Bio Exam 3

What is the central dogma of molecular biology?

DNA —> mRNA —> protein (genotype to phenotype)

What does the arrow from DNA to mRNA represent, and what enzyme does it?

Transcription, by RNA polymerase (uses the gene as a template to make an RNA copy

What does the arrow from mRNA to protein represent, and what machine does it?

Translation, by the ribosome (reads mRNA 3 nucleotides at a time)

What is a codon?

3 RNA nucleotides that specify 1 amino acid (ex: UGG = Trp)

Analogy: if mRNA is the sticky note or voicemail, what is DNA?

The permanent archive/record of the info

What are the extensions to the simple central dogma in LO 19.2?

rRNA/tRNA/other RNAs (transcribed, not translated); DNA replication; reverse transcriptase (RNA —> DNA). Protein is never a template

What do tRNAs and rRNAs do?

tRNA carries amino acids to the ribosome; rRNA is a major componet of the ribosome (both transcribed, neither translated)

What is an allele?

A different version of the same gene (any change in base sequence makes a new allele)

True or false: alleles differ only in their structural (coding) sequences, not regulatory sequences

False: they can differ in structural, regulatory, or both (regulatory-only differences can still change phenotype)

What do structural vs. regulatory sequences of a gene do?

Structural = code for the gene products; regulatory = control when, where, and how much it’s expressed

What is a nucleosome?

DNA double helix wrapped around a ball of 8 histone proteins (helps compact DNA to fit in the nucleus)

How do chromosome shapes differ between bacteria/archaea and eukaryotes?

Bacteria/archaea: circular; eukaryotes: linear

What are homologous chromosomes (homologs)?

Same genes at the same positions, but often different alleles (one inherited from each parent)

What are sister chromatids?

The 2 identical chromatids of one replicated chromosome, physically joined (often at the centromere)

What are non-sister chromatids?

Chromatids on different homologs in a pair (not joined; same genes, maybe different alleles)

A cell has 2 homologs of chromosome 1, and each has replicated. How many chromosomes and DNA molecules?

2 chromosomes, 4 DNA molecules (a replicated chromosome still counts as 1 chromosome)

What do haploid number (n) and ploidy mean?

n = number of different chromosome TYPES; ploidy = number of COPIES of each type (humans: n = 23, diploid, 2n = 46)

A drawing shows 8 chromosomes: 4 types, 2 of each. What is n and the ploidy?

n = 4, diploid (2n=8); replication state doesn’t change ploidy

Are triploid and trisomy the same thing?

No. Triploid = 3 fell sets (3N, polyploidy); trisomy = 3 copies of just one chromosome

Are triploidy and trisomy examples of aneuploidy (abnormal chromosome number)?

No, only trisomy is an example of aneuploidy; triploidy is an example of polyploidy.

Why must chromosome replication happen before mitosis? (LO 21.1)

So each daughter cell can get a full copy of every chromosome. Interphase makes the sister chromatids; mitosis then separates them

What is mitosis?

Distributes a copy of each chromosome to each daughter cell. Happens after replication and before the cell physically divides.

M phase vs, interphase?

M phase = mitosis + cell division. Interphase = growth + DNA replication. The two alternate in a cycle

Give the 5-step sequence for mitosis in the cell cycle

1) DNA replication (interphase) 2) prophase 3) metaphase 4) anaphase + telophase 5) cell division —> 2 daughter cells

What happens in prophase?

Replicated chromosomes condense and the nuclear envelope breaks down

What happens in metaphase?

Replicated chromosomes move to the middle of the cell

What happens in anaphase and telophase?

Anaphase: sister chromatids separate and move to opposite ends. Telophase: nuclear envelopes re-form around each set

Why do chromosomes condense before mitosis?

So they don’t tangle while being moved to the middle and pulled apart. Transcription stops too. Short and thick = condensed

How do chromosomes get pulled apart in mitosis?

Microtubules from either side attach to each sister chromatid at the point where they stay joined (kinesins help move them)

What are the daughter cells of mitosis like?

2 cells, genetically identical to each other and to the parent cell (2n→2n)

Do homologs pair up in mitosis?

No, each chromosome goes to the middle independently (single line)(homologs only come together in meiosis I)

Somatic vs, germ cells? (LO 21.1)

Somatic = body cells, undergo mitosis (growth/replacing damaged cells). Germ cells = give rise to gametes via meiosis → haploid cells

What are gametes?

Sperm and egg, they fuse to form a zygote

What is a zygote?

A fertilized egg: diploid, with 1 of each chromosome type from mom and 1 from dad. It divides by mitosis

How do bacteria and archaea reproduce?

No mitosis (1 circular chromosome), They copy the DNA, pull a copy to each end, and divide down the middle: asexual cloning

What is meiosis?

Reduction division: each daughter cell gets 1 chromosome from each homologous pair (2n→n). 1 starting cell → 4 daughter cells

Why does meiosis I reduce ploidy? (LO 22.1)

Homologous chromosomes separate, so each daughter cell gets only one replicated chromosome of each pair → haploid

What separates in meiosis I vs meiosis II?

Meiosis I: homologs separate. Meiosis II: sister chromatids separate (in each of the 2 cells from meiosis I)

What is the ploidy at the end of meiosis I?

Haploid (n), though the chromosomes are still replicated. (Diploid is a common mistake.)

When does DNA replication happen in meiosis?

Once, before meiosis I. NOT again before meiosis II.

