🧬 DNA, Chromosomes & Forensics – Study Guide (copy)
Chromosome | A tightly coiled structure of DNA and proteins that carries genetic information
Number of human chromosomes | Humans have 46 chromosomes (23 pairs)
Karyotype | A picture showing all chromosomes arranged in pairs
What a karyotype shows | Number, size, and shape of chromosomes
Normal human karyotype | 46 chromosomes total
Male karyotype | XY
Female karyotype | XX
Monomer of DNA | Nucleotide
Nucleotide components | Phosphate group, deoxyribose sugar, nitrogenous base
Adenine (A) | Pairs with Thymine (T)
Thymine (T) | Pairs with Adenine (A)
Cytosine (C) | Pairs with Guanine (G)
Guanine (G) | Pairs with Cytosine (C)
DNA structure | Double helix
DNA strands | Two strands
Complementary bases | A–T and C–G
Hydrogen bonds | Hold base pairs together
Sugar-phosphate backbone | The sides of the DNA ladder
Antiparallel strands | DNA strands run in opposite directions (5′→3′ and 3′→5′)
How to identify antiparallel strands | One strand goes up, the other goes down
🔁 DNA Replication
Purpose of DNA replication | To copy DNA so each new cell gets identical genetic information
Helicase | Unzips DNA by breaking hydrogen bonds
Primase | Adds RNA primers to start replication
DNA polymerase | Builds new DNA by adding complementary nucleotides and proofreading
DNA ligase | Connects DNA fragments together
Leading strand | Made continuously toward the replication fork
Lagging strand | Made in short pieces called Okazaki fragments
Why leading and lagging strands occur | DNA polymerase can only build DNA in the 5′→3′ direction
How to identify leading strand | Smooth, continuous strand
How to identify lagging strand | Fragmented strand with primers
Semi-conservative replication | Each new DNA molecule has one original strand and one new strand
Template strand | Original DNA strand used for copying
Complementary strand | New strand built to match the template
🧪 Roy Brown Case (Forensic Science)
Roy Brown case | A man wrongfully convicted of rape in the 1990s
Accusation | Rape based on bite mark and hair evidence
Bite mark evidence | Later shown to be unreliable and inaccurate
Hair analysis evidence | Not scientifically reliable; led to wrongful conviction
What freed Roy Brown | DNA testing proved his innocence
🧬 Creating a DNA Profile
Steps involved | Collect DNA sample → Cut DNA at STRs → Replicate using PCR → Separate with Gel Electrophoresis → Compare banding patterns
Purpose of DNA profiling | To identify individuals based on unique DNA patterns
STRs (Short Tandem Repeats)
Definition | Short repeating sections of DNA (like “GATA GATA GATA”)
Why use STRs | They vary the most between people and are easier to measure than the entire genome
How they vary person to person | Different people have different numbers of repeats at each STR location
Gel Electrophoresis
Purpose | To separate DNA fragments by size and create a banding pattern
How it works | DNA is placed in wells in a gel and pulled by an electric current
Charge of DNA | DNA is negatively charged, so it moves toward the positive end of the gel
Which pieces move faster | Shorter DNA fragments move faster and travel farther through the gel
How to compare DNA | Match banding patterns between samples — same pattern = same person
🧮 CODIS (Combined DNA Index System)
What it is | National FBI DNA database
Number of STRs used | 20 standard STR locations
Odds of two people matching | Less than 1 in a billion (except identical twins)