Ch 4- Genes and Cellular Function

DNA

A long, thread-like molecule with uniform diameter, varying length; carries genetic instructions.

46

How many DNA molecules (chromosomes) are in the nucleus of most human cells?

• Sugar (deoxyribose)

• phosphate group

• nitrogenous base

What are the three components of a nucleotide?

• A, T, C, G

• Purines: A, G (double-ring),

• Pyrimidines: C, T (single-ring)

What are the four DNA bases and which are purines/pyrimidines?

A pairs with T (2 H-bonds), G pairs with C (3 H-bonds); sequence of one strand determines the other.

What is complementary base pairing in DNA?

To carry instructions (genes) for protein synthesis.

What is the essential function of DNA?

~20,000 genes; ~2% coding DNA

How many genes do humans have and what percent of DNA is coding?

chromatin

Fine filamentous DNA complexed with histone proteins; forms chromosomes.

Clusters of 8 histones form a core particle; DNA wraps around like thread on a spool.

How does DNA wind around histones?

150 times thicker, 1,000 times shorter

How much thicker and shorter is chromatin compared to naked DNA?

sister chromatids

Two identical DNA filaments formed when a chromosome is copied before cell division.

kinetochore

Protein plaque on each side of a centromere; plays a role in cell division.

Ribose; uracil (U) replaces thymine

What sugar does RNA contain and which base replaces thymine?

• mRNA (messenger)

• rRNA (ribosomal)

• tRNA (transfer)

What are the three main types of RNA for protein synthesis?

gene

A segment of DNA coding for an RNA molecule that usually helps make proteins.

human genome

All DNA in one set of 23 chromosomes (~3.1 billion nucleotides)

genetic code

System where 4 DNA nucleotides code for amino acid sequences in proteins.

codon

3-base sequence in mRNA coding for an amino acid.

• Start: AUG (methionine)

• Stop: UAG, UGA, UAA

What are start and stop codons?

transcription

DNA → mRNA; copying instructions using RNA polymerase in nucleus.

translation

mRNA → protein; ribosomes read mRNA and tRNA delivers amino acids.

Initiation, Elongation, Termination

What are the three steps of translation?

Proteins that help newly synthesized proteins fold correctly.

chaperone proteins

Permanently off in some cells, temporarily on when needed; regulated by chemical messengers.

How can genes be turned on or off?

Prolactin triggers casein production in mammary cells.

Example of hormone-regulated gene activation?

Genes code for enzymes that catalyze synthesis; production occurs only when enzymes are active.

How are nonprotein compounds like testosterone regulated by genes?

1) Unwind DNA from histones

2) Helicase unzips strands

3) DNA polymerase synthesizes new strands

4) New histones assemble nucleosomes

What are the four steps of DNA replication?

semiconservative replication

Each new DNA molecule contains one old strand and one new strand.

About 1 per 1 billion bases; corrected by DNA Damage Response.

How often does DNA polymerase make errors?

mutations

Changes in DNA structure due to replication errors or environmental factors.

• Interphase (G1, S, G2),

• Mitotic phase (prophase, metaphase, anaphase, telophase), Cytokinesis

What are the phases of the cell cycle?

Cells exit the cycle and stop dividing temporarily or permanently.

What happens in G0 phase?

Produce two genetically identical daughter cells for growth, repair, and maintenance.

What is the purpose of mitosis?

Chromosomes condense, nuclear envelope disintegrates, spindle fibers form

What happens in prophase?

Chromosomes align at cell equator; spindle attaches to centromeres

What happens in metaphase?

Sister chromatids separate and move to opposite poles

What happens in anaphase?

Chromosomes uncoil, nuclear envelopes reform

What happens in telophase?

cytokinesis

Cytoplasm divides, forming two separate cells

Cyclins and cyclin-dependent kinases (Cdks)

What proteins regulate the cell cycle?

karyotype

Chart of all 46 chromosomes arranged by size and pairs

• Diploid (2n): 23 pairs (somatic cells)

• Haploid (n): 23 unpaired (sex cells)

What is the difference between diploid and haploid?

allele

Different form of a gene at the same locus

• Dominant: trait usually expressed, masks recessive

• Recessive: expressed only if both alleles are recessive

Difference between dominant and recessive alleles?

• Homozygous: two identical alleles

• Heterozygous: two different alleles

Homozygous vs heterozygous?

• Codominance: both alleles expressed equally

• Incomplete: intermediate phenotype

Codominance vs incomplete dominance?

• Polygenic: multiple genes affect one trait

• Pleiotropy: one gene affects multiple traits

Polygenic inheritance vs pleiotropy?

sex-linked traits

Traits carried on X or Y chromosomes, often affecting one sex more

epigenetics

Changes in gene expression without changing DNA sequence; can be inherited.