What is synapsis?

In meiotic prophase, homologs come together, held by bridging proteins. Each sister chromatid lines up exactly with a non-sister chromatid on the other homolog

What are the steps of crossing over?

1) break non-sister chromatids at the same point 2) cross the double helices 3) join the broken ends to the other chromatid’s ends

What is a recombinant chromosome?

One that is partly maternal, partly paternal, containing alleles from both

Why does crossing over matter?

Every chromosome in every gamete becomes a random mix of maternal and paternal alleles. It happens at random places, at least once per pair of non-sisters on average

What is independent assortment?

Maternal and paternal homologs line up at random at metaphase I, so a wide array of combinations ends up in daughter cells.

How many chromosome combinations from independent assortment?

2^n (n = number of homolog pairs). Ex: n→3 → 2³=8

What 2 things generate genetic variation in meiosis? (LO 22.2)

Crossing over (recombination) and independent assortment (random alignment at metaphase I)

How is meiosis II like mitosis, and how is it different?

Same process, except cells are haploid and sister chromatids are no longer identical (because of crossing over)

Do mitosis and meiosis both make 4 cells?

No. Mitosis: 1 round of division, 2 cells. Meiosis: 2 rounds, 4 cells

Most important difference between meiosis and mitosis?

Homologs synapse in meiosis but stay independent in mitosis

Human cell, track chromosomes and chromatids through meiosis (LO 22.1)

Before replication: 46 chromosomes (2n). After replication: 46 chromosomes, 92 chromatids. After meiosis I: 23 (n), 46 chromatids. After meiosis II: 23 unreplicated chromosomes (n)

Human somatic cell: track chromosomes and chromatids through mitosis

Before replication: 46. After replication: 46 chromosomes, 92 chromatids. After mitosis: each daughter has 46 unreplicated chromosomes (2n)

Sexual vs. asexual reproduction?

Sexual = meiosis + fusion of gametes → genetically unique offspring. Asexual = mitosis → clones of the parent

Why bother with costly sexual reproduction?

Genetic variation helps if the environment changes/disease strikes: at least one diverse offspring may survive. Disease may have driven its evolution.

What happens after meiosis in plants/algae and fungi?

Plants/algae: haploid cells divide by mitosis into haploid individuals, some of whose cells later become sperm and eggs. Fungi: meiosis, but no sperm/eggs

What happens with odd ploidy (3n, 5n)?

Only reproduce asexually (homologs can’t pair evenly for meiosis).

Transcription vs. translation?

Transcription: DNA is copied into mRNA in the nucleus. Translation: ribosomes read mRNA codons and build a polypeptide

How is RNA different from DNA, and what are the 3 types?

Single-stranded, with uracil instead of thymine. mRNA carries the message out of the nucleus; tRNA delivers amino acids; rRNA builds the ribosome

Homozygous vs, heterozygous?

A diploid carries 2 alleles of a gene: homozygous if they match, heterozygous if they differ

How does DNA go from chromatin to a chromosome?

DNA wraps around histones → chromatin → condensed chromosome. Condensed to divide; unwound to be transcribed

Chromosome vs. chormatid?

A chromatid is one copy of a replicated chromosome. You only have chromatids after replication; an X shape = a replicated chromosome

non-sister vs non-homologous?

Non-sister chromatids are on homologous chromosomes. Non-homologous chromosomes are different pairs with different genes

How are the 46 chromosomes organized?

22 pairs of autosomes + 1 sex pair (XX or XY)

What is trisomy? is it the same as triploidy?

Trisomy = 3 copies of one chromosome (2n+1), e.g. trisomy 21. Not triploidy (3n), which is 3 complete sets and lethal in humans

What was ‘blending inheritance’?

The idea that genetic material from sperm and egg fused into a new blended substance each generation

What was ‘inheritance of acquired characters’?

The idea that phenotype changes acquired during an organism’s lifetine get passed on to its offspring

What does ‘true-breeding ’ mean? (explains Mendel’s pure parents)

Homozygous: offspring always look like the parents because they carry 2 copies of the same allele. Mendel knew their exact genetic background

What are P, F1, and F2?

P = original parents. F1 = offspring of the P generation. F2 = offspring of F1s crossed with each other

Mendel crossed pure purple x pure white. What were the F1 and F2 results?

F1: all purple (white seems to disappear). F2: purple and white reappear, about 3:1 (75% purple, 25% white).

How did Mendel’s data disprove blending inheritance?

Blending predicts intermediate F1s and no return of the lost trait. Instead the F1 looked like one parent, and the other trait reappeared unchaged in F2

Mendel’s 4 key insights from 1-trait crosses?

1) Genes don’t blend 2) Each plant has 2 copies (alleles) of each gene 3) The 2 copies separate when sperm/eggs form (1 allele per gamete) 4) One allele is dominant, the other recessive

Why do ¾ of F2 plants show the dominant trait?

F1 (Aa) makes half A and half a gametes. 3 of 4 combinations get at least one A (purple); only 1 of 4 gets a + a (white)

Phenotype vs. genotype?

Phenotype = outward, observable appearance. Genotype = the combination of alleles

Dominant vs. recessive allele?

Dominant: its phenotype shows in heterozygotes. Recessive: its phenotype does NOT show in heterozygotes; it appears only with 2 copies.

Homozygous vs. heterozygous?

Homozygous = 2 copies of the same allele (AA or aa). Heterozygous = 2 different alleles (Aa)

Are the parents