replication. If DNA replication is complete, the cell will move on to mitosis.

M-spindle (Metaphase) checkpoint

This checkpoint occurs during metaphase of mitosis and checks for proper attachment of spindle fibers to all chromosomes at the centromere.

Homologous chromosomes

a pair of chromosomes in a diploid organism that carry the same genes and are inherited from different parents

Crossing over

occurs during prophase I during meiosis I. Its primary purpose is to exchange genetic material between homologous chromosomes.

Meiosis result

A cell divides twice during meiosis, resulting in 4 daughter cells with ½ the number of chromosomes.

Example of Haploid

if the diploid number in a liver cell of an organism is 52, then there would be 26 chromosomes in the egg of this organism, as an egg cell is haploid and contains ½ the number of chromosomes as a diploid cell.

Chromosome

a thread-like structure found in the nucleus of a cell, composed of tightly-coiled DNA and proteins

Gene

segment of DNA with information to direct the synthesis of a specific protein

Genome

all of the genetic information (DNA) in the cell

Characteristic

A heritable feature that varies among individuals (e.g., hair color)

Trait

Each variant for a characteristic (e.g., brown or red for hair color)

Locus

a gene's position on a chromosome

Allele

an alternative form of a gene found at a specific location (locus) on a chromosome

Genotype

DNA sequence contained in a gene (alleles)

Phenotype

the observable characteristics of an organism (includes visible features, chemical, and behavioral characteristics)

Gregor Mendel

a very important figure for establishing the development of genetics and inheritance

P generation

parental generation (true) breeding

F1 generation

the hybrid offspring of the P generation

F2 generation

produced when F1 individuals breed

Dominant allele

determines the organism's appearance, represented by a capital letter (A)

Recessive allele

has no noticeable effect on appearance, represented by a lower case letter (a)

Homozygous

two alleles at a particular locus are identical (e.g., BB or bb)

Heterozygous

two alleles at a locus may differ (e.g., Bb)

Punnett Square

a diagram that shows the possible combinations of sperm & egg

Complete Dominance

One allele (dominant) determines the phenotype & the effects of the other allele (recessive) are not seen in the phenotype.

Example of Complete Dominance

An individual with one allele for freckles will have freckles.

Example of Recessive Allele

Only someone with two recessive alleles will have albinism (no melanin).

Incomplete Dominance

Heterozygous alleles produce a phenotype that is distinct from individuals who are homozygous for one allele or the other.

Example of Incomplete Dominance

Flower color in the snapdragon: RR = red, rr = white, Rr = pink.

Codominance

Neither allele is dominant to the other; the phenotype of an individual who is heterozygous exhibits a combination of both fully expressed traits.

Example of Codominance

Cattle with red coats (R1R1), cattle with white coats (R2R2), and cattle that have a patchy red & white (R1R2).

Example of Human Codominance

Humans with A and B alleles (AB) have type AB blood.

Multiple Allelism

Occurs when there are more than two alleles of a gene in the population.

Example of Multiple Allelism

The ABO blood system has three alleles of blood-groups: IA (or A), IB (or B), i (or O).

Sex-Linked Inheritance

Genes located on the X or Y chromosome are called sex-linked genes.

Sex Chromosomes

Males have XY chromosomes, while females have XX chromosomes.

Inheritance of Sex Chromosomes

Mothers can only contribute an X chromosome, while fathers can contribute an X or a Y chromosome.

X-linked genes

Genes located on the X chromosome.

Inheritance of X-linked genes in males

Males only inherit X-linked genes from their mother.

Inheritance of X-linked genes in females

Females inherit X-linked genes from both mother and father.

X chromosome functions

Carries genes that affect many somatic (non-sex) structures.

Expression of X-linked genes in males

Males will always express the X-linked gene, regardless of whether it is dominant or recessive.

Females' genotypes for X-linked genes

Females may be homozygous dominant, homozygous recessive, or heterozygous.

Example of X-linked inheritance

Color blindness.

Dominant allele for color vision

Dominant allele on X chromosome (N) results in normal color vision.

Recessive allele for color vision

Recessive allele on X chromosome (n) results in red-green color blindness.

Female genotypes for color vision

XNXN or XNxn result in normal vision; xnxn results in red-green color blindness.

Male genotypes for color vision

XNY results in normal vision; xnY results in red-green color blindness.

X-linked diseases

Diseases caused by a mutant allele on the X chromosome, most often X-linked recessive.

Carriers of X-linked diseases

Females are usually carriers with one normal and one mutant allele, typically not affected phenotypically.

Affected females in X-linked diseases

A female may be affected if she has two copies of the mutant allele.

Prevalence of X-linked diseases

X-linked diseases are found more often in males than females.

Turner's Syndrome

Disorder associated with a single X chromosome (X).

Triple X Disease

Disorder associated with three X chromosomes (XXX).

Klinefelter's Syndrome

Disorder associated with an extra X chromosome in males (XXY).

Jacob's Syndrome

Disorder associated with an extra Y chromosome (XYY).

Edward's Syndrome

Disorder associated with Trisomy 18.

Down's Syndrome

Disorder associated with Trisomy 21.

Patau Syndrome

Disorder associated with Trisomy 13.

Pedigree Charts

Flow charts that display the patterns of genetic traits within a family.

Determining trait inheritance

Can determine whether a trait is dominant, recessive, or sex-linked.

Symbols in pedigree charts

Circle is a female, square is a male; shaded symbols show affected individuals.

Generational numbering in pedigrees

Generations are numbered I, II, III, etc.

Birth order in pedigrees

Individuals are shown in birth order.

Dominant trait inheritance

If two parents are affected (heterozygous), they may produce normal offspring.

Recessive trait inheritance

If two parents are normal (heterozygous), then they may produce affected offspring.

Sex-linked trait inheritance

Males are mostly affected; look for female carriers.

Analyzing a pedigree for recessive disorders

Pedigree shows affected individuals that come from unaffected parents.

Analyzing a pedigree for sex-linked disorders

Large number of affected individuals in one gender only (mostly males).

Dihybrid Cross

A cross that shows the possible offspring for two traits.

Gametes in a dihybrid cross

For a dihybrid cross of BbRR x BbRr, possible gametes could include: BR, Br, bR, and br.

Punnett Square for dihybrid cross

Contains 16 spaces.

Phenotype Ratio for dihybrid cross example

9:3:3:1.

Epistasis

The effect of a gene (gene B) is dependent on the presence or absence of mutations in another gene (gene E).

Modifier gene

Termed for a gene that affects the expression of another gene.

Phenotype ratio due to epistasis

Would be 9:4:3; not the 9:3:3:1 in a 'normal' dihybrid cross.

Variation due to epistasis

Results from the interaction between genes affecting phenotype.