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Meiosis

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  • Meiosis starts with 1 diploid cell and ends with two genetically unique haploid cells.  Each haploid cell is a gamete, or reproductive cell.  Non-reproductive cells are called somatic cells

    • Know what haploid (1n) and diploid (2n) mean
       

  • Know what happens in each of the four phases (prophase, metaphase, anaphase, telophase) in both Meiosis I and Meiosis II
     

  • Crossing over is important to understand.  Know the following:

    • It occurs in prophase I and creates recombinant chromosomes (has DNA from two different individuals)
       

    • what crossing over involves (synapsis, chiasmata, sister chromatids swapping “legs”, etc.)
       

    • The advantage of crossing over: it helps increase genetic variation
       

  • Clearly understand the differences between meiosis and mitosis

Albino rabbit example of inherited conditions

Albinism can be an inherited condition 

Intro to Genetics

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*Tip: There is a lot of new vocabulary in this section.  Here is a study guide we put together to help you study the terms.  It’s important to understand how the different terms and concepts connect with each other

 

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Understand what characters, alleles, and traits are, and the difference among the three:

  • Characters = a feature that can be inherited, such as eye color or earlobe shape
     

  • Trait = a version of a character, such as blue or brown eye color
     

  • Alleles = the genetic variants of a gene.  Every person has two copies of each chromosome, one from mom and one from dad.  That’s why we typically have two copies of a given gene

    • Dominant allele = will mask the presence of the recessive allele.  This is why in heterozygous genotypes (genetic composition for a character), the phenotype (physical appearance) is still the dominant trait

 

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Know the terms used to describe Mendel’s pea plant experiments.  Studying the terms in the context of the pea plants “story” will help you remember and understand them:

  • True-breeding parents and hybridization
     

  • P, F1, and F2 generations
     

  • Phenotype vs. genotype: understand the difference between them

 

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Understand the following two laws in genetics and know the difference between them.  Both of them refer to events in meiosis:

 

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Punnett squares are a tool used to determine genotype and phenotype frequencies among offspring, if the parents’ genotypes are known.  The following terms are used when discussing Punnett square results:

  • Homozygous dominant, homozygous recessive, and heterozygous genotypes.  For example, HH, hh, and Hh
     

  • Testcross.  This is a method of working backward to determine the genotype of a parent  

 

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*In addition to these terms, practice making Punnett squares and computing the genotype and phenotype probabilities using the multiplication and addition rule.  These types of problems are very common on school and AP exams.  Here is an example practice worksheet you can use.

 

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More complex inheritance patterns: codominance, incomplete dominance, multiple alleles, pleiotropy, polygenic inheritance, epistasis.  The last two terms refer to inheritance relationships involving more than one gene.  This study sheet from Biology Corner will help you study/review all these different terms

 

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Pedigrees = a diagram showing the family history of a particular gene inheritance across multiple generations

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*Tip: study several examples of pedigrees and see what information you can learn about the inheritance pattern.  Can you tell if it’s showing a dominant or recessive trait?

 

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Know the common examples of genetically inherited diseases and what type of inheritance pattern they exhibit:

  • Cystic fibrosis

  • Tay-sachs disease

  • Huntington’s disease

Chromosome structure

A chromosome is made up of chromatin (a mixture of DNA and protein).  Humans have 46 chromosomes totalImage courtesy of Vecteezy

Chromosomes and Inheritance

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Know the following terms and how to use pedigrees, and other tools/evidence to determine whether a gene is sex-linked or if two genes are linked

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  • Sex-linked genes = those on either the X or Y chromosome.  Usually they are on the X, because it’s larger and can contain more genes

    • EX: Hemophilia and color-blindness, both X-linked recessive 
       

  • Linked genes

    • Tend to be inherited together.  Generally, the closer together they are physically located on one chromosome, the higher the probability that they are inherited together
       

    • Understand how to compute recombination frequencies to determine if two genes are linked or not..  Recombination freq = # of recombinants / total offspring.
       

*Tip:  This concept is confusing to many students at first; I recommend carefully reading the textbook examples or this great Khan Academy article about genetic linkage and mapping
 

  • Understand linkage maps and what map units represent, how to interpret these maps
     

  • There are several chromosomal disorders that can occur.  Know the following types and some specific examples:

    • Nondisjunction

    • Aneuploidy, a result of nondisjunction.  EX: Down syndrome (trisomy 21)

    • Polyploidy, which is abnormal in humans but is actually naturally occurring in many common plant species, such as bananas (which are triploid)

    • Deletions, duplications, inversions, and translocations

DNA structure with double helix shape

DNA structure, synthesis, and function

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  • Know what Griffith’s mouse experiment and Hershey & Chase’s phage experiment showed, and how they were accomplished.  Questions relating to these show up often on exams

 

  • DNA structure: double helix, with hydrogen-bonded nitrogenous base pairs as the “rungs of the ladder”.  Sugar and phosphate backbone.  Antiparallel, meaning the 5’ end of one backbone is aligned with the 3’ end of the second backbone.  The antiparallel concept is easier to understand with a diagram, like this one from Lumen Learning
     

  • Understand the semiconservative model for DNA replication
     

  • How DNA replication differs in prokaryotes vs. eukaryotes.  One key difference is that prokaryote DNA is circular and thus can only have one origin of replication (involving a replication bubble) 
     

  • DNA Replication:  Understand all the steps and enzymes involved.  This short video of the process from Amoeba Sisters will really help you understand and remember everything.  Know the following terms:

    • Helicase, topoisomerase, primase (which synthesizes the RNA primer), DNA polymerase III, leading and lagging strands, Okazaki fragments (part of the lagging strand), DNA polymerase I, DNA ligase, nuclease

    • Understand why there is a leading and lagging strand
       

*Refer to the guide below for an overview of these different enzymes 

 

  • Chromosomes = made of DNA tightly packed together and wrapped around small proteins called histones.  Chromatin simply refers to the DNA and protein together

Protein synthesis

 

  • Understand the relationship between gene expression and protein synthesis
     

  • Understand the differences in protein synthesis in eukaryotes vs. prokaryotes (hint: remember that prokaryotes do not have a nucleus)

 

  • Know the three stages of eukaryotic protein synthesis, including where each happens and all the components involved:

  1. Transcription: from DNA to RNA, using RNA polymerase.  The promoter is a segment of DNA at which transcription begins. In eukaryotes, the promoter also includes a TATA box.  Occurs in the nucleus
     

  2. mRNA processing: spliceosomes splice out the introns (noncoding segments of DNA), keep the exons, and add a 5’ cap and poly-A tail to the RNA transcript.  Alternative splicing can also occur in mRNA processing.  These modifications result in the final mRNA, which is exported out of the nucleus to the cytoplasm
     

  3. Translation: from mRNA to polypeptide chain.  Occurs in the cytoplasm with the help of ribosomes and tRNA.  tRNA has anticodons that are complementary to the mRNA’s codons.  Also understand wobble 
     

*Tip: studying this diagram of the whole process from the textbook will really help

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  • For AP Bio, memorize the codons AUG, UAA, UGA, and UAG.  AUG is the start codon (starts translation) and the other three are stop codons

 

  • Types of mutations that can occur and disrupt protein synthesis:
     

    • Point mutations, which include base-pair substitutions.  Also know the 2 types of point mutations: missense and nonsense
       

    • Frameshift mutations, due to base-pair insertions or deletions

Regulation of Gene expression

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  • Understand the differences in gene expression for eukaryotes vs. prokaryotes:

    • Prokaryotes have special strings of genes called operons, which can turn gene expression on and off

    • Eukaryotes do not have operons.  Instead, they can modify their DNA with acetylation or methylation.  Eukaryotes also have distal and proximal control elements, transcription factors, and enhancers.  Know what all of these terms are

    • Eukaryotes also have special noncoding RNA molecules that help with gene expression.  Know the following types: siRNAs, miRNAs (microRNAs)

    • Eukaryotes also have homeotic genes, which specify the body structure placement/order.  For instance, in insects, the head comes first, then the thorax, then the abdomen, etc.
       

  • It is very important to understand how operons work in detail.  Know the different components and the difference between repressible vs. inducible operons.  Also know exactly how these two examples of operons work: trp operon and lac operon

    • Understand how the lac operon helps with both positive and negative gene regulation, using the molecules cAMP and CAP
       

  • Understand how cancer genes work, and the difference between oncogenes, proto-oncogenes, and tumor-suppressor genes.  Know the p53 and ras genes, what they do, and how damage to them can lead to cancer

In addition to knowing the concepts and terminology, it's also crucial to apply them by doing plenty of practice problems.  If you're struggling to find good problems to work through, check out our popular Practice Portal!  

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